Steric Effects of Phosphorus Ligands in Organometallic Chemistry and

Homogeneous Catalysis^

CHADWICK A. TOLMAN

Central Research and Development Department, E. I. du Pont de Nemours and Company, Experimental Station, Wilmington, Delaware 19898

Received June 2, 1976 (Revised Manuscript Received September 10, 1976)

Contents number of papers have appeared which show that steric effects
313
are generally at least as important as electronic effects and can
I. Introduction
dominate in many cases. Molecular structures, rate and equi-
A. Definition of Electronic and Steric Effects 314
librium constants, NMR chemical shifts, and even relative in-
B. The Electronic Parameter v 314
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frared intensities have been correlated with ligand cone an-

C. The Steric Parameter 9 314
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D. Limitations of the Molecular Models 315 gles.

There have been no reviews of phosphorus ligand steric ef-
II. Structures of Real Ligands and Complexes 316
fects, though Pidcock4 did devote a short section to the subject
III. Spectroscopic Properties and Electron
319 in his chapter in “Transition Metal Complexes of Phosphorus,
Distributions
A. NMR Chemical Shifts and Coupling Constants 319 Arsenic and Antimony Ligands”, published in 1973. For back-
B. Infrared Frequencies and Intensities 323 ground reading on steric effects in organic chemistry, the reader
C. Electronic Spectra 324 should see books by Brown5 and Taft.6 I have not attempted a
D. Electric and Magnetic Dipole Moments 324 comprehensive coverage of the literature but rather have se-
E. Electrochemistry and ESR 324 lected works, published through 1975, which best illustrate the
F. Ionization Potentials of Free Ligands 324 basic principles. Though phosphorus ligands are of primary in-
IV. Rates and Equilibria 325 terest, I have included some data on ligands coordinated by As,
A. Dissociation of Phosphorus Ligands 325 Sb, and Bi. For the convenience of the reader, values of v and
B. Dissociation of Other Ligands 325 0 are tabulated in Appendixes A and B. For purposes of com-
C. Metal-Metal Bond Cleavage 326 parison, Appendix C gives cone angles for a few ligands of other
D. Associative Reactions 326 types, such as H, halogen, alkyl, acyl, CO, and 7r-C5H5.
E. Ligand Exchange Equilibria 327 Abbreviations used include:
F. Oxidative Addition Reactions 328 Me methyl
Isomerism
G. 330 Et ethyl
V. Unusual Reactions and Products 334 Pr propyl
A. Displacement of Other Ligands 334 Bu butyl
B. Unusual Complexes of Arenes, Ph
335 phenyl (0 = Ph in figures)
Tetramethylethylene, C02, N2, and Hydrides Ar
335 aryl
C. Unusual Coordination Numbers
Bz benzyl
D. Products with Unusual Structures 335
E. Unusual Oxidation States 336
Cy cyclohexyl
VI. Homogeneous Catalysis 337 Np naphthyl
A. Reaction Rates 337 Cp cyclpentadienyl
Pz pyrazolyl
B. Product Distributions 337
Tol tolyl
C. Asymmetric Induction 338
339
L any monodentate phosphorus ligand
VII. Steric and Electronic Map
VIII. Steric Effects of Other Ligands 340 dmpe Me2PCH2CH2PMe2
IX. Summary 341
p3 CH3C(CH2PPh2)3
X an anionic ligand, generally a halogen
X. Addendum 341
DH dimethylglyoximato
XI. Appendixes 343
TPP tetraphenylporphyrin
XII. References 346
salen /V,/V'-ethylenebis(salicylideniminato)
p-CO a bridging carbonyl
I. Introduction acacen /V,A/'-ethylenebis(acetylacetoniminato) (see 41)
been recognized that changing substituents on
It has long diop 2,3-0-isopropylidene-2,3-dihydroxy-1,4-bis(diphen-
phosphorus ligands can cause marked changes in the behavior ylphosphino)butane (see 92)
of the free ligands and of their transition metal complexes. Prior v f'co(Ai) of Ni(CO)3L in CH2CI2 in cm-1
to 1970, nearly everything was rationalized in terms of electronic 0 the ligand cone angle in degrees
effects, although there were scattered references to steric ef- <5 an NMR chemical shift
fects. In that year, quantitative measures of electronic1 and A coordination chemical shift (<5compiex 5free)
—

steric2 effects were proposed—based on A-i carbonyl stretching P an electric dipole moment
frequencies (v) in Ni(CO)3L complexes, and ligand cone angles Ei/2 a half-wave potential in electron volts
(9) of space-filling CPK molecular models.3 Since then a large D Debyes
t In honor of Professor Richard C. Lord on the occasion of his 65th birth-
BM Bohr magnetons
day. S any substituent on phosphorus

313
314 Chemical Reviews, 1977, Voi. 77, No. 3 Tolman

ELECTRONIC STERIC

Figure 1. A schematic definition of electronic and steric effects.

PlOCHjCHjCl],
PM'W-.*"3 1 /Mil,CM.
PI0CH;CCI3!j

PWi /“
PM!3 PMe^f^ \ J
Figure 3. CPK molecular models of PfOChysCMe, PMe3, P(OPh)3, PPh3,
PCy3, and P(f-Bu)3.
PEI*,
\ p(op3

TABLE I. Selected Values of v (cm 1)

L V Av

P(p-Tol)3 2066.7 0.1

Pl+l3 HjC P(0-ToI>3 2066.6
PMe3 2064.1 2.4
2070 2080
v (cm"1) PEt3 2061.7 2.5
P('-Pr)3 2059.2 3.1
Figure 2. The semiquantitative ability of various ligands to compete for P(f-Bu)3 2056.1
coordination on Ni(0) plotted against v, from ref 2.

Frequencies for a large number of ligands are given in Appendix

Phosphorus ligands with different substituents are generally A. The additivity of substituent contributions Xi (also given in
written with the substituents given in order of increasing size,
Appendix A) shown in eq 1 makes it possible to estimate v for
for example, PHMe2, PMe2Ph, or PPh(f-Bu)2.
a variety of ligands for which it has not been measured.1

A. Definition of Electronic and Steric Effects For PX1X2X3 i/= 2056.1 ± Ex; (1)
1=1
By effect I
changes in molecular properties as a result
mean:
of changing part of a molecule That v is indeed a measure of electronic effects—not affected
electronic—as a result of transmission along chemical by crowding of the Ni(CO)3 by the substituents on phosphorus—
bonds, for example, changing from P(p-C6H4OCH3)3 to is suggested by the near identity of values for P(p-Tol)3 and
P(p-C6H4CI)3 P(o-Tol)3 in Table I and by the small and regular decreases in v
steric—as a result of forces (usually nonbonding) between on replacing FI by Me. [The decreasing electronegativity of the

parts of a molecule, for example, changing from P(p- alkyl phosphines going down the series is, however, partly due
C6H4CFI3)3 to P(o-C6H4CH3)3. Special cases involve
to alkyl-alkyl repulsions, as we shall see in section III.F.j
bonding between parts of a molecule, as in going from
P(OEt)3 to P(OCH2)3CMe or on changing n in a chelate C. The Steric Parameter 0
complex [Ph2P(CH2)„PPh2] M. The ligand cone angle 0 was introduced after it became clear
A nonverbal definition is shown in Figure 1. that the ability of phosphorus ligands to compete for coordination
It is important to realize that steric effects can have important positions on Ni(0) could not be explained in terms of their elec-
electronic consequences and vice versa. For example, in- tronic character (Figure 2).2 The ligands P(OCFI2)3CMe, PMe3,
creasing the angles between substituents will decrease the P(OPh)3, PPh3, PCy3, and P(f-Bu)3 show a decreasing binding
percentage of s character in the phosphorus lone pair. Changing ability in that order. CPK molecular models of these ligands
the electronegativity of atoms can also affect bond distances (Figure 3) clearly show an increase in congestion around the
and angles.7 Thus, electronic and steric effects are intimately bonding face of the P atom in the same order. The steric pa-
related and difficult to separate in any pure way. A practical and rameter 0 for symmetric ligands (all three substituents the same)
useful separation can be made, however, through the parameters is the apex angle of a cylindrical cone, centered 2.28 A (2.57
v and 0. cm) from the center of the P atom, which just touches the van
der Waals radii of the outermost atoms of the model (see Figure
B. The Electronic Parameter v 4). If there are internal degrees of freedom (e.g., rotation about
P-C bonds), the substituents are folded back, as shown in Figure
Strohmeier8 showed that phosphorus ligands can be ranked 3, to give a minimum cone. For values of 0 over 180°, mea-
in an electronic series (based on CO stretching frequencies) surements may be made more conveniently by trigonometry,
which is generally valid for a wide variety of monosubstituted as shown in Figure 5. Figure 6 indicates how an effective cone
transition metal carbonyls. For our electronic parameter v, we angle can be defined9 for an unsymmetrical ligand PX-|X2X3, by
choose the frequency of the At carbonyl mode of Ni(CO)3L in using a model to minimize the sum of half-angles shown in eq
CH2CI2. We could have chosen some other carbonyl complex, 2.
but Ni(CO)3L forms rapidly on mixing Ni(CO)4 and L in a 1:1 ratio
at room temperature, even if L is very large; the A-i band is sharp 0 =
(2/3) £ 0,72 (2)
and readily measurable with an accuracy of ±0.3 cm-1. f=i
Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 315

Figure 4. (a) Ligand angle measuring device; (b) the cone angle, from gands.
ref 2.

TAN a = h/d
&copy;= 180 + 2 a

Figure 5. Method of measuring cone angles larger than 180°.

TABLE II. Equilibrium Constants9 In Benzene at 25°:

Ki
NiL4 .
*
NiL3 + L

Original
L Kd, M 9,6 deg New 9, deg

P(OEt)3 <10-i°c 109 109

PMe3 <10_9c 118 118
P(0-p-C6H4CI)3 2 X 10_1° —121 128
P(0-p-Tol)3 6 X 10~1° ~121 128
P(0-/-Pr)3 2.7 X 10~5 114 130
PEt3 1.2 X 10-2 132 132
P(0-o-Tol)3 4.0 X 10“2 ~165 141
PMePh2 5.0 X 10“2 d 136
PPh3 e 145 145
9
Selected values from ref 9. 6 From ref 2. 0 At +70 °C. d 9 for un-
symmetrical ligands was not defined in ref 2. 9 No Ni(PPh3)4 was detected Figure 8. Strain energy in phosphorus ligand models as the ligand cone
in solution: ref 58. is compressed, from ref 9.

In the case of chelating diphosphines, 0,72 can be taken as the

D. Limitations of the Molecular Models
angle between one M-P bond and the bisector of the PMP For ligand models with fixed geometries [such as pf3,
angle.9 P(OCH2)3CMe, or P(f-Bu)3], or with only a few internal degrees
A much better correlation of binding ability of ligands on Ni(0) of freedom (PMe3 or PPh3), 9 can be rapidly and confidently
with 9 is shown in Figure 7. [The values of 9 used (Appendix measured to ±2°. With more complex ligands, it may be difficult
B) are not in all cases those reported originally.] to decide when a minimum cone has been reached. It may be
There is an approximate group additivity relationship for cone possible, as shown in Figure 8, to get still smaller angles by in-
angles of unsymmetrical PX^Xs ligands, which assumes that troducing strain (indicated by nonparallel faces between atoms).
0,72 will be the same as in PX,-3. Physically this means that the The question arises; How much strain in a model is realistic in
orientation of substituent X, which minimizes 9 in eq 2 will be terms of the behavior of real molecules? To answer, we must
the same which minimizes 9 of PX/3. Mathematically, 9 for turn to chemical experiments.
PMePhs is two-thirds of the way between PMe3 (118°) and PPh3 NiL4 dissociation equilibrium constants (Table II) show that
(145°) or 136°. Ka is not sensitive to changing para substituents in aryl phos-
316 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

TABLE III. SPS Angles3

ph3
93.8
PFs Ni(PF3)4 H3B-PF3 opf3
96.3 98.4 100 101.3
P('0-C6H4)3P OP(o-C6H4)3PO
97.0" 100.5b
PMe3 OPMe3
98.9 ~106
P(OCH2)3CH2Br OP(OCH2)3CMe
100.1 c 103.7d
PCI3 OPCI3
100 103.8
PPh3 Au(PPh3)3+ OPPh3
103 103.83 107
P(?-Bu)3 NiP(f-Bu)3Br3_
105.7' 107.0®
3
From ref 12 unless noted otherwise. b D. Schomburg and W. S. Shel-
drick, Acta Crystallogr., Sect. B, 31, 2427 (1975). c D. S. Milbrath, J. P.
Springer, J. C. Clardy, and J. G. Verkade, J. Am. Chem. Soc., 98, 5493
d
(1976). D. M. Nimrod, D. R. Fitzwater, and J. G. Verkade, ibid., 90, 2780
(1968). Reference 11. 'Estimated from 31P NMR data in ref 53. Ref-
e

erence 24.

might be expected. The meshing of three PPh3's (9 = 145°) in

a plane occurs in the structure11 of Au(PPh3)3+ (Figure 10); there
are graphite-like interactions (about 3.4 A spacings) between
rings 1 and 6, 4 and 9, and 2 and 7.
Cone angles of the CPK models are based on a 2.28 A M-P
bond length and on tetrahedral angles about phosphorus. In real
moleules both parameters are likely to vary. We have found,
however, that variations in M-P or P-C bond lengths in the
models of 0.1 A seldom change 9 by more than 3 or 4°—not
much more than the uncertainties in the measurements, and
certainly not enough to cause gross distortions in the steric scale.
Real M-P bond lengths vary from about 2.12 [Ni(PF3)4] to 2.55
A [WCI4(PMe2Ph)2] in a recent tabulation of structures;12 2.28
A is about in the middle of the range. In any case, one is usually
comparing complexes of one metal, where the metal covalent
radius is fixed.
Angles between substituents on real phosphorus ligands are
phites. If we assume that steric effects dominate, the original generally less than tetrahedral and can be changed by crowding,
values of 9 are clearly not in the right order for the experimental as we shall see in the next section, which describes steric ef-
Kd\ PPh3 > P(0-o-Tol)3 and P(0-/-Pr)3 > P(0-p-Tol)3. Closer fects on structures.
inspection of the models shows that 114° for P(0-/-Pr)3 requires
a great deal of strain; an unstrained value is near 130°. The 165°
II. Structures of Real Ligands and Complexes
for P(0-o-Tol)3 was measured2 with all three methyl groups
pointing toward P; pointing them away gives an essentially Angles between substituents on trivalent phosphorus (SPS
strain-free model at 141°. The similarity of Ka's for PMePh2 and angles) are invariably less than 109.5°, as seen in the first col-
P(0-o-Tol)3 and the fact Kd is larger for P(0-p-Tol)3 than for PMe3 umn of Table III, but approach tetrahedral as the substituents
suggest that strain-free models give a more realistic picture of increase in size. Coordination to a transition metal, BH3, or O
the size of real ligands. usually opens the angle by 3 or 4°.
In cases where 9 is difficult to measure with models, a value This opening can be seen in the structure of P(CFI20)3-
can be estimated from a simple experiment in which Ni(CO)4 is PFe(C0)3P(CFl20)3P13 in Figure 11. Notice that the carbonyl
heated with an eightfold excess of L in a sealed tube.10 The groups bend away from the larger PC3 ligand (9 = 114°, de-
degree of substitution (DS) of CO by L can be readily estimated termined by the methylene hydrogens) toward the smaller P03
from the IR spectrum in the carbonyl region.2 Figure 9 shows (9 = 101°). The Fe-P03 bond is also shorter by 0.07 A, but it
the rather good correlation of DS with 9, larger ligands replacing is unclear to what extent the effect is steric or electronic. The
fewer CO's. The value of 1.25 for PCy3 is the average of three same problem arises in the structures of Cr(CO)5PPh3 and
determinations, with the average deviation indicated by error Cr(CO)5P(OPh)3, where the Cr-P bond lengths differ by 0.11 A
bars. On this basis, PCy3 can be assigned a cone angle of 170°, (2.422 and 2.309 A).14 In both Cr complexes, the four equatorial
rather than the original model-based value2 of 179 ± 10°. carbonyl groups are bent toward the axial CO to give average
Several other types of experiments to be described also show angles of about 88.5°.
that PCy3 is significantly smaller than P(f-Bu)3, which has an The M-P bond is 0.075 A shorter (2.406 vs. 2.481 A) in
accurately measurable 9 = 182°. It is such revised values of frans-CpMo(CO)2[P(OMe)3]l15 than in frans-CpMo(CO)2(PPh3)l.16
9 which appear in Appendix B. More electronegative substituents on P are expected to give a
One difficulty with the ligand cone idea is that even symmetric shorter M-P bond because they put more phosphorus s char-
real ligands do not have cylindrical symmetry. They can mesh acter into the bond. This is nicely shown by the 0.07-A con-
into one another and achieve higher coordination numbers than traction (2.265 to 2.191 A) in the Fe-P distance on going from
Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 317

Figure 11. Structure of P(CH20)3PFe(C0)3P(CH20)3P, from ref 13.

TABLE V. Structural Data3 on Square-Pyramidal 5-Alkyl-5H-
TABLE IV dibenzophosphole Complexes (2)

Bond Lengths, A
Compound d{Co-P), A
R = Me R =
Et
M-P(l) 2.312 (5) 2.342(5)
CpNi(jU-CO)2Co(CO)2PEt3 2.236 (1)a
M-P(ll) 2.248 (6) 2.257 (5)
CpNi(M-CO)2Co(CO)2P(p-C6H4F)3 2.242 (3)b
2.269 (2)c M-P(lll) 2.306(6) 2.309(5)
7r-MeC5H4Ni(M-CO)2Co(CO)2PPh2Cy
a b M-Br(1) 2.534 (3) 2.542(3)
F. S. Stephens, J. Chem. Soc., Dalton Trans., 1067 (1974). I. L. C.
c M-Br(2) 3.026 (3) 3.143 (3)
Campbell and F. S. Stephens, ibid, 340 (1975). I. L. C. Campbell and F.
S. Stephens, ibid., 337 (1975).
Angles, Deg
P(I)MP(II) 98.40(19) 98.55 (18)
P(I)MP(III) 168.57 (19) 166.87 (17)
CpFe(CO)2P(CF3)2 to its oxide CpFe(CO)2P(OXCF3)2.17 [The CPC
P(II)MP(III) 92.62(20) 93.01 (20)
angle opens from 94.5 (3) to 96.4 (3)°.] 86.21 (13)
Br(2)MP(l) 85.90(14)
The,substituents on the phosphorus atoms of 118 are similar 108.34(15)
Br(2)MP( II)
in size, but very different electronically. The Pt-P(CF3)2 bond is 88.09 (5)
Br(2)MP(lll)
3
K. M. Chui and H. M. Powell, J. Chem. Soc., Dalton Trans., 1879
(CF3)2 2.168(3) (1974).
,P ,CI
2.369(3)
91° TABLE VI. Structural Data3 on Cu(l) Complexes
2.317(3)
P Dist
Ph, 2.244(2) Cu-P, A ZPCuP, deg
Complex
1
Cu(N03)(PPh3)2 2.25 131.2 (1)
shorter by 0.07 A and its trans Pt-CI bond by 0.05 A. These ef- 2.29 140 (1)
Cu(N03)(PCy3)2
fects must be largely electronic. A very similar structure was Cu(B3H8)(PMePh2)2 2.25,2.26 128 (1)
found for the Pd analogue.19 Cu(B3H8)(PPh3)2 2.27,2.29 120.0 (1)
A role for steric effects is, however, evident in Table IV, where 3
S. J. Lippard and G J. Palenik, Inorg. Chem., 10, 1322 (1971).
.

the Co-P bond lengths increase in the order of ligand size, not
electron-acceptor character. PPh3 to PCy3 in the nitrate complexes increases the Cu-P dis-
Data for a pair of electronically similar complexes 2 (R = Me tance by 0.04 A and the PCuP angle by 9°. Going from PMePh2
or Et) are given in Table V. The more bulky ethyl ligand gives to PPh3 in the borane complexes increases the distances by only
longer M-P bonds (as much as 0.03 A) and longer M-Br (as much about 0.02 A. In this case, the interaction with the borane is more
as 0.12 A). The cis PMP and Br(2)MP angles increase by as much
important than between the phosphines, and the larger phos-
as 4°. phine has a smaller PMP angle. (The PMePh2 ligands have their
smaller Me groups toward the B3H8.)
Palenik and co-workers20 have reported a nice example of
steric effects on structure in the series [Ph2P(CH2)nPPh2[-
Pd(SCN)2 with n = 1, 2, or 3, shown in Figure 12. Not only does
the PMP angle increase with increasing n, but the binding mode
of the thiocyanates goes from N,N to N,S to S,S.
The 0.06-A longer M-P distance in frans-Ptl2(PCy3)2 than in
frans-PtBr2(PEt3)2 (Table VII) has been attributed to greater steric
crowding in the iodide complex. [The Pt-I distance is slightly
longer (0.014 A) than the value calculated from the Pt-Br dis-
tance and the difference in covalent radii of I and Br.] The shorter
Table VI shows the effects of changing phosphorus substit- Pt-P distance in frans-FI2Pt(PCy3)2 approaches that in Pt-
uents on the structures of some Cu(l) complexes. Going from (PCy3)2.
318 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

Figure 13. Structure of RhCI(PPh3)3, from ref 26.

Figure 14. Structure of c/s-PtCl2(PMe3)2, from ref 28.

TABLE VII. Pt-P Distances in frans-PtX2L2 Complexes

Complex d(Pt-P), A Ref

Pt(PCy3)2 2.231 (4) 23

H2Pt(PCy3)2 2.25 (1) 22
PtBr2(PEt3)2 2.315 (4) 21 Figure 16. Structure of HRh(PPh3)3, from ref 33. The H presumably lies
Ptl2(PCy3)2 2.371 (2) 21 on the three-fold axis below the Rh.

The unusually crowded molecule NiP(f-Bu)3Br3- 24 shows an analogous but less crowded lr(Ph2PCH2CH2PPh2)2+ does, as
exceptionally long Ni-P bond of 2.48 (1) A, 0.20 A longer than does lr(PMe2Ph)4+. The x-ray structure of both dioxygen com-
the more normal value of 2.28 A in NiPPh3l3-.25 Average BrNiBr plexes have been determined.300
and INil angles are 108.7 and 114.0°, respectively. The mean It is common in hydride complexes to find distortions away
CPC angle of 107.0° in NiP(f-Bu)3Br3_ is one of the largest re- from idealized geometries because of bending of ligands toward
ported for a phosphine complex. An even larger value of 114.5° the less sterically demanding hydride.31 An example is trans-
in HP(f-Bu)3+ in the same crystal has been attributed24 to the HPdCI(PEt3)232 (Figure 15). The phosphines have rotated to
much smaller steric requirements of -H compared to -NiBr3. minimize interaction of the methylene groups with the Cl. Going
This example does illustrate how the geometry within a phos- to frans-HPdCI[P(/-Pr)3]2 increases the mean PMCI angle from
phine can be significantly altered by contacts with other atoms 95.3 to 96.3°.31 With a smaller metal atom in frans-HNiCI[P(/-
in a complex. The P(f-Bu)3 cone angle of 184 ± 2° in the anion, Pr)3]2, the angle is still larger, 98.3°.31
determined from the x-ray structure, is in good agreement with The structure of HRh(PPh3)4 (Figure 16) has a threefold axis,
the 182° measured using CPK models. but it is so far distorted from a trigonal bipyramid that it is nearly
There are many examples of structures where deviations from tetrahedral.33 In HRh(PF3)(PPh3)3, where the smaller PF3 is trans
idealized geometries are attributable to steric effects. One ex- to H, the average angle between axial and equatorial phosphines
ample is the nonplanar arrangement of the heavy atoms in is 99.3°.34 Axial-equatorial phosphine angles of 89° are found
RhCI(PPh3)326 (Figure 13). The short Rh-P bond trans to Cl re- in 3.353
flects a reduced trans influence of Cl relative to P (certainly an
electronic effect), and results in a greater 1JRhP in the 31P NMR
spectrum.27 The heavy atoms are also not coplanar in cis-
PtCI2(PMe3)228 (Figure 14). Steric crowding between the two
phosphines is more severe than between the two halogens. The
Pt-P length is 0.03 A longer than in PtCI3(PEt3)-,29 where the
deviations from an idealized square-planar geometry are much
less (mean CIPtCI angle 89.4 (2)°).
The crystal structure of lr(PMePh2)4+ shows a large tetrahedral The equatorial CO's in 4 are bent slightly toward the Mn-Mn
distortion away from the idealized square plane.303 Significantly, bond (Table VIII) and staggered to reduce repulsive interactions.
the compound does not react with 02 or CO, although the closely Phosphines of moderate bulk replace CO’s in the less crowded
Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 319

TABLE VIM. Structural Data for Mn2(CO)1() and Its Derivatives TABLE X. Grim’s Group Contributions3 to S(31P)

Mean
For PR1R2R3 <5(31P)caicd
=
£ GC,

angle Dist Dist

MnMnC, deg Mn-Mn, A MnP, /
R, GC, R, GC,

(CO)5Mn-Mn(CO)6a 86.2 ± 1.4 2.92 Me 21 C-C5H9 0

b
(CO)5Mn-Mn(CO)4PMe2Ph 87.5 2.90 2.24 Neopentyl 18 Cy -2
PMePh2(CO)4Mn-Mn(CO)4P- 86 2.90 2.23 /-Bu 15 sec-Bu -3
MePh2c Pr, Bu 11 /-Pr -6
d
PEt3(CO)4Mn-Mn(CO)4PEt3 87.6 ± 2.6 2.91 2.25 Et 7 f-Amyl -21
a
L. F. Dahl and R. E. Rundle, Acta Crystallogr., 16, 419 (1963). b M. Bz 4 f-Bu -23
Laing, E. Singleton, and R. Reimann, J. Organomet. Chem., 56, C21 (1973). Ph 3
c
From ref 35b. d M. J. Bennett and R. Mason, J. Chem. Soc. A, 75 3
From ref 41.
(1968).

TABLE IX. 31P Chemical Shifts3 of Phosphines TABLE XI. 31P Chemical Shifts3 of Phosphites

Compound ZSPS,5 deg <5(3'P),dppm 0, deg ZOPO, <5(31P),C

Compound deg ppm 9, deg
ph3 93.8 +240d 87
b 101
98.9 +62 118 P(OCH2)3CMe 100.1 —90.9d
PMe3
PEt3 +20.1 132 P(OMe)3 -139.7 107

PPh3 103 +6.0 145 P(OEt)3 -137.6 109

-20.0s 160 P(0-/-Pr)3 -137.5 130
P(/-Pr)3
105.7' -63.3 182 P(0-f-Bu)3 -138.1s 172
P( f-Bu)3 b
3
In ppm upfield of 85% H3P04.
b
From ref 12. c From ref 53, unless
a
Negative values are ppm downfield from 85% H3PO4. Angle for
c W.
noted otherwise. d From ref 37, p 238. 8 C. A. Tolman, unpublished result P(OCH2)3CCH2Br from Table III. From G. A. Olah and C. McFarland,
d
in THF. 'Estimated from 1JPc in ref 53. j. Org. Chem., 36, 1374 (1971), except where noted otherwise. From ref
2. e Neat liquid: C. A. Tolman, unpublished result.

axial positions, but seem not to affect the structures much.

AsMe2Ph is sufficiently less sterically demanding than PMePh2
that the disubstituted complex is diequatorial rather than diax-
ial.3Sb

C0 m
/°°
CO—Mn—CO
I

CO, .CO
\ /
Mn

c°/io'S'co

III. Spectroscopic Properties and Electron

Distributions
A. NMR Chemical Shifts and Coupling Constants remain essentially constant. The strained bicyclic phosphite,
however, is ~50 ppm to higher field. Verkade46 believes that
1. 31P Chemical Shifts this is not due to an unusually small OPO angle in the cage. Un-
The electronegativity of substituents on phosphorus and the fortunately, accurate OPO angles for the acyclic phosphites are
angles between them are the two most important variables de- not available.47 A 50-ppm chemical shift difference for phos-
termining 31P chemical shifts and coupling constants.36 The large phites corresponds to a change in OPO angle of about 3°.48
range of phosphine chemical shifts in Table IX is striking, in view Gorenstein49 has recently proposed an empirical correlation
of the similar electronegativities of the substituents, but can be between 31P chemical shifts of phosphates and OPO bond an-
understood in terms of opening of SPS angles by steric crowding gles. The correlation is, however, not monotonic.
of the substituents. The cone angles (defined by the outermost Coordination chemical shifts (A = <5oornpiex <5free) depend
—

edges of the ligands) also increase with the downfield shift of on the nature of the metal and on the change in SPS angles on
<5(31P). coordination.50 Angle opening on coordination is consistent with
Grim’s group contributions41 (GC,) to 31P chemical shifts of the usually observed downfield shift (negative A). The magnitude
tertiary phosphines (Table X) can be similarly understood, though of A tends to be less for larger ligands, as seen for trans-
he first explained them in terms of hyperconjugation. RhCI(CO)L2 complexes in Figure 17. This is because SPS angles
31P shifts to high field can be anticipated if structural con- of ligands with large substituents generally open less on coor-
straints require small CPC angles, as in 5 to 7. In 8, where the dination.
angles are constrained to 60°, the chemical shift is +450 ppm,45 In chelating diphosphine complexes, A depends on ring size
the highest <5(31P) known. (Table XII). Large downfield shifts are general for five-membered
Chemical shifts of acyclic alkyl phosphites (Table XI) are in- chelate rings.51 If a phosphorus is part of two or three five-
sensitive to changes in the bulk of the alkyl. The oxygens provide membered rings as in 9 and 10, more negative values of A (in
enough flexibility in the free phosphites that the OPO angles can parentheses) can be found.52
320 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

Figure 18. Dependence of the % s character of the phosphorus lone

pair on the SPS angle, from ref 38, p 360.
Figure 17. Correlation of change in 31P chemical shift (ref 50) on
coordination with 9 for RhCI(CO)L2 complexes.
TABLE XIV. P-P Coupling Constants9 in Diphosphines

TABLE XII. 31P Data9 on [Ph2P(CH2)nPPh2]W(CO)4 Complexes Diphosphine 1JPP, Hz

n ~
1 2 3 Me2PPMe2 -180
Me( f-Bu)PPMe( f-Bu) -290
PP^l
^complex* +23.6 -40.1 0.0 Me2PP(f-Bu)2 -318
Ppm
^free> +23.6 + 12.5 + 17.3 (f-Bu)2PP(f-Bu)2 -451
9
A, ppm 0.0 -52.6 -17.3 FI. C. E. McFarlane and W. McFarlane, Chem. Commun., 582

1Jwp, Hz 202 231 222 (1975).

9
S. O. Grim, W. L. Briggs, R. C. Barth, C. A. Tolman, and J. P. Jesson,
Inorg. Chem., 13, 1095(1974). TABLE XV. 31P NMR Data9 for SeP(C6H4X)3 and P(CeH4X)3

X 1JpSe> Hz ^PSe <5P

TABLE XIII. Coupling Constants in L and HL+

p-CI 753 +2.2 +3.1
1Jpc in 1Jph in H 735 (0.0) (0.0)
Compound ZSPS9 PR3, Hz6 HL+, Hz m-CH3 726 -0.2 -0.2
p-CH3 724 + 1.6 +2.5
P(OCH2)3CMe 100.1 898 9 p-och3 719 +3.8 +4.6
P(OMe)3 827 9 o-CH3 708 +7.6 +24.6
9
P(0-/-Pr)3 796" From R. P. Pinnell, C. A. Megerle, S. L. Manatt, and P. A. Kroon, J. Am.
571 e Chem. Soc., 95, 977 (1973). Chemical shifts are in ppm upfield of the un-
P(o-C6H4)3CH (6)
PH3 93.8 548' substituted parent.
PMe3 98.9 -13 505'
PEt3 + 14 2. Coupling to 31P
PPh3 103 +21® 510®
P(/-Pr)3 + 19 455' Opening of SPS angles increases the phosphorus s character
P(f-Bu)3 +34 in the P-S bonds and decreases it in the lone pair (see Figure
PMe„+ 109.5h +56 18). This is reflected in the coupling constants shown in Table
PPh4+ logs'1 +909.' XIII. Mann53 used 1JPC in P(f-Bu)3 to estimate a CPC angle of
(MeO)PMe4 120' + 128' 105.7°.
9 6
From Table III unless noted otherwise. From ref 53 unless noted There are clearly also electronegativity effects in Table XIII.
c
otherwise. L. J. Vande Griend and J. G. Verkade, Phosphorus, 3,13 (1973). Because of the greater electronegativity of oxygen compared
d G. A.
OlahandC. W. McFarland, J. Org. Chem., 36, 1374(1971). 9 Ref-
to carbon, protonated phosphites show a larger1JPH than pro-
erence 42. 'Reference 40, p 18. 9These values are anomalously high
tonated phosphines with the same SPS angles.
because of sp2 C hybridization. h Tetrahedral by symmetry. ' T. A. Albright,
W. J. Freeman, and E. E. Schweizer, J. Am. Chem. Soc., 97, 2946 (1975). Decreasing s character in the P-P bond of the diphosphines
' For
equatorial Me groups of the trigonal bipyramid, from H. Schmidbaur,
shown in Table XIV causes a marked lowering of 1JPP (more
W. Buchner, and F. H. Kohler, ibid., 96, 6208 (1974). negative) as Me is replaced by f-Bu. The signs and magnitudes
were determined by 1H|31Pj double resonance techniques.
The P-Se coupling in Table XV shows both electronic and
A (31P) (-80.3) (-157.6) steric effects. More electronegative para substituents give higher
Ph coupling constants (more s character in the lone pair). The low
value of 1VPse for SeP(o-Tol)3 is presumably caused by CPC
angle opening; however, the high-field shift of P(o-Tol)3 is un-
usual. (It may involve an interaction of the ortho methyl groups
with the P lone pair, or a special feathering of the benzene
rings.)
Metal-phosphorus coupling constants depend on both elec-
tronic and steric factors. Grim and co-workers54 found that1JWP
in LW(CO)5 complexes is dominated by electronic effects,
Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 321

TABLE XVI. NMR and IR Data3 for LW(CO)5 Complexes

Strongest
1
Jwp "CO
L (±8 Hz) (±1 cm-1)

PPh3 280 1942

PBuPh2 250 1938
PMePh2 245 1939
PPh2(f-Bu), PPh2(/-Pr), PEtPh2 240 1937, 1937, 1938
PBu2Ph 235 1937
PBu3 200 1934
3
From ref 54. See also J. G. Verkade, Coord. Chem. Rev., 9, 1
(1972).

TABLE XVII. NMR Coupling Constants3

Jptp, Hz 2Jpp, Hz
1

Complex X PMe3 PEt3 PMe3 PEt3

Figure 19. Correlation of d(Pt-P) with Vrp in Pt(ll) and Pt(IV) complexes
of PEt3 and PBu3, from ref 55. trans-PtX4L2 Cl 1516 660
1461 587
Br 1550 645
1471 574
judging from a good correlation of1JWP with the frequency of frans-PtX2L2 Cl 2379 2408 510 436
the strongest vco band (Table XVI). Steric effects in their study Br 2336 2336 514 449
may be masked, however, by the large uncertainties in VWP and 3
R. J. Goodfellow and B. F. Taylor, J. Chem. Soc., Dalton Trans., 1676

by the limited range of cone angles employed (130-157°). (1974).

Pidcock and co-workers55 have shown a good inverse cor-
relation (Figure 19) between Vw and Pt-P bond lengths in PEt3 taining P-NR2 groups were too small, tentatively attributed to
and P6u3 complexes of Pt(ll) and Pt(IV). They suggest that the opening of the PMP angles by steric interactions.
decreasing for PEt3 trans to Cl in 11 to 13 may be due pri- Metal-phosphorus coupling constants depend on structural
constraints in chelates (see Table XII). A marked effect can be
marily to steric distortions.
seen in comparing 16, 17, and 18. The PPtP angles in the tri-
cr Cl L+ 2233 phosphine of 18 are all restricted to ~90°, tending to concen-

Cl—It—L Cl—Pt —L
\Jr^ PPh3

Cl—jt—L 4?
i-n!
Cl L
13
PhcR PPh, V ifV \
0-0.
/
pxRpxp

X
11 12
1
Jptp 3704 Hz 3520 3449 ^p/FV
Ph3p PPh3 Ph2 Ph2 Me
Table XVII shows substantially smaller1 Jp,P for PEt3 than for
PMe3 in the crowded PtX4L2 complexes. 2JPP is always ~70 Hz 16 17 18
less for PEt3. 437058 364459 309659
The coupling constants56 in 14 and 15 show similar effects.
Here the weakening of the upper Pt-P bond on going from trate Pt s character in the fourth Pt-P bond. It is noteworthy that
P(p-Tol)3 to P(o-Tol)3 is reflected in a strengthening of the lower 18 is dissociatively stable up to 60 °C whereas 16 is completely
one. The changes in 1JptP (from Figure 19) correspond to dissociated to Pt(PPh3)3 and PPh3 at +25 °C.58
changes in bond length of 0.004 A. The small value of 'Jpp59 in 21 is due to the small PPtP angle
of 73°.60 The Pt-P bond lengths [2.30 (1) A] are normal.
P(p-Tol)3

V
2822 Ph. PPh3
CH3PtNC OCH,
\ / Ph\
'pip
2856 Ph
/\ PPh3 Ph P Ph
/\ P
PMe2Ph
19 20 21
14 1
1748 1749 1492
Jp,p
2JP 413

P(o-T ol)3 3. 1H Chemical Shifts and Coupling Constants

'ptp
2749

2943
l
CHoPtNC OCHq
Perhaps the most surprising steric effect on 1H NMR chemical
shifts is shown (Figure 20) by the methanol protons in the
[(CH3OH)Co(DH)2L]+ complexes of Trogler and Marzilli.61 Near
independence of electronic effects is suggested by the similar
PMe2Ph shifts for P(CH2CH2CN)3 and PBu3 (v = 2077.9 and 2060.3 cm-1,
15 respectively). There seems to be little steric effect until 0
reaches ~120°, presumably where the interaction with the
2J,pp 405
planar dimethylglyoximato ligand becomes important. The au-
Values of 2JPP in M(CO)4L2 complexes have been correlated thors have tried to fit their data with a model which assumes that
with the electronegativity of atoms attached to P.57 However, the increased shielding for larger L arises from a distortion of
the absolute values of 2JPP in c/s-M(CO)4L2 complexes con- the ring from planarity, but the calculated shifts are much smaller
322 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

YW
1.34

Figure 21. 'H chemical shifts in crowded aromatic hydrocarbons, from

+ ref 64.
Figure 20.1H chemical shift of methanol CH3 in [(CH3OH)Co(DH)2L]
complexes (ref 61) against 0.
TABLE XX. 13C NMR Data3 for Carbonyl Carbons in CpMn(CO)2L
TABLE XVIIi. 1H NMR Data3 for HA and H„ in CpFe(CO)LCH2R

L 6(13CO), PPm 2Upo, Hz

A5, 3UPHa 3UPHB,
R L ppm Hz Hz
P(OPh)3 228.8 36
P(OMe)3 229.5 34
Ph P(OMe)3 0.49 5.0 9.6
PPh3 232.8 23
PMePh2 0.74 5.0 9.8
PBu3 231.1 26
PPh3 0.81 3.9 10.7 3
G. M. Bodner, Inorg. Chem., 13, 2563 (1974).
SiMe3 P(OMe)3 0.72 3.6 11.5
PPh3 1.01 2.0 13.7
3
Reference 66.

TABLE XIX. 1H NMR Coupling Constants (Hz) and vptH (cm 1) in

3
frans-HPt(PEt3)2L+

L 1UptH ^PtH

P(OPh)3 872 2090

P(OMe)3 846 2067
PPh3 890 2100 23 24 25
PEt3 790 2090
3
J. P. Jesson, “Transition Metal Hydrides”, E. L. Muetterties, Ed., Marcel With the largest R and L (bottom of Table XVIII), essentially only
Dekker, New York, N.Y., 1971, p 100. rotamer 23 is populated. The large difference between 3JPHa and
3UPHb suggests that one is a gauche coupling, the other a trans.
23 is most likely because R is gauche to only one bulky group
than those observed.62 Interestingly, the colors of the compounds
(L) whereas it is gauche to two (L and Cp) in 25. With smaller L
change from yellow to dark brown as L gets larger!
(or R), rotamers 24 and 25 can be appreciably populated so that
The methyl protons in M(CO)5P(Tol)3 complexes (M = Cr, Mo,
or W) are shifted slightly downfield (0.04 ppm) with respect to 3JPHa (which is an average over rotamers) increases while 3JPHb
decreases. The explanation is supported by temperature stud-
the free phosphines in the meta and para isomers. The ortho
ies.
isomers are shifted upfield by ~0.23 ppm, apparently because
Values of Yrh in frans-FIPt(PEt3)2L+ complexes 26 (Table
of proximity to the metal or carbonyl groups.63 A related effect
XIX) tend to increase for more electronegative L. Flowever PPh3,
in crowded aromatic hydrocarbons64 can be seen in the methyl
chemical shifts in Figure 21. +
PEt3
The ortho hydrogens nearest the metal in 22 appear at un-
usually low field (8 9.33).65 The x-ray structure shows very short H—Pt—L
Pd-H distances of 2.76 ± 0.03 A.
PEt3
26
the largest ligand, is out of line. The large implies an ex-
ceptionally strong trans Pt—H bond. This complex also has the
highest Pt—H stretching frequency of the series.
22
The chemical shift difference (A<5) between the inequivalent 4. 13C Chemical Shifts and Coupling Constants
methylene protons, and the vicinal coupling constants 3VPH, in A series of ligands similar to that in Table XIX was used in a
CpFe(CO)LCFI2R complexes depend on the size of L (Table study of 13C NMR chemical shifts and coupling constants in
XVIII). Baird66 has explained the results in terms of the effect of CpMn(CO)2L complexes (Table XX). Again PPh3 is out of line;
the sizes of R and L on the populations of the rotamers 23 to 25. 2Jpc is abnormally low.
Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 323

TABLE XXI. Infrared Stretching Frequencies of ivh3 in HlrCI2(CO)L2 TABLE XXIII. Relative IR Intensities8 in Mn2(CO)gL
Complexes
L l\l /2 L l\l /2

''IrH, ''IrH,
L cm-1 L cm-1 PF3 0.05 PBu3 0.3
P(OPh)3 0.3 PPh3 0.6
PMe3 2169 PPr2(t-Bu) 2242 8
J. P. Fawcett, A. J. Poe, and M. V. Twigg, J. Organomet. Chem., 61,
PMe2Ph 2191 PBu2(f-Bu) 2244 315 (1973).
PMe2(f-Bu) 2208 PMe(f-Bu)2 2254
PMePh2 2217 PCya6 2276
TABLE XXIV. Angles (a) between CO Groups, and CO Stretching
PEt2(f-Bu) 2227 PEt(f-Bu)2 2300
Force Constants in PzB(Pz)3Mn(CO)2L Complexes8
PPh3 2237 PPh( f-Bu)2 2300
8 From ref 67
unless noted otherwise. b F. G. Moers. J. A. M. deJong,
L a, deg kco, mdyn/A
and P. M. H. Beaumont, J. Inorg. Nucl. Chem., 35, 1915 (1973).

PCI3 44.4 15.54

TABLE XXII. P—O Stretching Frequencies in Oxides P(OPh)3 45.6 15.06
PBu3 46.9 14.46
1
Compound ''p—o, cm PPh3 43.8 14.60
PCy3 42.7 14.40
OPtOCFyaCCsPh, 13278 8
From ref 69.
OP(OBu)3 1260"
OP(o-C6H4)3N 1250°
electron density in Ni(0) compounds.68 (This is also reflected in
OP(o-C6H4)3CH 1240d
a reduced basicity of the NiL4 complexes toward H+.)
OP(o-C6H4)3PO 1233 8
1195' The P=0 stretching frequencies of the oxides of phosphorus
OPPh3
a L.
J. Vande Griend and J. G. Verkade, private communication from ligands (Table XXII) show similar effects. Note the decrease in
Verkade. b S. C. Goodman and J. G. Verkade, Inorg. Chem., 5,498 (1966). frequency in the triptycene derivatives as the covalent radius
c
D. Hellwinkel and W. Schenk, Angew. Chem. Int. Ed. Engl., 8, 987 (1969). of the bridgehead atom is increased in size.
d
Reference 43. 8 Reference 42. 1F. A. Cotton, R. D. Barnes, and E. Ban- The relative IR intensities of the two highest frequency car-
nister, J. Chem. Soc., 2199 (1960). bonyl stretching modes and u2 in Mn2(CO)9L depend on the
size of L (Table XXIII). The larger ligands may increase the PMnC
B. Infrared Frequencies and Intensities angle.

Changes in bonding as a result of increasing ligand size can A

\
A
T /
A
be expected to affect vibrational spectra. One of the most
striking examples is Shaw’s67 observation of an increase in i
/ \
V
in the series of complexes of type 27 in Table XXI.
V1 v2

Schoenberg and Anderson69 have used the relative intensities

L of symmetric and asymmetric carbonyl vibrations in PzB(Pz)3-
/| Mn(CO)2L complexes to calculate angles (a) between the CO
groups (Table XXIV). Larger ligands decrease a, increasing the
27 symmetric to asymmetric intensity ratio. The CO force constants
The equilibrium constant for going to the six-coordinate show the expected dependence on electronic effects.
Steric effects can change the number of bands observed in
complex (27) in eq 3 decreases as L becomes larger,67 even
the IR. M(CO)5L complexes of Cr, Mo, and W show a single E
though some of the larger ligands are better donors. Apparently
the IrH bond becomes stronger going down the table; the Ir-P carbonyl mode (at about 1945 cm-1) when L = P(p-Tol)3 or
bonds must become weaker in the same order. P(m-Tol)3. With P(o-Tol)3, however, the band is split into a doublet
with a separation of ~10 cm-1.63
HCI + lrCI(CO)L2 The Ni(CN)2L3 complex (L = 5-methyl-5H-dibenzophosphole,
HlrCI2(CO)L2 (3)
31) shows three CN stretching bands in its mull IR spectrum at

The carbonyl stretching frequencies in 28, 29, and 30 show

the effect of increasing constraints on reducing the metal

'P'
Me
31

2118, 2108, and 2102 cm-1.70 Its 5-ethyl analogue shows only
one, at 2118. The UV absorption spectra of the crystals are also
very different. A single-crystal x-ray study shows that the com-
pounds have different stereochemistries: square pyramidal 32
vco(Ai 2076 2082 (5-Me) and trigonal-bipyramidal 33 (5-Et).

/
C—R
Ni(CO)3P^
n—'
o- NC—Ni—CN
30 NC L T
;co(A-i) 2087 32 33
324 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

TABLE XXV. Effect of Replacing P(n-Pr)3 by P(/-Pr)3 on Spectra of TABLE XXVI. Percent s Character In Co(ll) Complexes by ESR
frans-NIX2L23
L Co(TPP)La Co(salen)Lb
-Ar, cm 1

PF3 68
Band X =
NCS Cl Br P(OMe)3 55 49
P(OEt)3 51
III 600 1400 1750 P(OBu)3 48
IV 1250 1350 PMe3 36 33
From ref 72. PHMe2 34
PBu3 31
PMe2Ph 28
C. Electronic Spectra PEt3 27
PPh3 24
Dobson, Stolz, and Sheline, in their review of group 6B metal 3
B. B. Wayland and M. E. Abd-Elmageed, J. Am. Chem. Soc., 96, 4809
carbonyl derivatives, point out that derivatives with small b
(1974). B. B. Wayland, M. E. Abd-Elmageed, and L. F. Mehne, Inorg. Chem.,
phosphines are often colorless, while those of larger ones are 14, 1456(1975).
+
yellow.713 The deepening of color of [(CH3OH)Co(DH)2L]
complexes as the size of L increases was mentioned pre- TABLE XXVII. UV Photoelectron Data on Gaseous Ligands
viously.
Bennett and co-workers71b found a red shift of the lowest L IPi, eV L IPi, eV
frequency ligand field band in trans-[CrL2(NCS)4]2- complexes
in a sequence which must involve ligand size: PMe3 (19 950), ph3 10.583 PBu3 8.11b
PEt3 (19 500), and PEt2Ph (19 050 cm-1). Complexes with P(OCH2)3CEt 9.8b P(/-Pr)3 8.05b
bulkier ligands PPh3 and PCy3 could not be made. P(OMe)3 9.253 P(f-Bu)3 7.71 b
Electronic transitions of frans-NiX2L2 complexes show a PMe3 8.65a P(m-C6H4F)3 8.32b
P(vinyl)3 8.48b PPh3 7.92c
marked red shift when P(n-Pr)3 is replaced by P(/'-Pr)3. The
PEt3 8.31 b P(p-C6H4OCH3)3 7.48 b
frequencies of bands III and IV at ~25 000 and ~19 000 cm-1 3 6
O. Stelzer and E. Unger, Chem. Ber., 108,1246 (1975). M. A. Weiner
decrease by an amount which depends on X (Table XXV). The c
and M. Latlman, private communication from Weiner, M. A. Weiner, M.
shift is greater for larger X. The authors72 attribute the shifts to
Lattman, and S. O. Grim, J. Org. Chem., 40, 1292 (1975).
lengthening of the Ni-X bonds. A similar red shift has been re-
ported more recently for the lowest frequency transition in tet- series where p = 3.4 D (Cl), 3.7 (Br), and 4.1 (I), or in trans-
rahedral NiBr2L2 complexes: PMePh2 (11 680), PEtPh2 (11 360), EtPt(PEt3)2X, where n
=
3.7 (Cl) and 4.15 (I).78 The anomaly
and PPh2(f-Bu) (10 800 cm-1); here, however, the effect was should be small or absent in frans-CH3Pt(PMe3)2X. An effect of
attributed to lengthening Ni-P bonds.73 Probably both occur. PPtP angle on electric dipole moment is seen in comparing
The complexes NiCI2(PEt3)2, NiBr2(PEt3)2, and NiCI2(PCy3)2 c/s-Ph2Pt(PEt3)2 (7.2) and c/s-Ph2Pt(Et2PCH2CH2PEt2) (8.4
are red while NiBr2(PCy3)2 is olive green; all are diamagnetic in D).78
the solid state.74 The magnetic dipole moment of NiP(f-Bu)3l3- (3.07 BM) is
Ligand size can also affect the electronic spectra of NiX2L2 anomalously small when compared to NiPPh3l3- (3.46).79 The
complexes in solution by shifting the square-planar-tetrahedral corresponding complexes with te smaller bromide ion, which
equilibrium, to be discussed in section IV.G.2. are less crowded, show essentially the same moments for
Larger ligands also dissociate more readily, as we shall see P(f-Bu)3 (3.73) and PPh3 (3.68) complexes.80
in section IV. A. Thus, Ni(PMe3)4 is a light yellow crystalline solid
and gives light yellow benzene solutions. Ni(PEt3)4 crystals look E. Electrochemistry and ESR
very similar but give permanganate violet solutions, because of
a high concentration of NiL3.9 Similarly Ni(Me2PCH2CH2PMe2)2 Steric effects in electrochemistry have barely been explored.
is light yellow9 while Ni(Cy2PCH2CH2PCy2)2 is violet;75 one end Baird813 has found that the Co(lll) complexes Co(salen)L2+ and
of the bulky diphosphine remains uncoordinated, even in the Co(DH)2L2+ become easier to reduce in the order of increasing
solid. ligand size: PMe2Ph < PBu3 < PMePh2 < PPh3.
Electrochemical studies on oxidation of M(CO)2[Ph2P-
frans-Pdl2(PMe2Ph)2 exists in two crystalline modifications,
one yellow (34) and one red (35).76 35 shows much shorter ortho (CH2)„PPh2]2 (M =
Cr, Mo, W) show that the complexes with n
H-Pd distances (2.84 vs. 3.28 A).
= 1 are more easily oxidized than those with n = 2.82
ESR studies on Co(ll) phosphine complexes (Table XXVI) show
that the percent s character in the P-donor orbital generally in-
creases with more electronegative substituents on P. Large
substituents, however, show abnormally low s character, most
clearly seen for PPh3.
P—Pd—P F. Ionization Potentials of Free Ligands
Me' j / |
Me
Me 1
Me The advent of UV photoelectron spectroscopy has made it
35 possible to measure ionization potentials with high accuracy.
The first ionization potential (IP^ of free phorphorus ligands
D. Electric and Magnetic Dipole Moments nearly always arises from the phosphorus lone pair. It might be
expected that electron-withdrawing groups would increase IP-i,
The electric dipole moment of paramagnetic Nil2(PPh3)2 is which is indeed the case (Table XXVII). Going from P(p-
8.5 D, much larger than the 5.9 D of NrBr2(PPh3)2.77 This result C6H4OCH3)3 to P(m-C6H4F)3 increases IP1 by 0.84 eV. The data
cannot be explained in terms of the electronegativities of the in Table XXVII suggest, however, that there is also an important
halogens, but is consistent with a greater tetrahedral distortion steric effect on IP-|. Thus, the ionization potential decreases in
with the larger halide. the series PMe3, P(vinyl)3, and P(m-C6H4F)3 even though the
A similarly anomalous trend is seen in the frar>s-CH3Pt(PEt3)2X substituents on P become more electronegative. This and the
Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 325

TABLE XXVIli. Equilibrium Constants3 in CH3CN at 60 °C for the TABLE XXX. Relative Rate Constants3 in CHCI2CHCI2 at 40 °C for the
Reaction Reaction:
K k
CpMo(CO)2LCOCH3 ^
CpMo(CO)3CH3 + L c/'s-Mn(CO)4BrL —
Mn(CO)3BrL + CO

L K X 104, M L KX 104, M L ^"rel L ^Vel

PMe2Ph 0.1 PPh3 8.3 P(OBu)3 0.3 PBu3 1.3

PBu3 0.3 PCy3 50 SbPh3 0.6 AsPh3 2.3
PMePh2 1.3 PPh2(/-Pr) 90 P(OPh)3 0.6 PPh3 8.4
P(p-C6H4OCH3)3 5.3 P(o-Tol) b CO (1.0)
3
P(p-Tol)3 6.1 J. D. Atwood and T. L. Brown, J. Am. Chem. Soc., 98, 3155 (1976).
3
From ref 86. 6
Very large.

TABLE XXIX. Equilibrium Constants3 in 95% EtOH at 25 °C for the

Reaction:
K
Co(salen)L2+ ^ Co(salen)L+ + L

L KX 104, M L KX 104, M

PMe2PH 0.13 PMePh2 9.1

PBu3 3.7 PPh3 610
3
From ref 81.

marked decrease in IP3 in the series PH3, PMe3, PEt3, P(/-Pr)3,

and P(f-Bu)3 can be largely attributed to the decreasing per-
centage of s character in the lone pair as larger substituents
force open the SPS angles. The high ionization potential of
P(OCH2)3CEt relative to P(OMe)3 is consistent with a smaller OPO
angle in the cage.

IV. Rates and Equilibria

Ligand size can have a marked influence on rates and equi-
libria. This section will emphasize quantitative solution studies Figure 22. Rate constants in CH3CN at 60 °C for the reaction:
CpMo(CO)2(L)COCH3
—
CpMo(CO)2(L)CH3 + CO, from ref 86.
on simple reactions. Section V deals with the isolation of unusual
complexes and section VI with homogeneous catalytic sys- ciation (24.7 kcal/mol for PMe2Ph and 14.9 for PMePh2 (deter-
tems. mined by NMR line shapes) show a marked steric labiliza-
tion.81
A. Dissociation of Phosphorus Ligands Rates of dissociation of HCoL4 complexes increase in the
steric order P(OMe)3 < P(OEt)3 < P(OPh)3;87 equilibrium mixtures
We saw earlier (Table II) that steric effects dominate the in competition experiments strongly favor the smaller phos-
dissociation equilibria of NiL4 complexes. Ka at 25 °C increases
phites.
by a factor of 10® on going from P(0-p-Tol)3 to P(0-o-Tol)3, an
Rigo and Turco88 have concluded that steric effects dominate
increase in 9 from 128 to 141°. This corresponds to a decrease
equilibria of type
in AHd of 10 kcal/mol.9 The rates of L dissociation also show
a steric effect; kd is 100 to 1000 times faster for P(0-o-Tol)3 than
MX2L3 MX2L2 + L (5)
for P(0-p-Tol)3 at 25 °C. Electronic effects are also important;
phosphines dissociate faster from Ni(0) than phosphites of where M = Ni or Co and X = CN or halogen. We have confirmed
similar size.9 this in the nickel dicyanide system and find that Ks increases in
Musco and co-workers83'84 have studied the equilibria the sequence PMe3 < PEt3 < PCy3.89
PdL4 ^ PdL3 ^ PdL2 (4) The extent of L dissociation will, of course, depend on the
steric requirements of the other ligands in a complex. We find
and find that the extent of L dissociation is dominated by steric a reduced tendency to dissociate L in the analogous HNiL3CN
effects, increasing in the order PMe3 PMe2Ph ~ PMePh2 < ~

complexes,90 presumably because H is less sterically demanding

PEt3
~
PBu3
~
PPh3 < PBz3 <
< PCy3 < PPh(f-Bu)2. 31P
P(/'-Pr)3 than CN. The extent of phosphine dissociation is less for
NMR studies84 also show steric effects on the rates of exchange
HRuCI(PPh3)3 than for RuCI2(PPh3)3,91 presumably for a similar
of free and coordinated phosphines. reason.
Vapor pressure osmometry shows that L dissociation from
CuXL3 (X = halogen) increases in the steric order PMe2Ph ~
B. Dissociation of Other Ligands
PMePh2 < PPh3 and AsMe2Ph < AsMePh2.85 CuX(AsPh3)3 could
not be made. CO substitution in c/'s-MnBr(CO)4L involves rate determining
Equilibrium constants of Barnett and Pollmann86 for L disso- loss of the CO cis to both Br and L. Relative rates (Table XXX)
ciation from CpMo(CO)2LCOCH3 are shown in Table XXVIII. increase s with the size of phosphorus ligand and in the order
P(o-Tol)3, the largest ligand used in the study, is completely SbPh3 < AsPh3 < PPh3.
dissociated. Figure 22 shows the effect of increasing the size of L on the
Equilibrium constants for L dissociation from Co(salen)L2+ rates of CO dissociation from CpMo(CO)2LCOCH3. A small
(Table XXIX) show a larger steric effect; K increases by a factor electronic effect is evident in the increase in rate on going from
of 4700 from PMe2Ph to PPh3. Activation energies for disso- P(p-C6H4OCH3)3 to PPh3. Rates of chelation of (R2P-
 326 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

TABLE XXXI. Rate Constants9 In n-Octane at 124 °C for the Reaction:

(diphosphine)Cr(CO)5 —* CO + chelated (diphosphine)Cr(CO)4

1
Diphosphine 104k, sec

Me2PCH2CH2PMe2 0.75
Ph2PCH2CH2CH2PPh2 26
Ph2PCH2CH2PPh2 34
Ph2PCH2PPh2 55
9
J. A. Connor, J. P. Day, E. M. Jones, and G. K. McEwen, J. Chem. Soc.,
Dalton Trans., 347 (1973).

TABLE XXXII. Equilibrium Constants9 In C2H4CI2 at 25 °C for the

Reaction:

CoX2L2(CO) —
CO + CoX2L2

KX 104, M

Figure 23. Rate constants for ring closure in nonane at 110 °C, from L X = NCS Br Cl
ref 92.
PEt3 4.0 8.1
CH2CH2PR2)Mo(CO)s complexes92 (Figure 23) increase with the 4.3 12.7
P(n-Pr)3
size of R, but the slope is less. Steric acceleration can be un- 10 500
PEt2Ph
derstood in terms of a rate-determining loss of CO, followed by PEtPh2 20 97 1470
rapid ring closure. The rates of chelation of [R2P(CH2)„PR2] PPh3 110 3000 b
Cr(CO)5 complexes also depend on the value of n (Table PCy3 450 b b
9 b
XXXI). From ref 94. Very large.
Herskovitz93 has found that the dissociation constants for eq
6 and 7 increase in the sequence R = Me < Et < /-Pr, obviously RuBr2[Ph2P(CH2)3PPh2]2 in 1,2-dichloroethane dissociates
a case of steric control. to Br- and RuBr[Ph2P(CH2)3PPh2]2+. Less crowded
RuX2[Ph2P(CH2)nPPh2]2 complexes with n 1 or 2 do not, nor
=

(C2H4)lr(R2PCH2CH2PR2)2+ —
C2H4 + lr(R2PCH2CH2PR2)+ (6) does the complex with n = 3 and X = Cl.97 Attempts to prepare
CO + lr(R2PCH2CH2PR2)2+ the n = 4 complex RuCI2[Ph2P(CH2)4PPh2]2 failed, giving instead
(CO)lr(R2PCH2CH2PR2)2+ ^ (7)
Ru2CI4[Ph2P(CH2)4PPh2]3.
Reversible CO dissociation from CoX2L2(CO) complexes in-
creases with the size of L (Table XXXII). The dissociation se- C. Metal-Metal Bond Cleavage
quence NCS < Br < Cl must involve both electronic and steric A special class of dissociation reactions involves cleavage
factors, since Br is larger than Cl. The authors94 argued against of metal-metal bonds. Drakesmith and Whyman98 have found
steric effects of L on the basis that earlier work95 on MX2L3 that lr4(CO)4L3 clusters are much more easily broken down under
dissociation showed that differences in K were due to differences CO and H2 pressure with L = PPh3 or P(/-Pr)3 than with PEt3, PPr3,
in AHd, not ASd. We now know that steric effects can have a or PBu3. Since Hlr(CO)3P(/-Pr)3 forms so easily, the P(/-Pr)3
major influence on AHd.9 system is particularly active in olefin hydroformylation."
Steric factors can influence rates of radiation-induced dis-
RuX2L„ (X = halogen, n = 2, 3, or 4) complexes participate
sociation reactions. Irradiation of 36 in the presence of PPh3 in several equilibria to form binuclear complexes linked by two
gives 37 rather than the less crowded 38,96 indicating more rapid or three halide bridges. Only with bulky phosphines such as PPh3
or P(p-Tol)3 are the mononuclear species favored; then RuX2L4
complexes are completely dissociated in solution to RuX2L3 and
100
|_

Poe101 has reported that activation energies for homolytic

cleavage of the Mn-Mn bond decrease in the sequence
(CO)5Mn-Mn(CO)5 > (CO)5Mn-Mn(CO)4L > L(CO)4Mn-Mn(CO)4L
and L = P(OPh)3 > PBu3 > PPh3, suggesting a weakening of the
Me
metal-metal bond by crowding transmitted through the car-
36 bonyls.
hv Sufficiently large ligands should stabilize odd-electron mo-
PPh3
nonuclear fragments. Indeed, while [P(OMe)3]4Co-Co-
[P(OMe)3]4 is a stable diamagnetic dimer,87 Co(PMe3)4 exists
as a paramagnetic monomer,102 as do Co[o-C6H4(PEt2)2]2103
and Co(N2XPPh3)3.104 Steric crowding probably accounts for why
V(CO)6 does not dimerize and thus provides one of the rare ex-
amples of a stable paramagnetic carbonyl complex.105 V(dmpe)3
has also been prepared.106 steric hindrance to radical recom-
bination is a well-established phenomenon in organic chemis-
try.107

loss of CO from the most hindered position. The preferential

D. Associative Reactions
kinetically controlled formation of c/s-M(CO)4L2 complexes in The rate of carbonyl substitution on 39 by L to give 40 in Table
light-induced reactions of M(CO)sL with L may be largely steri- XXXIII depends on L and is cleanly second order, implying an
cally controlled. associative reaction. There is an electronic factor in the rate
Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 327

TABLE XXXIII. Rate Constants9 in Toluene for the Reaction: TABLE XXXV. Competitive Rate Constants9 in Heptane at ~70 °C in
the Reactions:
39 + L ->40 + CO
*-i(PPh3)
Fe(CO)3C(OEt)MePPh3 -<—

L 10k, s“1 M-1 o ML)

Fe(CO)3C(OEt)Me —*- Fe(CO)3C(OEt)MeL
PBu3 12.3 30
PEtPh2 9.6 35
PHPh2 8.5 35 L k2/k_t
P(OMe)3 1.23 35
0.65 35 PBu3 3.0
PPh3
0.58 35 PCy3 1.0
PCy3
0.069 35 P(OPh)3 0.3
P(OPh)3 9
70 D. J. Darensbourg and H. L. Conder, Inorg. Chem., 13, 374 (1974).
P(o-Tol)3 b
6
9
P. C. Ellgen and J. N. Gerlach, Inorg. Chem., 12, 2526(1973). No
TABLE XXXVI. Rate Constants9 In THF at 27 °C for the Reaction:
reaction in 140 h.

TABLE XXXIV. Rate Constants9 In Decalin at 50 °C for the Reaction LW(CO)5 + PhCH2MgCI c/s-LW(CO)4C(OMgCI)CH2Ph
of Ru3(CO)12 with L

10 4k2, 104k2, 103k, 103k,

L sec-1 M-1 L sec-1 M_1 L s_1 NT1 L s_1 M-1

456 17 P(OEt)3 25.7 PBu3 8.5

PBu3 P(OPh)3
191 13 PMe3 19.1 PPh3 5.0
P(OEt)3 PPh3
161 <1 PMe2Ph 14.0 PCy3 1.42
P(OCH2)3CEt PCy3
9
From ref 108. PMePh2 10.4 P(o-Tol)3 1.75s
9
M. Y. Darensbourg, H. L. Conder, D. J. Darensbourg, and C. Hasday,
b
J. Am. Chem. Soc., 95, 5919 (1973). The product was exclusively the
constants; the phosphites react more slowly than expected on
trans isomer.
the basis of size alone. P(o-Tol)3 fails to react at all, even under
forcing conditions. TABLE XXXVII. Rate Constants9 in Et20 at 25 °C for the Reaction:
k
LW(CO)5 + 1/4(MeLi)4
—
c/s-LW(CO)4C(OLi)Me

102k, 102k,
L S—1 M_1/4 L s-1 M_1/4

P(OPh)3 6.56 PPh3 3.96

PBu3 4.74 PCy3 2.27
9
From ref 110.

A kinetic and equilibrium study111 of carbamoyl complex

formation (eq 8) from Mn and Re carbonyls showed that the rate
Poe and Twigg108 have shown that the second-order rate term
in the reaction of L with Ru3(CO)12 (Table XXXIV) depends on
both electronic and steric effects. PPh3 and especially PCy3 react
more slowly than expected on the basis of a plot of log k2 against
CO—M—CO
I/0 +
2RNH2 CO—M—C
l/0/
half-neutralization potentials. A number of other systems show CO7! ,
/I
CO L
X
NHR
the same behavior: Co(NO)(CO)3, Fe(NO)2(CO)2, Mn(NO)(CO)4,
CpRh(CO)2, and M(CO)e (M = Cr, Mo, and W).108
+ RNH3+
1H NMR line shape effects indicate slow ligand exchange with
HPdCI(PCy3)2 but fast exchange with the less crowded (8)
HPdCI(PCy3XPBu3) and HPdCI(PBu3)2.109 We have similarly found decreases in the order: PMePh2 > PMe2Ph > PPh3. The order
slow associative exchange of PCy3 with HNi(PCy3)2CN, but a very of the first two is expected from electronic factors but the rate
fast reaction of PEt3 with HNi(PEt3)2CN (by 31P NMR).90 with PPh3 is abnormally slow, suggesting steric inhibition of the
Competitive rate constants for reactions of the carbene in- associative reaction. The rate also decreases with increasing
termediate Fe(CO)3C(OEt)Me with various L relative to PPh3 are bulk of the amine in the order R = sec-Bu > /-Pr > Cy > f-Bu,
shown in Table XXXV. The discrimination of this uncrowded and decreasing the size of the metal from Re to Mn. Equilibrium
species is very small. P(OPh)3 reacts more slowly than expected constants decrease with increasing bulk of phosphine or
on the basis of size alone. amine.
Another type of associative reaction is the formation of car- Rates of reaction of KH with HCoL4 complexes decrease in
benes by the reaction of LW(CO)5 with PhCH2MgCI (Table the order P(OMe)3 > P(OEt)3 > P(OPh)3.87
XXXVI), where the steric effect is larger (the ratio of PBu3 to PCy3
rates is 6.0). For a given L, rates decrease in the order Mo
E. Ligand Exchange Equilibria
> Cr, consistent with more crowding on a smaller metal.
Rates of reaction of LW(CO)s with (MeLi)4 to give cis- There are many situations where ligands compete for coor-
LW(CO)4C(OLi)Me are first order in W and one-fourth order in dination sites on a transition metal, it was a study2 of competition
(MeLi)4, implying formation of monomeric MeLi prior to nu- equilibria among phosphorus ligands for Ni(0) which first led me
cleophilic attack.110 Rate constants in Table XXXVII show a to realize the importance of steric effects. Ligand exchange in
smaller steric effect than those in Table XXXVI, consistent with eq 9 shows a bonding order L' = P(OMe)2Ph > PMe2Ph P(OEt)3 ~

the smaller size of MeLi compared to PhCH2MgCI. > PEt3 > PEt2Ph PPr2Ph PBu2Ph > AsMe2Ph > AsEt2Ph.112
~ ~
328 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

TABLE XXXVIII. Equilibrium Constants9 In Hexane for the Reaction: TABLE XXXIX. Equilibrium Constants9 In Toluene at 35.4 °C for the
Reaction:
c/s-Mo(CO)4(PPh3XNHC5H10) + L- c/s-Mo(CO)4(PPh3)L + NHC5H10
W(CO)5(NH2Ph) + L ^ W(CO)5L + NH2Ph
L K T, °C
L K L K
PPh3 ~30 25
AsPh3 1.9 36.5 225 17.1
PBu3 PPh3
P(o-Tol)3 <0.09 25
P(OCH2)3CEt 180-360 11.2
9
AsPh3
C. L. Hyde and D. J. Darensbourg, tnorg. Chem., 12, 1286 (1973). 198 7.5
PCy3 SbPh3
P(SCH2)3CMe 45.3 P(OPh)3 6.6
CO . CO 31.2 1.1
I
P(OBu)3 BiPh3
I/
H—Ru—L + L'
I/
H—Ru—L' + L
9
R. J. Angelici and C. M. Ingemanson, Inorg. Chem., 8, 83 (1969).
(9)
L/lCl /\
L
Cl
TABLE XL. Percentage of Ionic Product9 in Acetone at Room
Temperature for the Reaction:
PEt2Ph, PPr2Ph, or PBu2Ph C7HeRhCIL + L^ C7HeRhL2+ + Cl“

With the exception of P(OEt)3, the order of the phosphorus li- % ionic product
gands is that of increasing 9. The PPh3 in (o-Tol)NiBr(PPh3)2 is
15
quantitatively replaced by PMePh2, which in turn is replaced by PMe2Ph
PMePh2 1
PMe2Ph.113 Larger phosphines, P(o-Tol)3 and P(o-C6H4OCH3)3
PPh3 0
do not react. P(/-Pr)3 and PCy3 in Cr(CO)4C(OMe)MeL are dis-
PPhCy2 0
placed by PEt3 or PBu3.114 A greater degree of substitution oc- 9
R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc., 93, 2397
curs when P(OMe)3 reacts with H2Fe(PMePh2)4 than when it (1971).
reacts with H2Fe[P(OEt)3]4.115
The length of the methylene chain in chelating diphosphines
markedly affects their coordinating ability. 31P studies116 of
competition for coordination to Ni(0) show decreasing bonding +
CH3CN
ability in the order Ph2P(CH2)3PPh2 Ph2P(CH2)2PPh2 >
~
HCo[P(0^^)3]4
Ph2P(CH2)4PPh2 » Ph2PCH2PPh2. The last may have an inad- R R
equate bite.
Competition of phosphorus ligands with ligands of other types
+ D
is also strongly influenced by the size of L. One of the best
—
HCo[P(o/Q\3]3(CH3CN) P(o(0>)3 (1

studied other ligands is CO. Figure 9 showed the results for Ni(0).
Reimann and Singleton117 have studied the reactions of Re- Olefins compete more effectively with smaller ligands. Ks
Br(CO)5 with 15 different phosphorus ligands and have found that for eq 12 are about 104 for L = P(0-o-Tol)3 and 10-4 for L =
rates and equilibria of CO substitution are controlled by the size
P(0-p-Tol)3.123 (An x-ray crystal structure124 of (CH2=CHCN)-
of L. For example, PEt3 and PEtPh2 give equilibrium mixtures of
Ni[P(0-o-Tol)3]2 shows that the acrylonitrile is indeed coordi-
ReBr(CO)3L2 and ReBr(CO)2L3, while PPh3 gives only disubst- nated by its double bond.} The similarity of this 108 factor to the
itution. MnBr(CO)5 behaves similarly except that trisubstitution ratio of NiL4 dissociation equilibrium constants (Table II) indicates
is even more difficult on the smaller metal. MnBr(C0)L4 can be that the strain energy in Ni[P(0-o-Tol)3]4 is completely relieved
made with the very small ligand P(OMe)3.118 when the first P(0-oTol)3 is removed; removal of the second
Complexes of weakly held ligands can be favored by in- ligand from NiL3 requires essentially the same energy for L =
creasing the size of the phosphines. Thus Ni(PEt3)4 forms an N2 P(0-o-Tol)3 as for L = P(0-p-Tol)3.
complex [(N2)Ni(PEt3)3] in solutions under nitrogen,119 because
a phosphine dissociates readily to form a coordinatively unsat-
CH2=CHCN + NiL4 (CH2=CHCN)NiL2 + 2L (12)
urated 16-electron complex. Ni(PMe3)4 shows no tendency to
dissociate an L or to coordinate N2. An N2 complex forms in the It is understandable that all known (C2H4)NiL2 complexes have
cyclohexylphosphine-nickel system, but the Ni is three-coor- L’s with 9 5; 130°. The strength of a Ni-ethylene bond is only
dinate in an N2 bridged dimer: N2[Ni(PCy3)2]2.120
slightly stronger than a Ni-P,125 so smaller L’s preferentially form
Electronic factors are also involved in the formation of N2 NiL4 complexes.
complexes, which require a metal with very good back-bonding Competitions of group 5 donor ligands with piperidine and
ability. Thus Pd(PEt3)3 and Pt(PEt3)3 do not form a detectable aniline are shown in Tables XXXVIII and XXXIX.
amount of N2 complex at 50 psig, nor does Ni[P(0-
Though the Ks in Table XXXIX generally decrease as the
»Tol)3]3.119 phosphorus ligand becomes more electronegative, the small
Nitriles do coordinate to Ni(0) complexes containing phos- P(OCH2)3CEt does better and the large PCy3 worse than expected
phites.121 The equilibrium constant for eq 10 with L P(0-o-
=
on the basis of electronic effects alone. Note the order PPh3 >
Tol)3 is ~10 at 25 °C. With L =
P(0-p-Tol)3, is too small to be
AsPh3 > SbPh3 > BiPh3. Recently Nasielski and co-workers126
AC

readily measured (~10-7). (These values were estimated from have pointed out that the Ks are not based on true thermody-
the equilibrium constants121 for CH3CN + Ni[P(0-o-Tol)3]3 ^ namic concentrations but rather on pseudostationary states in
(CH3CN)Ni [P(0-o-Tol)3] 3 at various temperatures and the known a photochemical reaction.
NiL4 dissociation constants.9} Data on competition of chloride ions with phosphines are
given in Table XL (C7H8 = norbornadiene). The percentage of
CH3CN + NiL4 (CH3CN)NiL3 + L (10) ionic product was determined by conductivity measurements.

Equilibrium 11 lies far to the left for R = H, but far to the right F. Oxidative Addition Reactions
for R = Me, i-Pr, or Ph.122 The HCoL4 complexes can be pre-
pared from the two smaller phosphites but not the larger Oxidative addition in the broadest sense includes all those
ones. reactions where one or two odd-electron fragments (usually the
 Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 329

TABLE XLI. Equilibrium and Rate Constants* in Acetone at 25 °C

under Air at 1 Atm for the Reactions:
K
Co(acacen)NO + L ^ LCo(acacen)NO

k
LCo(acacen)NO + 1/202
—*
LCo(acacen)N02

102/c X
L K, M-
1
[02],s-1

PMe2Ph 37 11.1
PEt3 23 6.67
PBu3 19 6.67
PEtPh2 5.4 5.75
P(OBu)3 1.87 0.8
P(OPh)3 No oxidation observed
PPh3 No oxidation observed
PCy3 No oxidation observed
*
From ref 130.
Figure 24. Rate constants in benzene at 25 °C for the reaction
TABLE XLII. Rate Constants* in Benzene at 25 °C for the Reaction: 2Co(DH)2L + PhCH2Br — PhCH2Co(DH)2L + BrCo(DH)2L, from ref
k
129.
lrCI(CO)L2 + PhCH2CI —
PhCH2lrCI2(CO)L2
130
[L] Both a preassociation equilibrium constant and a rate
constant (Table XLI) could be determined by analyzing the data.
k, M
1 1
L s
The ineffectiveness of PCy3 in promoting the oxidation was at-
1.5 X 10~3
tributed to its low coordinating ability. Larger size hurts both K
PEtPh2
5.5 X 10-4 and k, especially the former.
PEt2Ph
P(p-C6H4OCH3) 3.3 X 10“4 Ugo and co-workers131 examined electronic and steric effects
PPh3 1.2 X 10~4 of L in oxidative addition reactions of lrCI(CO)L2 complexes.
P(p-C6H4CI) 2.0 X 10“5 Selected data are shown in Table XLII. While the effect of
electron-withdrawing substituents in slowing the rate is clear,
*
From ref 131.
the steric effects are not obvious. Unfortunately the study was
latter, formed by cleavage of a bond in X-Y) are added to a restricted to a very narrow range of ligand size (136-145°).
transition metal, with an attendant increase in its formal oxidation Vaska and co-workers132 have recently reported results of rate
state.127 We can anticipate steric inhibition in direct associative and equilibrium studies which include L = PCy3 and P(o-Tol)3
reactions, and steric acceleration if prior dissociation of L or (Table XLIII). Both k and K are hurt by making L very large.
other ligand from an 18-electron complex is required as a first P(/d-ToI)3 acts slightly larger than P(p-Tol)3. Note that the de-
step.128 crease in K on going from P(m-Tol)3 to PCy3 is larger for 02 than
Data of Halpern and Phelan129 for the associative reaction of for Fl2, presumably reflecting the greater steric requirements
benzyl bromide with Co(DH)2L are shown in Figure 24. Larger of 02 in the product.
L's inhibit the reaction. There is also an electronic effect: While eq 13 proceeds at 0 °C when L = PMe3, no reaction
electron-withdrawing P(p-C6H4CI)3 is slower than P(p- when L =
occurs PEt3.133
C6FI4OCH3), and P(OMe)3 falls below the line defined by the
Fe(CO)3L2 + CH3I -*
Fel(CO)2L2(COCH3) (13)
phosphines.
Rates of oxidation of Co(acacen)NO (41) with air depend on lrCI(CO)[PMe2(f-Bu)]2 undergoes rapid oxidative addition with
the concentration of added L, reaching a limiting value at high a variety of small molecules. lrCI(CO)[PEt2(f-Bu)]2 reacts much
less readily, but will react with Cl2 and 02. lrCI(CO)[PMe(f-Bu)2]2
reacts only very slowly with Cl2 and 02 and lrCI(CO)[PEt(f-Bu)2]2
does not react at all.134
A remarkable case of steric inhibition of oxidative addition
has been reported by Otsuka and co-workers.65 Pd[PPh(f-Bu)2]2
reacts readily with 02 to form an 02PdL2 complex, as expected
for this highly coordinatively unsaturaed complex. The still more
crowded Pd[P(f-Bu)3]2 is stable in air! Apparently there is not
41 room even for a relatively small 02 molecule.

TABLE XLIII. Rate and Equilibrium Constants* in Chlorobenzene at 30 °C for the Reaction: XY + ^
frans-lrCI(CO)L2 (XY)lrCI(CO)L2

XY =
H2 XY =
02 XY =
HCI

L k, s
1
M 1
10~3K, M"1 102k, s_1 M~1 10~3K, IVT1 k, s
1
M 1 '

PPh3 1.2 31 5.9 28 1.1 X 104

P(p-Tol)3 1.7 44 9.6 21 2.4 X 104
P(m-Tol)3 0.69 25 4.4 7.0 2.7 X 103
PCy3 0.0066 2.4 0.038 0.066 <4.0
P(o-Tol)3 No reaction in 3 h No reaction in 18 days 0.79
at 740 mm at 700 mm
From ref 132. b
Data in benzene.
330 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

TABLE XLIV. Rate Constants9 in CH3C00D/D20/CDCI3 (5/1/5) at TABLE XLVI. Equilibrium Constants9 in Benzene at 25 °C for the
100 °C for the Reaction: Reaction:
K
Pt2Cl4[P(CH2CH2CH3)R2]2 Pt2CI4[P(CH2CH2CH2D)R2]2 c/s-MCI2L2 ^
frans-MCI2L2

10^A'obscfi 104A'obsd> L M =
Pd M = Pt
L s-1 L s-1
SbMe3 1.5 0.25
PPr3 1.8 PPr2-f-Bu 4.3 SbEt3 15.7 1.9
PPr2Ph 1.9 PPr(f-Bu)2 48 SbPr3 25.3 4.0
PPrPh2 2.0 SbBu3 26.0 3.8
9
from ref 138. SbPh3 5.8
9
J. Chatt and R. G. Wilkins, J. Chem. Soc., 70 (1953).
TABLE XLV. Equilibrium Constants9 in Benzene at 25 °C for the
Reaction: TABLE XLVII. Equilibrium Constants 9 in Benzene at 25 °C for the
Reaction:
K
cis-PtX2L2 ^ trans-RX2L2 K
cis-CH3COMn(CO)4L —
frar)S-CH3COMn(CO)4L

L X =
Cl X =
I

L K L K
PEt3 12.3
PPr3 29.5
P(OCH2)3CMe ~0b SbPh3 ~0b
PBu3 25.5
P(OMe)3 0.11 AsPh3 0.11
P(pentyl)3 28.4
PMe2Ph ~0 PPh3 0.42
AsMe2Et 55
PEt3 ~0
AsEt3 175 9 b
C. S. Kraihanzel and P. K. Maples, tnorg. Chem., 7, 1806 (1968). Too
AsPr3 ~650 little of the trans isomer to detect.
AsBu3 ~340
AsBu2Ph 9.6
(CH2)4PPh2] reacts with excessPhCOBr to give a mixture
SbEt3 1.9
of NiBr2[Ph2P(CH2)4PPh2], Ni(CO)[Ph2P(CH2)4PPh2]2, and
SbPr3 4.0
3.75
PhCOPh.
SbBu3
9
From ref 142. Ni[Ph2P(CH2)4PPh2]2 reacts readily with HCN in a 1:1 ratio
to give HNiCN[Ph2P(CH2)4PPh2]2 (one diphosphine monoden-
frarts-RhCI(CO)[PMe2(o-C6H4OCH3)]2 undergoes oxidative tate).90 Ni[Ph2P(CH2)2PPh2]2 and Ni[Ph2P(CH2)3PPh2]2 under
addition with a variety of small molecules such as HCI, Mel, CCI4, the same conditions react to only a small extent.141
and Cl2. The more crowded frans-RhCI(CO)[P(f-Bu)2(o-
C6H4OCH3)] 2 does not react with them but rather demethylates G. Isomerism
with loss of the elements of CH3 and Cl.135
Rates of ortho-metalation reactions of CH3Mn(CO)4L com- 1. Cis-Trans
plexes increase with the size of L in the sequence PMe2Ph (no The most familiar cis-trans isomerism is in MX2L2 complexes
reaction) < P(OPh)3 < PPh3 < P(o-Tol)3.136 Steric acceleration of Pd and Pt. Table XLV shows some data published several
of intramolecular oxidative addition reactions is now firmly es- years ago by Chatt and Wilkins142 which show that the trans/cis
tablished, especially by the extensive work of Shaw and co- ratio is generally favored by bulkier L and by iodide over chloride.
workers.137 Two factors are probably involved: (1) more ready The stabilizing effect of the phenyl group on c/s-PtCI2(AsBu2Ph)2
dissociation of other ligands to give coordinative unsaturation, was said to be partly electronic. An earlier study of stilbene
and (2) a close proximity to the metal of the bond to be broken complexes by the same authors (Table XLVI) showed a marked
as a consequence of steric crowding. increase in K when Me was replaced by Et, and smaller changes
A study by Masters and co-workers138 shows the effect of on going to Pr or Bu. SbPh3 gave more c/s-PdCI2(SbPb3)2 than
steric crowding on the rate of H-D exchange on C-3 of the propyl expected on the basis of size alone. The less crowded com-
group in Pt2CI4(PPrR2)2 complexes (Table XLIV). plexes gave a higher percentage of cis isomers.
Increasing the bulk of the ligand also stabilizes four-membered Recent studies by Verstuyft and Nelson143 on PdX2L2 com-
ring intermediates, as shown by an increase in the rate of ex- plexes [L = PMe2-p-C6H4Y or PMe(p-C6H4Y)2] show that the
change at C-2 relative to C-3.138 trans/cis ratio is favored by (1) making L bulkier, (2) changing
An example of the effect of structural constraints on an oxi- X“ from N3~ to Cl- to I-, (3) making Y more electronegative,
dative addition reaction is the decreasing equilibrium constants and (4) going to less polar solvents. Unfortunately the range of
(a factor of >30) for eq 14 in the sequence L = P(OEt)3 < PO- 0 is very limited.
Me(OCH2)2CMe2 < P(OCH2)3CR.68 MX2[P(0-Tol)3]2 complexes tend to favor the trans isomer
on going from P(0-p-Tol)3 or P(0-m-Tol)3 to P(0-o-Tol)3, from
H+ + NiL4 ^ HNiL4 (14)
Cl or Br to I, and from Pt to Pd.144
In this case, of course, the constraint reduces the electron The rates of cis-trans equilibration in MX2L2 complexes also
density on nickel. A greater Ni 2p3/2 binding energy (0.5 eV) for depend on the size of L. The much slower rate of
Ni[P(OCH2)3CMe]4 than for Ni[P(OEt)3]4 is observed by PdCI2[PMe2(o-Tol)]2 compared to PdCI2[PMe2Ph]2 has been
ESCA.139 attributed to steric interference of a rate-determining associative
The chain length in Ph2P(CH2)nPPh2 complexes can have a reaction.145 Catalysis by added phosphines is a general feature
marked effect on reactivity. For example, Ni[Ph2P(CH2)4PPh2]2 of these reactions. A decreasing efficiency in the order PMe2Ph
reacts with PhBr at 50 °C to give PhNiBr[Ph2P(CH2)4PPh2] 2 while ~
PBu3 > PMe2(o-Tol) » PMe(o-Tol)2 > PPh3 indicates that
Ni[Ph2P(CH2)3PPh2]2 does not react under these conditions.140 smaller ligands are better isomerization catalysts.
With the more reactive PhCOBr, both complexes react at 70 °C Selected trans/cis isomer ratios in CH3COMn(CO)4L com-
but give different products. PhCONiBr[Ph2P(CH2)3PPh2] is stable plexes are shown in Table XLVII. P(OMe)3 has a greater tendency
for at least 40 h, but the more crowded PhCONiBr[Ph2P- to go trans to the relatively bulky acyl than would be expected
Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 331

TABLE XLVIII. Equilibrium Constants* in Toluene-dg at 60 °C for the TABLE XLIX. Equilibrium Constants* in CDCI3 at 25 °C for the
Reaction: Reaction:
K
c/s-(carbene)M(CO)4L —
frans-(carbene)M(CO)4L

Carbene L K

M = Cr
Me(OMe)C: PMe3 0.34
PEt3 0.52
PBu3 0.54
0.56 P(OPh)3 <0.02 <0.02 0.03 1.7 5.6
P(octyl)3
1.69 SbPh3 0.04
P(/-Pr)3
PCy3 1.82 PBu3 <0.02 0.13 0.48 7.1
P(p-Tol)3 0.19 PMe2Ph 0.36 0.93*
PPh3. 0.22 PPh3 <0.02 0.04 1.16 0.59*
12.5C >50
P(p-C6H4F)3 0.31 P(OMe)3 0.11 0.34 3.3 0.19 5.9
Me(OEt)C: PBu3 0.50 aJ. W. Faller and A. S. Anderson, J. Am. Chem. Soc., 92, 5852
Et(OMe)C: PBu3 0.87 (1970). *in o-dichlorobenzene. C|n toluene.
Ph(OME(C: PBu3 0.94
/-Pr(OMe)C: PBu3 2.22

W
M =

Me(OMe)C: PBu3 0.09

Ph(OMe)C: PBu3 0.24
*
H. Fischer and E. 0. Fischer, Chem. Ber., 107, 673 (1974).

on the basis of size alone. Reaction of c/s-CH3COMn(CO)4L

complexes with L gave only 42 at equilibrium when L =
P(OCH2)3CMe or PHPh2, a mixture of 42 and 43 when L =
P(OMe)3, and only 43 for the largest ligands PMe2Ph, P(OPh)3, librium in Table XLIX. K does increase regularly in the sequences
PEt3, or PMePh2.146 PPh3 would not give a CH3COMn(CO)3L2 Cl < Br < I and H < Me < Bz.

complex. While Rh(CO)2L3+ complexes have configuration 47 in so-

lution (two carbonyl bands in the IR) when L = PMePh2 or As-
O cn3 MePh2, they have 48 when L = P(p-Tol)3, PPh3, AsPrPh2, or
VCH’ AsPh3.148 Attempts to prepare a tris phosphine complex with
l/co
L—Mn—CO L—Mn—L
I /“ P(o-Tol)3 failed, giving only 49.

/I
CO
c0 CO
42 43

Equilibrium 15 shifts toward 45 in the sequence R = R' = H

< R = H, R' = CH3 < R = R' = CH3. With Ph2PCH(CH3)PPh2,
the only isomer observed is 46. If the CH3CO is replaced by the
smaller CH3, only the fac isomer analogous to 44 is seen even
if R =
R' = The isomer distribution in eq 16 (L = PR3, AsR3, or SbR3)
CH3.147
depends on both steric and electronic effects.149 Equilibria shift
to the right in the series R = Me < Et Pr ~ Bu < n-pentyl <
~

n-octyl < isopentyl < aBu < sec-alkyl, SbR3 < AsR3 < PR3, and
X = CF3 < Br < F ~ H < Me < OMe when L = AsEt2(p-C6H4X).
R' The more crowded isomers 50 and 51 or ones with two L’s on
the same Co were not seen.

R'
(15)

Trans/cis ratios in (carbene)M(CO)4L complexes are larger

for larger L, larger carbene, and smaller M (Table XLVIII). The CO
aryl phosphine results are anomalous relative to the alkyl c^co
phosphines but do indicate a greater trans/cis ratio for more L—Co — —
Co—L (16)
electronegative L.
Both electronic and steric factors are involved in the equi-
A
CO CO CO
332 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

TABLE L. NIX2(PRPh2)2 Isomers Isolated3 TABLE Lll. Equilibrium Constants3 in CH2CI2 at 31 °C for the
Reaction:
R X =
Cl Br 1
K
planar NiBr2L2 —
tetrahedral NiBr2L2
Me p T T
Et p P,T T
Pr p P,T T L e K Ref
Bu P.T P,T T
n-Amyl P,T T T PMePh2 136 1.78 153
i- Bu P P,T T PEtPh2 140 2.03 155
/-Pr P P,T T,G PPrPh2 140 2.33 155
sec-Bu P P.T T,G PPh3 145 b 153
f-Bu T T PPh2Cy 153 2.45 153
3
From ref 151. Abbreviations: P, planar; T, tetrahedral; G, a green isomer PPhCy2 161 0.14 153
of unknown structure. PCy3 170 0.00 153
b
From ref 153. Very large.
TABLE LI. Equilibrium Constants3 in CDCI3 at 25 °C for the Reaction:
L and X which can be relieved by a tetrahedral distortion toward
K
planar NiX2(PMePh-p-C6H4Y)25=1 tetrahedral NiX2(PMePh-p-C6H4Y)2 53, where the LNiX angles are opened from 90 to 109.5° (and
L
Y X =
Cl Br I X.
X.
/
Ni

cf3 0.15 0.56

L'
\X
Cl 0.23 0.75 1.22
H 0.41 1.43 2.70 52 53
Me 0.45 2.12 3.00
the LNiL angle is decreased from 180 to 109.5°). Still larger
OMe 0.56 2.33
increases in 0 will lead to a severe repulsive interaction of L with
NMe2 1.00 4.88 11.5
a
L. H. Pignolet, W. D. Horrocks, Jr., and R. H. Holm, J. Am. Chem. Soc., L, favoring an opening of the LNiL angle back toward 52. The
92, 1855(1970).
value of 9 for optimum stability of the tetrahedral form must be
around 145°; PCy3 is too large. A square-planar geometry for
0 0 0 0 NiCI2(PCy3)2 has been established by an x-ray study.154
LaMar and Sherman155 reported a second-order acceleration
of the planar-tetrahedral interconversion for NiBr2(PMePh3)2 with
added phosphine, along with exchange of free and coordinated
phosphine. PPh3 exchanges more slowly with NiBr2(PPh3)2. The
still bulkier PPh2Cy does not exchange with NiBr2(PPh2Cy)2 and
its addition has no effect on the interconversion rate.153
A steric effect is clearly shown in the planar-tetrahedral
equilibrium of NiX2[Ph2P(CH2)nPPh2] complexes. Increasing n
2. Square-Planar/Tetrahedral from 2 to 3 for X = Cl increases Kfrom ~0 to 0.75 in CH2CI2 at
While PtX2L2 and PdX2L2 complexes usually form both cis and 22 °C.156 An earlier study on the Br and I complexes gave a
trans complexes, the planar complexes of the smaller cation similar result.157 Attempts to make NiX2(Ph2PCH2PPh2) com-
Ni(ll) are invariably trans; for certain X and L there can be an plexes failed; the complexes isolated contained two monoden-
appreciable fraction of a paramagnetic tetrahedral isomer. For tate diphosphines.
small R (e.g., Me or Et), NiX2(PR3)2 and NiX2(PR2Ar) complexes
are trans square planar and diamagnetic in the solid state and 3. Ambidentate
in solution. NiX2(PRAr2)2 complexes give a measurable amount
of both isomers in solution while NiX2(PAr3)2 are essentially Unsymmetrical diphosphines are capable of preferential
100% tetrahedral in both solution and solid states.150 bonding at one end. Reaction of Ph2PCH2CH2PMePh with
In the NiX2(PRPh2)2 series, Hayter and Humiec151 were able
(C6H5NH2)W(CO)5 in a 1:1 ratio gave predominantly (90%) the
isomer with the -PMePh end coordinated. Reaction of
to isolate the crystalline isomers shown in Table L. Both planar
Ph2PCH2CH2PPh(/-Pr) gave predominantly (85%) the PPh2 bound
and tetrahedral isomers are present in the crystalline unit cell
isomer.158 The smaller end of the diphosphine is preferred in
of NiBr2(PPh2Bz)2.152
each case.
The tendency to favor the tetrahedral form in the sequence
Reaction of P(OCH2)3P with (C6H5NH2)W(CO)5 in a 1:1 ratio
X = Cl < Br < I is shown in the solid state in Table L and in so-
at room temperature [or with Cr(CO)6 or Mo(CO)6 under UV ra-
lution in Table LI. The latter clearly shows the importance of
electronic effects in the phosphine on K. The tetrahedral isomer diation] gave predominantly the isomer with the smaller P03 end
coordinated.159 It was possible, however, to isolate both isomers
is favored by electron-donating L. This is in accord with our ob-
by irradiation of Fe(CO)5,13 where the crowding is presumably
servation, based on ESCA measurements,139 that the Ni(ll) in less.
a tetrahedral isomer is more electron deficient than in the cor-
Changing the size of phosphorus ligands on a metal can affect
responding planar one. It cannot, however, explain the increase the coordination of other ambidentate ligands, as we saw for SCN
in K in the sequence PR3 < PR2Ph < PPh3, which must therefore
in Figure 12. Carty and co-workers160 have concluded that steric
be determined primarily by steric effects.
effects also dominate the bonding mode in frans-Pd(SCN)2L2
Some results of a study by Que and Pignolet153 are shown in
Table Lll. They concluded that steric effects are relatively un- complexes. The crystal structures show S bonding with L =
P(OPh)3 but N bonding with the larger PPh3.
important. Their conclusion is, however, based on the ques-
tionable assumption that steric effects must be monotonic with
4. Other
increasing 0.
Consider a frans-NiX2L2 complex 52 with small L. Increasing An extremely important problem in many homogeneous
the size of L (or X) will cause a repulsive interaction between catalytic reactions is how to control the formation of primary and
Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 333

TABLE Llll. Structural Preference of [CuXL]4 Complexes3 TABLE LIV. Equilibrium Constants3 at 27 °C for the Reaction:
K
L =
PEt3 PPh3 58= 59

X =
Cl 56 56
Br 56 57 R K
I 56 57
3
From ref 163. H 0.81
Me 0.89
secondary alkyl products. Frequently the distribution depends Cl 1.75
on equilibria in solution. Bennett and Charles161 found that re- 3
From ref 164. R' =
Me
actions of secondary (R2) acyl chlorides with lrCI(PPh3)3 (eq 17)
TABLE LV. 31P Chemical Shifts3 in 4-R'( cyclohexyl )PMe2 Phosphines

6(31P),
R2COCI + R' Structure T, K ppm
lrCI(PPh3)3

c/s-t-Bu 58 300 54.8

cis-Me 58 183 57.2
always led to isolation of primary (R1) alkyl products. Using cis-Me b 300 49.0
models, they concluded that the instability of the intermediate cis-Me 59 183 44.2
secondary alkyl complexes is due to unfavorable steric inter- H 60 300 42.7
actions of the alkyl group with the triphenylphosphines. trans-Me 60 300 42.5
trans-t-Bu 60 300 42.5
Though they did not look at complexes with smaller phos- 3 6
From ref 164. Exchanging between 58 and 59.
phines, they did investigate the reactions of acyl chlorides with
[lrCI(CO)(cyclooctene)2]2 to give 54.161 In this less crowded changing R' from Me to f-Bu in 58. 4-c/s-Me(cyclohexyl)PMe2
is in rapid exchange between structures 58 and 59 at 300 K and
gives an intermediate averaged chemical shift at this tempera-
ture.

structure, 2-methylpropanoyl chloride gave the R = /-Pr com-

plex. An equilibrium mixture containing ~50% /-Pr and ~50%
n-Pr could be obtained by refluxing 90 min in benzene. The
equilibrium between butyl isomers favors R = n-Bu.
Otsuka and co-workers162 prepared complexes 55 in order
to resolve racemic tertiary phosphines such as PPh(a-naph-
thyl)(p-C6FI4OEt). When PPh(a-naphthyl)(o-Tol) was used, iso-
mers attributed to restricted rotation of the o-tolyl group were
obtained which did not interconvert up to 80 °C.

Cl

Equilibrium 20 is shifted to the right as R gets larger in the

series Me Et < Ph < /-Pr < f-Bu.165 Room-temperature 31P
~

NMR spectra show exchange averaged chemical shifts.

(CuXL)4 has a cubane structure 56 for small X and L but goes

over to a step structure 57 as they become larger (Table
Llll).163
X-Cu R

5. Stereochemical Nonrigidity
Intramolecular motions can cause NMR signal averaging and
temperature-dependent line shape effects. We saw earlier (Table
XVIII) how steric effects can influence rotamer populations and
average NMR parameters. This section will deal mainly with
56 57 steric effects on rates and activation energies.
Barriers (Table LVI) to intramolecular exchange in ML5
Equilibrium 18 depends on R' and R.164 It is completely to the complexes generally increase with the size of L.166 For fixed L,
left with R' = f-Bu (R = H, Me, Cl). For R' = Me, it shifts to the they tend to be larger for the smaller first-row metals (Table LVII).
right (Table LIV) as the size of R increases. Equilibrium 19 is It has not been possible to prepare ML5 complexes with P(OPh)3
completely to the left for R' = H, Me or f-Bu.164 (Both R' and PR2 (9 = 128°) or larger L.
can take the less crowded equatorial positions). 31P Chemical Barriers in HML5 complexes decrease with increasing size
shifts (Table LV) depend on the isomer and show an effect on of L and are smaller for first-row metals (Table LVIII). Both trends
334 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

TABLE LVI. Barriers3 to Intramolecular Exchange in RhL5* (~10%), and 64 (~1 %). The IR spectrum (cyclohexane) shows
Complexes distinct vco bands in decreasing intensity assignable to the three
rotamers at 1996,1975, and 1950 cm-1. The decreasing stability
AG*,
kcal/mol along the series is consistent with the greater size of Cl (0 =
L r, K
102°) compared to CO (0 = 92°).
P(OCH2)3CMe 7.8 153 Mann, Masters, Shaw, and Stainbank172 had earlier reported
P(OMe)3 7.5 200 freezing out three rotamers of frans-RhCI(CO)[PR(f-Bu)2]2 (R
P(OEt)3 9.9 208 =
Me, Et, Pr) in the low-temperature 31P NMR. Rotamers could
P(OCH)3(CH2)3 10.3 210 not be frozen out with the smaller ligands PMePh2, PMe2(f-Bu),
P(OBu)3 11.1 228 or PPr2(f-Bu). Two rotamers can be seen in the 0 °C 1FI NMR
a
From ref 166. spectrum of frans-PdCI2[PH(f-Bu)2]2.173
A nonequivalence of H, and H4 at low temperature in 65 with
TABLE LVII. AG* (kcal/mol) in M[P(OCH2)3CMe]5 Complexes3
L = PCy3 has been attributed to restricted rotation about the Pt-P
oo + o Ni 8.3 bond.174 This behavior was not observed with the smaller L =
Rh+ 7.8 Pd 5.7 PPh3.
lr+ 8.4 Pt 7.0
3
P. Meakin and J. P. Jesson, J. Am. Chem. Soc., 96, 5751 (1974).

TABLE LVIII. AG* (kcal/mol) in HML5 Complexes3

Pt
HCo [P(OCH2)3CPr] 4 L
HFe(PF3)4- <5 HCo(PF3)4 5.5
HCo[P(OEt)3]4 <6 HNi [P(OEt)3]
HRh[P(OCH)3CPr]4 10.5
HRj(PF3)4- 7.0 HRh(PF3)4 9.0
HRh[P(OEt)3]4 7.25
Hlr[P(OCH2)3CPr]4 10.3
HOs(PF3)4- 8.0 Hlr(PF3)4 10.0 V. Unusual Reactions and Products
3
P. Meakin, E. L. Muetterties, and J. P. Jesson, J. Am. Chem. Soc., 94,
5271 (1972), except for the P(OCH2)3CPr complexes, by E. M. Hyde, J. R.
Changing the size of ligands frequently gives an unexpected
reaction or unusual product. We have seen several examples
Swain, J. G. Verkade, and P. Meakin, J. Chem Soc., Dalton Trans., 1169
in earlier sections. Here we are concerned primarily with prod-
(1976).
ucts isolated from stoichiometric reactions. Section VI deals with
catalytic reactions.
are consistent with the tetrahedral jump model; the heavy atoms
need to move less if they are more nearly tetrahedral. A. Displacement of Other Ligands
Rearrangement barriers in H2ML4 complexes (M Fe or Ru)
=

do not change in a systematic way with steric or electronic PPh3 displaces H2 in eq 21. The larger (and more basic) PCy3
changes in L.167 The higher barriers for Ru (for example, 20.1 gives eq 22.175
kcal/mol for FI2Ru(PMe2Ph)4 vs. 13.9 for the Fe complex) are, * +
[H2Os(CO)(NO)(PPh3)2] PPh3
however, consistent with the tetrahedral jump model. The un- +
—
[Os(CO)(NO)(PPh3)3] + H2 (21)
usual coupling constants observed in the iron complexes
[VPH(cis) > JPH(trans)j were attributed to their greater tetrahedral [H2Os(CO)(NO)(PCy3)2]+ + PCy3
distortion.167 — + HPCy3+
HOs(CO)(NO)(PCy3)2 (22)
Activation energies (in parentheses) for interconverting cis-
and frans7HWCp(CO)2L decrease as the size of L increases: Reactions of various monodentate phosphorus ligands with
PMe3 (15.5 kcal/mol), PEt3 (14.7), PPh3 (14.3).168 On the other (RC=CR')Co(CO)4 (2:1 ratio in refluxing benzene) gave disub-
hand, the barrier is less for PPh3 than for PCy3 in [H2Os(CO)- stituted (RC^CR')Co(CO)2L2 complexes for L = P(OMe)3, PBu3,
(NO)L2]+.169 P(/-Bu)3, PPh3, PPh2(sec-Bu), PPhCy3 and PCy3. P(/-Pr)3,
Rotation of f-Bu groups on P can be frozen out at low tem- P(sec-Bu)3 and P(o-Tol)3 did not react at all, even in refluxing
perature. Some measured activation energies include W- toluene. Under these conditions, excess P(OMe)3 gave te-
(CO)6PPh2(f-Bu) (8.3), P(f-Bu)3 (8.6), BH3P(f-Bu)3 (10.4), and trasubstituted (RC=CR')Co(CO)2L4.176
SP(f-Bu)3 {10.5).170 Several years ago, King177 tried to prepare M(CO)3[P-
English and Bushweller have determined the first activation (NMe2)3]3 (M
=
Cr, Mo, W) by the reaction of the phosphine with
energy for rotation about a metal-phosphorus bond (AG*
=
12.6 (cycloheptatriene)M(CO)3. The reaction gave frans-M(CO)4L2
kcal/mol at -32 °C) in frans-RhCI(CO)PCI(f-Bu3)2.171 The instead. He attributed the failure to give trisubstitution to a special
low-temperature limit 31P{ 1H| spectrum shows 62 (~90%), 63 electronic effect (interaction of P with the N lone pairs). Actually
P(NMe2)3 is very similar sterically (and electronically) to P(/-Pr)3.
A similar failure to achieve trisubstitution has been found more
recently for L = PCy3 by Moers and Reuvers,178 who also ob-
tained only f/-ans-M(CO)4L2. In contrast, the smaller PMe3 easily
forms cis-M(CO)4L2, M(CO)3L3 and even M(CO)2L4.179
Schoenberg and Anderson180 found that the degree of pho-
tochemical CO substitution on pyrazolylboratotricarbonylman-
ganese(l) complexes 66 and 67 (Table LIX) decreases in in-
creasing 0, or on putting methyl groups on the pyrazolyl
rings.
The maximum degree of substitution found for group 6B
hexacarbonyls under UV irradiation is shown in Table LX. The
largest ligand replaces only four CO's from the smallest
64 metal.
Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 335

TABLE LIX. Degree of Photochemical Substitutiona of

Pyrazolylboratotricarbonylmanganese( I) Complexes

L 66 67

pf3 1,2 1,2

P(OMe)3 1,2 1

P(OPh)3 1,2 1

PMe3 1 1

PCI3 1 0
PBu3 1 1

PPh3 1 0
P(/-Pr)3 1 0
PCy3 1 0
P(C6F5)3 0 0
a
From ref 180.

TABLE LX. Maximum Degree of Photochemical Substitution a on

M(CO)e

L Cr Mo W

PF2(OPr) 6 6 6
PF(OMe)2 6 6 6
P(OMe)3 5 6 5
P(OMe)2Me 5 5 5

P(OMe)Me2 5 5 5
PMe3 4 5 5
a
From ref 203.

earlier report189 of the preparation of this and other H2PtL2

complexes by the AIR3 reduction of Pt(acac)2 appears to be
incorrect.188

B. Unusual Complexes of Arenes, C. Unusual Coordination Numbers

Tetramethylethylene, C02, N2, and Hydrides
With small L, the preferred coordination number for zerovalent
Unusual complexes (arene)Ni[Cy2P(CH2)nPCy2] (arene =
Ni, Pd, and Pt is 4.2,9 This gives the metal an inert gas configu-
benzene, naphthalene, or anthracene; n = 2 or 3) and (naph- ration of 18 electrons. ML3 complexes can be isolated with
thalene)Ni(PCy3)2 can be prepared with very bulky ligands.181 PEt3,83,119 (P(0-oTol)3,19° PPh3,83,191 or PBz3.83 Still larger li-
Similar complexes of Me2PCH2CH2PMe2 or PMe3 are unknown
gands P(/-Pr)3,83 PCy3,23,83,192 PPh(f-Bu)2,83 and P(f-Bu)365 can
because of the stability of Ni(Me2PCH2CH2PMe2)2 or Ni-
give 14-electron ML2 complexes. X-ray structures have been
(PMe3)4. reported for Pd[PPh(NBu)2]2,65,193 Pd(PCy3)2,193 and Pt-
The reaction of tetramethylethylene (TME) with Ni(Cy2P-
(PCy3)2.23
CH2CH2PCy2) gives the olefin complex, whose x-ray crystal These ML2 complexes appear to be the first structurally
structure has been determined.182 The similar reaction of TME characterized exceptions to the 16- and 18-electron rule.128
with the more crowded Ni(PCy3)2 does not go. Ethylene forms Unusual five-coordinate Pt(ll) complexes PtX2L3 (X = Br, I)
stable complexes in both cases. can be prepared with L = PMe2Ph but not with PEt3 or PPh3.194
Carbon dioxide complexes are still rare, presumably because
CoBr2L3 complexes can be prepared when L = PHPh2 but not
of the weakness of bonding in most cases. The crystal structure
PHCy2; PMe3 or PMe2Ph but not PMePh2, PPh3, or PCy3.195
of (C02)Ni(PCy3)2 has been reported.183 The authors state that
Co(CN)2L3 can be prepared when L = PHCy2 or PMePh2 but not
the stability of (C02)NiL2 complexes depends on the basicity of
PPh3 or PCy3.195 While a number of ML5n+ complexes are
L. It is clear, however, that steric effects are also very impor- known with L = P(OCH2)3CMe, P(OMe)3, or P(OEt)3, none have
tant. been prepared with L as large as PMe3 (9 = 118°) or P(OPh)3
The role of steric and electronic factors in the formation of
(128°).166 Co(PMe3)4+ does not react with PMe3 to form a
(N2)NiL3 complexes was discussed in section IV.E. Green and five-coordinate complex but does add PHMe2196 to give
Silverthorn184 have similarly used a large, electron-donating
Co(PMe3)4(PHMe2)+.
phosphine to prepare (N2)Mo(C6H6)(PMeCy2)2. Smaller ligands Though Ir(lll) complexes are usually six-coordinate (with 18
under the same conditions give Mo(C6H6)L3 complexes.
electrons), reactive 16-electron HlrCI2L2 complexes can be
While the reaction of phosphines with [(cyclooctene)2RhCI]2
prepared with L = PMe(f-Bu)2, PEt(f-Bu)2, and PPr(f-Bu)2.197
is a common method for preparing RhCIL3 complexes, a similar
Similarly, the unusual HMCI(CO)L2 complexes (M = Ru, Os) can
reaction of PCy3 under nitrogen gives (N2)RhCI(PCy3)2.185 be prepared when L = PCy3.198 A similar preparation with PPh3
Unusual transition metal hydrides can be stabilized through
gives HMCI(CO)L3.199 NReCI2L„ complexes form with n = 3
steric effects. Thus, while HNiBrL2186 and HPdBrL2187 with L = when L = PMe2Ph, PEt2Ph, PPr2Ph, and PMePh2 but n = 2 when
PEt3 decompose rapidly at room temperature, the corresponding L =
PEtPh2, PPrPh2, or PPh3.200
complexes with L = P(/-Pr)3 or PCy3 are stable and isolable.
Attempts to reduce trans-HPtCIL2 to trans-H2PtL2 leads to D. Products with Unusual Structures
decomposition with small L = PMe2Ph, PEt3, AsEt3, or PPh3.
H2PtL2 compounds can, however, be prepared with PMe2(f-Bu), A number of complexes of bulky phosphines have been found
PEt2(f-Bu), PBu2(f-Bu), PBz2(f-Bu), PPr(f-Bu)2, PBz(f-Bu)2, and with novel structures or modes of bonding. A bridging cyclo-
PCy3.188 The more bulky the L, the more stable the complex. The pentadienyl group occurs in 68 [L = PPh3, P(/-Pr)3, or PCy3]; the
crystal structure of trans-H2Pt(PCy3)2 has been reported.22 An x-ray structure has been determined for L = P(/-Pr)3.201
336 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

L—Pd Pd-

V 68
Though Mo(PF3)6202 and Mo[P(OMe)3]6203 are known, re-
duction of MoCI3(THF)3 in the presence of excess PMe2Ph gives
“Mo(PMe2Ph)4’'204 with structure 69.205 M(CO)4L2 complexes
(M = Cr, Mo or W) can be readily prepared from M(CO)6 with L
=
P(p-Tol)3 or P(m-Tol)3, but no disubstituted products are
formed with P(o-Tol)3. Attempts to force the reaction of M-
(CO)5P(o-Tol)3 with excess phosphine gave 70 instead.63
Prolonged heating of RhCI3 in 2-methoxyethanol with P(p-Tol)3
or P(m-Tol)3 gives the RhCI(CO)L2 complexes. With P(o-Tol)3,
71 is obtained.206 Reaction of [RhCI(CO)2]2 with Ph2P(CH2)„PPh2 gives 81 when
n =
1, 3, or 4 but 82 when n = 2.215

.CO .Cl
Rh Rh
.Cl
cr CO"
Ph2P. ,PPh2
‘CO
^(CH2)^
Reaction of (C10H7)FeH(dmpe)2 with P(OPh)3 or PEt3 gives 81 82
LFe(dmpe)2 complexes (and C10H8). A similar reaction with PPh3 Attempts to prepare NiBr2[P(f-Bu)3]2 by the usual reaction
gives 72.207 of NiBr2 with the phosphine216 in alcohol gave instead [HP(f-
Reduction of Fe(ll) salts in the presence of P(OMe)3 gives +
Bu)3] [NiBr3P(f-Bu)3]-.24
Fe[P(OMe)3]5.208 A similar reaction with PMe3 gives
“Fe(PMe3)4”, whose structure, by NMR, is 73.209’210 E. Unusual Oxidation States
While the reaction of RhCI3-3H20 with PEt3 gives the RhCI3L3
complex,217 similar reactions of PCy3,218 PEt(f-Bu)2,219 or P(o-
Tol)3206 give the paramagnetic Rh(ll) complexes RhCI2L2.

A
Me2P—Fe—PMe3
lrCI2(PCy3)2 has also been prepared.206 The lr(ll) complex 83 has
been prepared from lrCI83- and P(f-Bu)2(o-C6Fi4OMe) and its
x-ray structure determined.220 Note that the bulky P(f-Bu)2 groups
PMe, adopt mutually trans positions.
PMe3
73
Flexakis(trifluoromethyl)benzene reacts with (trans-stU-
bene)PtL2 with ring opening of the arene to give 74 with L =
PMe3. The reaction with L = PEt3 gives 75 instead.211 Presum-
ably 74 is too crowded to form with the larger phosphine.

NiX(PPh3)3221 (X =
Cl, Br or I) and [NiX(PCy3)2]2222 (X = Cl
or Br), both containing bulky L, are among the few examples
reported of Ni(l) complexes containing monodentate phos-
phines.
Ni(l) complexes NiXp3 [X = Cl or Br, p3 = CFI3C(CFI2PPh2)3]
have been prepared from the reaction of NiX2 with p3 in the
presence of NaBFi4.223 Nil2 reacts directly with p3 to give Nilp3,
whose x-ray structure has been determined.224 NiCI(Cy2P-
Cyclic polyphosphines (PR)„ form the five-membered ring 76 CFi2CFi2PCy2) and Nil(Cy2PCFI2CH2PCy2) have also been re-
with R = Me, Et, Bu, or Ph but 77 with R = /-Pr, Cy, or f-Bu.212 ported.225
Unfavorable steric crowding can be relieved by trans alternation The nickel atoms in 84 are formally Ni(l), but the complex itself
of R groups in 77. is diamagnetic; the diphosphine shown is Cy2PCFI2CFI2PCy2.
Unusual large ring compounds 78-80 can be formed using
(f-Bu)2P(CH2)ioP(f-Bu)2;213 the bulky f-Bu groups prevent the
phosphorus atoms from taking up mutually cis positions. A long
chain is required to form mononuclear complexes. Thus, 79
could not be preparsed with (f-Bu)2P(CH2)5P(f-Bu)2. Some x-ray
structures have been reported.214 84
Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 337

Using p3, Sacconi and co-workers226 have isolated a related TABLE LXI. Relative Hydrogenation Rates* of Styrene by L +

complex, 85, and determined its crystal structure. [RhCI(C2H4)2]2 (P:Rh = 2.1:1) in Benzene at Ambient Temperature
and 1.1 atm of H2

Rel Rel
L rate L rate

PPh3 2.8 Ph2P(CH2)5PPh2 0.5

85 PEtPh2 (1.00) Ph2P(CH2)6PPh2 0.22
Ph2PCH2PPh2 0.5 c/s-Ph2PCH=CHPPh2 0.07
VI. Homogeneous Catalysis Ph2P(CH2)2PPH2 0.04 Ph2PCH2OCH2PPh2 0.03
Ph2P(CH2)3PPh2 1.75 diop 0.17
Over the past 20 years, the development of homogeneous Ph2P(CH2)4PPh2 0.25
catalysis has had a major impact on the growth of organometallic a
From ref 234.
chemistry, much of which has been justified on the basis of the
TABLE LXII. Hydroformylation of 1-Hexene at 160 °C and 1000 psig;
insight it gives into catalytic reactions. Several have proven H2:CO = 1.2:1. Cobalt Carbonyl Catalyst
practical for large-scale (over 100 million pounds per year) in-
dustrial syntheses of organic compounds. While our ability to L %a L %a
design catalyst systems is still in its infancy, it is clear that
phosphorus ligands are often involved and that their steric and PBu3 89.6 PEt2Ph 84.6
electronic characters play extremely important roles. Learning PEt3 89.6 PEtPh2 71.7
to control catalytic reactions to give high yields of desired PPr3 89.5 PPh3 62.4
85.0
products under mild conditions will become increasingly im- a
P(/-Pr)3
Linear aldehyde and alcohol. From ref 239.
portant as the supply of petroleum for energy and feedstocks
dwindles in the years ahead.

A. Reaction Rates
H
The rate of hydrogenation of cyclohexene by RhCIL3 catalysts
increases in the sequence L = P(p-C6H4F) « PPh3 < P(p- L—Col
C6H4OCH3),227 indicating that electron donation by aryl phos- 'L
I
phines increases activity, probably by increasing the rate and L
extent of oxidative addition by H2.27 Replacing phenyl by ethyl,
90
however, decreases the rate in the sequence PPh3 > PEtPh2 >
PEt2Ph > PEt3.227 The difficulty here occurs at the next stage Catalytic hydrogenation of benzene to cyclohexane is three
of the cycle, where L dissociation from the 18-electron H2RhCIL3 times as fast with 7r-C3H5Co[P(OMe)3]2P(0-/-Pr)3 as with
is required before the olefin can coordinate.128 The tripod 7r-C3H5Co[P(OMe)3]3.233
phosphine complex 86 reacts readily with H2 but is not a hy- The rate of styrene hydrogenation by rhodium catalysts con-
drogenation catalyst228 because the phosphorus atoms in the taining Ph2P(CH2)nPPh2 depends markedly on the value of n
dihydride do not dissociate easily. Rapid dissociation of the PPh3 (Table LXI).234 Quite different relative rates are obtained if the
trans to H in 87 has been demonstrated by NMR line-shape substrate is changed to a-acetamidocinnamic acid.
The rate of addition of active hydrogen compounds to buta-
PPh3 diene in a Ni(acac)2/P(OR)2/NaBH4 system increases in the order
R = Me < Et < /-Pr.235 Using preformed NiL4 catalysts, only a
H—Rh-—PPh3 trace of morpholine reacted in 1 h at 100 °C when L = P-
H (OEt)2Ph, while 96% reaction was observed with P(0-/'-Pr)2-
PPh3 Ph.236
87 Both the rates and product distributions of butadiene cy-
clooligomerization with Ni(0)/L catalysts depends on L.237a The
studies, but the H2RhCI(PPh3)2 intermediate has not been de- same is true in reactions involving added ethylene.2376
tected spectroscopically.27 H2RhCIL2 complexes can, however,
be isolated with bulky ligands229 like PMe(f-Bu)2, P(f-Bu)3,230 or
230 B. Product Distributions
PCy3.185 They are active hydrogenation catalysts.185
A HRhL4 catalyst an order of magnitude more active than The major product of propylene dimerization in a 7r-C3H5Ni-
HRh(PPh3)4 or RhCI(PPh3)3 for 1-hexene hydrogenation has been LAICI3 catalyst system changes from 2-methylpentenes to
reported using L = 5-phenyl-5H-dibenzophosphole (88). The 2,3-dimethylbutenes on increasing the size of L in the series
enhanced activity was attributed to the rigid, bulky nature of the PMe3, PEt3, P(/-Pr)3.237c
ligand.231 It has been known for some time that the addition of phos-
phines to the cobalt carbonyl catalyzed hydroformylation slows
the reaction but gives a higher percentage of the desired linear
aldehyde and alcohol products.238 Tucci239 observed the product
distributions shown in Table LXII.
A more extensive study of HRh(CO)4_„L„ catalyzed hydro-
formylation by Pruett and Smith240 showed that both steric and
electronic effects of L are important. Table LXIII shows that in-
creased electron donation by para-substituted aryl phosphites
88 decreases the percentage of linear aldehyde. A greater decrease
The greater activity of 89 than 90 in olefin hydrogenation has is caused by making L more bulky. The role of ligand size can
been attributed to the more ready dissociation of an L = P(OPh)3 be understood by referring to Scheme I, where R2 and R1 refer
in the former.232 Hydrogenation is inhibited in both cases by the to isomeric secondary and primary alkyls. Increasing the size
addition of P(OPh)3. of L should increase K-t, K2, and K3. (Greater crowding in a
338 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

TABLE LXIII. Hydroformylation of 1-Octene at 90 °C and 80-100 pslg, TABLE LXV. Percentage of Methylenecyclopropane Trimers2
H2:CO = 1:1. HRh(0 )4-„ L„ Catalyst Produced by NI(COD)2/L(1:1) in Benzene at 25-60 °C

L %2 L %2

P(0-p-C6H4CI)3 93 P(OBu)3 81
P(OPh)3 86 P(0-o-ToI)3 78
P(0-p-C$H4Ph)3 85 P(0-o-C6H4Ph)3 52
P(0-p-C6H40CH3)3 83 P[0-2,6-C6H3(Me)2] 47
2
Linear aldehyde. From ref 240. A B

% A % B
TABLE LXIV. Percentage of Products2 Produced for Morpholine and
Butadiene with N!CI2/L/NaBH4 (1:2:1) at 20 °C In 6-9 h PEt3 b 92 2
PPh3 89 6
P(:Pr)3 78 18
PCy3 72 23
P(«-Pr)2 (r-Bu) 18 69
P(r-Bu)3 0 80
a P.
Binger and J. McMeeking, Angew. Chem., Int. Ed. Engl. 1 2,
995 (1973). bp. Binger, private communication.
L > CD c D
terms of crowding in the (olefin)PdL2 complexes which are the
P(0-;-Pr)2Ph 13 77 8 1 presumed primary products.
P(0-o-Tol)3 3 27 64 6
PPh3 2 6 75 17 C. Asymmetric Induction
P(0-o-C6 H,Ph)3 85 15
a R. Baker, A. A discussion of steric effects on catalytic reactions would not
Onions, R. J. Popplestone and T. N. Smith, J.
Chem. Soc., Perkin Trans. 2, 1133 (1975). be complete without mentioning the rapidly growing area of
asymmetric induction. In 1968, Knowles and Sabacky242 re-
SCHEME 1
ported the first catalytic asymmetric hydrogenation, employing
a Rh catalyst with the optically active ligand (—)PMePh(/-Pr). Later

R2Rh(CO)L3 ^ R1Rh(CO)L3 Dang and Kagan243 prepared the optically active amino acid
N-acetylphenylalanine (eq 23) using a Rh catalyst containing the
LJc° Ot O optically active diphosphine diop (92). Enantiomeric excesses
of up to 96% have been achieved using 93 as the catalyst.244
R2Rh(CO)2L2 R1Rh(CO)2L2
R2CHO-
L|c° t
If
-R1CHO PhCH=CCOOH +
H2
—
PhCH2CHCOOH (23)

R2Rh(CO)3L ^ R1Rh(CO)3L NIHCOMe NHCOMe

k*
L1IC° L|c° k*
^
_

R2Rh(CO)4 R1Rh(CO)4

complex decreases the stability of a secondary alkyl more than

a primary one.) But K^> K2> K$> K4. Increasing the size of
L Improves the ability of CO to compete with L for coordination
sites and increases the fraction of Rh present as RRh(CO)3L and
RRh(CO)4. In the limit of very large L, the product distribution will
be that of a phosphine-free system.
Table LXIV shows the effect of changing L on the product
distribution in the reaction of morpholine with butadiene. Larger
L's compete less favorably with butadiene for coordination, al-
lowing the butadiene to dimerize to a greater extent.
The trimers produced in the oligomerization of methylene-
cyclopropane are markedly affected by the size of L, showing
a smooth transition to cyclic products as the size of L increases
(Table LXV). Apparently the hydride transfer step required to give Though Knowles and co-workers have argued that electrostatic
open chain products becomes less favorable relative to car- interactions of the methoxy group with the substrate are involved
bon-carbon coupling. in this case, it is clear that steric effects are very important and
Trost and Strege241 have shown that the position of attack of perhaps solely responsible for asymmetric induction in systems
nucleophiles on 91 can be shifted to the primary carbon of the without methoxy substituents.245
ir-allyl by increasing the size of L. The result was explained in Table LXVI shows the effects of varying the structure of some
optically active monodentate phosphines on the enantiomeric
excess (l configuration) of a-amino acid produced by the hy-
(PdL2+ drogenation of prochiral olefins 94 and 95 by [(1,5-cycloocta-
diene)RhL2]BF4.
A catalyst for asymmetric hydrogenation can be prepared
from the reaction of RuCI2(PPh3)3 with (+)diop to give 96.246 The
91 reaction of RuCI2(PPh3)3 with Ph2PCH2CH2PPh2 gives a coor-
Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 339

TABLE LXVI. Enantiomeric Excess in Asymmetric Hydrogenation3 by

21I0
((1,5-COD)RhL2]BF4 at 25 °C

ph2 2100

L Substrate (atm) ee%

P(0CWl,l3
• 1
P(0.®CN)’ PI°P^
2090 mojOcii, P(( PfC6F5)3*
PMePrPh 94 3.5 28 •
P(0&copy;1,

P(0CH?),CMe PI0*l3 p(0O)j .p(0&copy;),

-r
&copy;

PMePh(o-C6H4OMe) 94 3.5 586 .p(OCH,CH2CI)3 ^p(05

p(0'OcHj
94 0.7 90 2080 -T p(ob)j»
PMeCy(oC$H4OMe) •
P(0Me)j
95 3.5 47 P(OEt) • •
PMeBz(o-C6H4OMe) •PfOEH^
P(0-<]3 •^2^5
95 3.5 49 • •P(OC03
PMe(/-Bu)(o-C6H4OMe) 2070
P0Me*2

PMe(/-Pr)(o-C6H4OMe) 95 3.5 85 ,PM'^2

PEI?* •P(CH?*)3
PMeCy( o-CeH4OMe) 95 0.7 88 P*&copy;CH,)3

2060 pBu .PEt3 P(NM«3)3.

PMeCy(2Me-4-BrC6H3) 95 3.5 74 ,pK h
a
W. S. Knowles, M. J. Sabacky, and B. D. Vineyard, Adv. Chem. Ser., •PC,, •?[+),
No. 132, 274 (1974). b This run was by the anion procedure. The others 2050
100 HO 120 130 I40 150 160 J70 I80 190
were by the free acid procedure.

Figure 25. Steric and electronic map.

H2SiPh(1-Np) + CH3COCH3
(+)diop
-HSi *
(OCHMe2)Ph( 1 -Np) (24)
i’(cyclodiene)2RhCl]2

VII. Steric and Electronic Map

If a measurable parameter Z is dominated by steric effects,
the dependence can be readily shown by plotting Z against 9,
such as in Figures 7 and 9. [Z might be the log of a rate constant,
an infrared stretching frequency, a metal-phosphorus coupling
95
constant, etc.] In cases where Z is dominated by electronic
effects, a plot of Zagainst v is appropriate. In the general case,
Z may depend on both steric and electronic effects. In addition
to displaying the dependence graphically, it might be desirable
to describe it in terms of its percentages of steric and electronic
character, as Swain and Lupton249® have done with electronic
96 field and resonance effects in organic chemistry. A step in this
direction is what I call the Steric and Electronic Map of phos-
dinatively saturated complex RuCl2(Ph2PCH2CH2PPh2)2 which
is inactive under similar conditions. phorus ligands, shown in Figure 25. The position of any ligand
on the map can be determined by its values of v and 0, deter-
Asymmetric hydroformylation of olefins using optically active
mined from the Appendixes or by a few very simple experiments.
phosphines has been demonstrated.247 Though little work has
been reported involving steric effects in the phosphines, it is A parameter Z can be represented by a vertical height above
clear that the size of substituents on the olefin is very important the map. Enough experimental values of Z will define a three-
in controlling which face of the olefin preferentially coordi- dimensional surface—a sort of landscape. A pure steric effect
nates. will give a surface sloping east or west but not north and south.
A purely electronic effect will give a north-south slope. If the
Asymmetric hydrosilylation of carbonyl compounds has been
used to prepare asymmetric alkoxy silanes (eq 24).248 The surface is a plane, it can be represented by
preferred configuration of the product can be understood in terms Z= aQ + bv+ c (25)
of minimizing steric interactions in the intermediates; 97 is more
stable than 98. The percentage of steric character then might be defined by
% steric character =
100[a/(a + b)] (26)
In the more general case of a nonplanar surface, the % steric
character at a particular point could be defined in terms of 8Z/8Q
and bZ/bv.
An example of a surface can be seen in Figure 26, where the
positions of the points above the plane represent the first ion-
ization potentials of the electron pair of a number of free phos-
phorus ligands, as determined by UV photoelectron spectros-
copy. The device shown, which I call a “steric and electronic
box", can be readily constructed from metal rods, beads, and
11/4 in. thick styrofoam sheet available at many hobby and craft
stores. Figures 27 and 28 show surfaces defined by 13C NMR
chemical shifts of the carbonyls in Ni(CO)3L complexes and
enthalpies of reaction of phosphorus ligands with trans-
[MePt(PMe2Ph)2(TFIF)]+. The former is obviously electronically
and the latter sterically controlled.
A surface defined by the percentage of a desired product in
a homogeneous catalytic reaction could be of great help in se-
lecting the best ligand to use in a particular system. The surface
340 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

Figure 28. AH for reactions of frar7S-[MePt(PMe2Ph)2(THF)]PF6 with

Figure 26. IP! of free phosphorus ligands in the gas phase. Data from excess L, from ref 250. The height in cm is one-half the number of
references in Table XXVII. The height in inches is the number of electron kcal/mol.
volts above 7.0.
of E, the M-E bond strengths can be very different and generally
fall rapidly in the order P > As > Sb > Bi.
Ligand cone angles can be defined for ligands other than those
bonded by group 5 donor atoms. A few values for common types
are given in Appendix C. The importance of steric effects in
compounds not containing group 5 atoms is illustrated by the few
following examples.
The structure of HFe(CO)4~ (101)255 shows a greater bending
of equatorial carbonyls toward the hydride than does FiMn(CO)5
(102),256 whose metal has a larger covalent radius. A still greater
deviation from idealized geometry is shown by FICo(PF3)4.267
H
1.57 OC n me
ICO
1.75A-^>e^CO 1.85 A—'''Mn
OC r mean
mfi 99.1c OC^P^CO
C mean 97.1°

Figure 27.5(13C) of the carbonyls in Ni(CO)3L complexes, from ref 249b. 101 ? 102
The height in cm is the chemical shift (downfield from internal Me4Si)
in ppm beyond 180. The oxidation potentials of complexes 103 depend on the size
of the macrocyclic tetraamine ring; increasing the ring size from
could, of course, depend on variables such as temperature, 14 to 16 makes it more difficult to go from Ni(ll) to Ni(lll) by about
solvent, and ratios of reactants. 0.3 eV.258 Busch and co-workers have also shown that the fit

VIII. Steric Effects of Other Ligands

Though steric effects of ligands bonded by atoms other than
phosphorus are strictly beyond the scope of this review, a few
comments should be made. Other group 5 donors (ES3) con-
taining As, Sb, and Bi are expected to be slightly smaller than
their P analogs. Models show that increasing M-E or E-C bond 103
lengths by ~0.1 A decreases cone angles by 3 to 5°. Pauling's of the cation in the “hole” formed by the four N donors also af-
tetrahedral covalent radii for P, As, Sb, and Bi are 1.10, 1.18, fects the UV absorption spectrum.259 Increasing the M-N bond
1.36, and 1.46 A.251 Mean SES angles are also likely to be lengths beyond an optimum value (by increasing ring size)
smaller for heavier group 5 donors. For example, the mean CSbC causes red shifts in the UV transitions, reminiscent of effects
angle of 101.8° in Fe(CO)4SbPh3252 is 2.4° smaller than the CPC seen in section III.C. It is intriguing to note that the UV spectra
angle in Fe(CO)4PHPh2.253 The Sb-Fe bond (2.47 A) is 0.23 A of transition metal complexes can be changed by subjecting
longer. The smaller size of L = SbPh3 has been used to explain them to high pressures.260
the formation of isomer 99 while AsPh3 and PPh3 form 100.254
Brintzinger261 and co-workers have claimed that the insertion
Ph2 CO of WCp2 into aromatic C-FI bonds, not observed for the Cr and
ph2 CO CO Mo analogues, is largely due to reduced steric crowding in the
"p\l/Mo
transition state of the larger metal.
The activation energy to bridge-terminal H interchange is
L much larger in Cp2VH2BFI2 than in Cp2NbFl2BH2. Marks and
r,,co Kennelly2623 attribute this to the larger ionic radius of Nb. Sig-
nificantly the Cp2M(allyl) complex is ?j1 for M = V and rp for M
100 = Nb.
It should be borne in mind that, though carbonyl stretching Formation of Schrock's alkyl idene (eq 28) in the reduction of
frequencies in M(CO)n(ER3)m complexes are nearly independent TaR3Cl2 complexes by thallium cyclopentadienide must de-
Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 341

TABLE LXVII. Equilibrium Distribution 3 of Products In the Reaction of

HZrCICp2 with RC= =CR'

R R' R R'

R R' (Zr)
X H H
X (Zr)6

H Bu >98 <2
Me Et 89 11
Me Pr 91 9
Me /-Bu >95 <5
Me i- Pr >98 <2
Me t- Bu >98 <2

a D. W. Hart, T. F. Blackburn, and J. Schwartz, J. Am. Chem.

Soc., 97, 679 (1975). * (Zr) =
ZrCICp2.

pend262b on the conflicting steric requirements of the groups R

and Cp, as seen in the following sequence of reactions:

TaMe3CI2 ~jjcp"^ CpTaMe3CI Cp2TaMe3 (27)

Figure 29. The 02 complex of Collman’s “picket fence porphyrin”,
Ta(CH2Ph)3CI2 CpTa(CH2Ph)3CI showing the fourfold disorder in the 02 position, from ref 263.
XH2Ph
Cp2Ta (28) in x-ray crystal structures. Some efforts along these lines are
^CHPh underway in our and other265 laboratories. Hopefully someone
no reaction in the near future will do the statistical analysis necessary to put
Ta(CH2CMe3)3CI2 T|Cp» (29)
the separation of phosphorus ligand steric and electronic effects
Neopentyl groups are too large to permit even the first stage of on a sound mathematical basis. The idea of ligand cone angles
reduction. should be extended to other types of ligands (such as those in
Addition of HZrCICp2 to unsymmetrically disubstituted acet- Appendix C), and tested experimentally wherever possible.
ylenes gives a mixture of vinyl zirconium derivatives in which There is a great need at the present time for measurement of
the steric bulk of the alkyl substituents on the acetylene deter- steric effects on heats of reaction. Calorimetric studies are in
mines the direction of addition (Table LXVII). The bulky ZrCICp2 their infancy.266 Other areas in need of research are the role of
preferentially goes on the smaller end of the acetylene. steric effects in rates of reactions, electronic structure and
Collman’s “picket fence porphyrin’’263 (Figure 29) and Bal- spectra, and electrochemistry. The application of steric effects
dwin's “capped’’ porphyrin complex264 both utilize organic to the control of homogeneous catalytic reactions is probably
superstructure on the porphyrins to protect the Fe-coordinated the most important area and holds great promise for future re-
02 in a manner analogous to the biological protein. The possi- search.
bilities of modifying chemical behavior through changing mo-
lecular structure have hardly begun to be explored. X. Addendum
Since writing this review, a number of relevant articles have
IX. Summary
come to my attention. In particular, a review2673 on tertiary
Steric effects are extremely important to structures, spec- phosphine ligands by Mason and Meek discusses steric effects
troscopic properties, and chemical behavior of phosphorus li- and independently arrives at a number of the conclusions pre-
gands and their complexes. Increasing the size of substituents sented here. A book entitled “The Chemistry of Phosphorus”
on P will tend to: by Emsley and Hall267b has only a short section on steric effects
•
open the SPS angles and the angles between L and other but provides broad coverage for all sorts of phosphorus chem-
ligands on a metal; istry. For the convenience of the reader, I have listed the fol-
• increase the bond lengths of M to P and to other ligands; lowing additions in the order in which they would have appeared
• reduce the s character in the phosphorus lone pair, thus in the text, with an appropriate section number.
decreasing 1JMP and shifting <5(31P) to low field; II. An x-ray crystal structure shows that trimesitylphosphine
• increase the basicity of the lone pair; (mesityl = 2,4,6-trimethylphenyl) has a mean C-P-C angle of
• favor lower coordination numbers (possibly involving M-M 109.7°,2683 the largest value reported for a free phosphine.
bond cleavage); Trimesitylmethane, with a smaller central atom, has an even
• favor coordination of other ligands which are in competition
larger (C-C-C) angle of 115.9°.268b CPK models show that
for coordination sites; P(mesityl)3 has a ligand cone angle of 212°. The 31P chemical
• favor intramolecular oxidative addition reactions; shift has an unusual value of +36.6 ppm, and the DS (Figure 9)
• favor isomers which are less crowded; is 0!268c
• increase the rates of dissociative reactions and decrease The x-ray crystal structure of Mo(CO)5[P(CH2)3(NCH2)3] has
the rates of associative ones. If the L’s become large enough, been solved to give a ligand cone angle of 102 ± 0.5° for the
they can interfere with the coordination of other ligands which symmetric phosphine cage.269
are normally strongly held, such as CO or 02. X-ray studies of CuCI(PPh3)3 and CuCI(PMePh2)3 have been
In order to reach conclusions about the relative importance determined and compared, and the role of steric effects on the
of steric and electronic effects in a particular system, a wide structures of 31 phosphine or arsine complexes of Cu(l) sum-
range of ligand types should be used. The Steric and Electronic marized.270
Map should be helpful, in conjunction with values of v and 9 from Graziani and co-workers271 have found that increasing the
Appendixes A and B, and the additivity relationships. size of the methylene chain (n = 0, 1, and 2) in complexes of the
It would be helpful to have cone angles from actual ligands type
342 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

N N complexes. A similar result was obtained in a study of Ni-

(CH2)n
V PPh3
(CN)2[P(OMe)nMe3-n]2 complexes.278
CoCI2L2
high spin
+ L ^ CoCI2L3
low spin
(31)

PPh3
IV.D. A kinetic study of the reaction of A/-methylaniline with
c/Xc
N N
c/s-PdCI(CN-p-G6H4Me)(L) complexes to give carbene com-
plexes (by attack of the amine on the isocyanide carbon) shows
decreases the P-Pt-P angle from 101 to 93° and increases the that the rates depend on both steric and electronic properties
Pt-P bond length from 2.29 to 2.33 A. of L.279 The reaction is favored by small L’s which are good
The orange dithioformate complex Ru(S2CH)(PMe2Ph)4+ (I) electron acceptors.
isomerizes on heating to a purple complex (II) in which a bulky IV.E. Steric factors control the ligand exchange equilibria
axial phosphine (P) has been pushed from Ru to C.272 Substi- observed when (o-Tol)NiBr(PPh3)2 is treated with other phos-
tuting the equatorial phosphines by the smaller L = P(OMe)3 phorus ligands, as shown by 31P NMR studies.280
causes the axial phosphine to move back to Ru, giving the yellow IV. G. Anderson and co-workers281 have shown by 1Hj145Ptj
INDOR measurements on solutions of frans-Pt(CNS)2[As-

\l/\ C—H w Me3-nEtn]2 complexes that the coordination mode of the thio-
cyanate ligands is sensitive to the size of the arsines. As n in-

/JV\
Ru 1 creases from 0 to 3, the distribution changes from 69% S,S to

/JV I 11
83% N,N bonded. The 195Pt chemical shift is diagnostic for the
coordination mode.
V. While most trialkylphosphines, including PEt3 and PCy3,
form adducts with CS2, P(f-Bu)3 does not.282 This further em-

-T-+-
\I/\ Ru C
phasizes that PCy3 acts smaller than P(f-Bu)3. While most trialkyl
phosphites react readily with diethyl peroxide to form pentoxy

/JV III
phosphoranes,283 P(0-f-Bu)3 does not react under the same
conditions.284
V.B. The very bulky ligands PPh(f-Bu)2 and P(f-Bu)3 react with
H3lrCI6 to give the dihydrides H2lrCIL2, while the less bulky li-
complex III. Treatment of II with the larger L' =
P(OMe)2Ph gives gands PMe(f-Bu)2 and PEt(f-Bu)2 give the monohydrides
only the purple C-bonded IV. HlrCI2L2.285 The authors propose that the larger ligands favor
H over Cl because a hydride ligand causes less steric strain.

Lwp Ru C.
V.C. Otsuka and co-workers286 have published the details of
the preparations and x-ray structures of their PdL2 and PtL2
complexes [L = P(f-Bu)3, PPh(f-Bu)2, PCy3and P(/-Pr)3], P(/-Pr)3
is small enough to allow isolation of Pt[P(/-Pr)3]3, from which
iv one ligand can be readily removed. The still smaller P(0-o-Tol)3
gives only a three-coordinate Pt complex. P(f-Bu)3 is so large
III.A. Values of J(PNP) in that even the small molecule 02 does not react with Pd[P(f-
Bu)3]2.65 The authors do, however, define cone angles incor-
rectly.286
V.D. An unusual cis dihydride of platinum, H2Pt|o-
[(f-Bu)2PCH2]2C6H4), has been prepared with a bulky chelating
Ph^ >Ss,CI
diphosphine. It fails to react with HCI even on prolonged treat-
ment.287
depend markedly on the size of R as follows: R = Me (+334), We have been able to make the five-coordinate Fe(0) complex
Fe[P(OCH2)3CEt]5 by treatment of Fe(COD)2 [COD
=
Et (+158), /-Pr (-29), and f-Bu (-35).273 1,5-cy-
III.B. Verkade and co-workers274 have shown that NO clooctadiene] with the phosphite.288 A similar reaction with the
stretching frequencies in (NO)NiL3+ complexes (where L is an bulkier P(OPh)3 gives a product analyzing for '‘Fe[P(OPh)3]4’’
acyclic, cyclic, or bicyclic phosphite) increase as the molecular whose spectroscopic properties and reactions show it to be
constraint is increased. They find a good correlation between V.289 Ligands of intermediate size [P(OMe)3, P(OEt)3 and P(0-
i^mo and 1JPH in the corresponding HL+. /-Pr)3] give (1,3-COD)FeL3 complexes.290
III. C. UV spectra of trans-lrCI(CO)L2 complexes show shifts
to shorter wavelength as the size of L increases.275 Values for
the longest wavelength band (in C6H6) are: P(p-Tol)3 (440), PPh3
(439), PCy3 (430), P(o-Tol)3 (418 nm).
IV. A. A thermochemical study276 of CoCI2L2 complexes by
differential thermal analysis indicates that more strongly elec-
tron-accepting phosphines tend to have stronger P-Co bonds;
however, steric bulk weakens the bonds. Thus in eq 30 L =
P(/-Pr)3 requires about 16 kcal/mol less than L = PEt3.
AH
CoCI2L2(I)->- CoCI2(s) + L(g) (30) Irradiation of solutions of Fe(CO)5 in the presence of
Ph2P(CH2)nPPh2 (n = 1, 2, 3, or 4) gave Fe(CO)4Ph2P(CH2)n-
1 only, however, three
A study277 of eq 31 by an accurate NMR susceptibility method PPh2Fe(CO)4. In the case of n
=

[L = P(OEt)3, P(OEt)2Et, P(OEt)Et2, or PEt3] has shown that K other products were formed: Ph2PCH2PPh2Fe(CO)4,
depends on both electronic and steric effects; the middle Ph2PCH2PPh2Fe(CO)3, and Ph2PCH2PPh2Fe2(CO)7.291 We have
members of the series give the most stable five-coordinate found that reactions of excess Ph2P(CH2)nPPh2 with (COD)2Ni
Steric Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No, 3 343

give Ni[Ph2P(CH2)„PPh2]2 for n

=
2, 3, or 4 but for n = 1 give cients by least squares, they are able to specify the percentage
(Ph2PCH2PPh2)2Ni(Ph2PCH2PPh2) in which two ligands are mo- of electronic and steric control in each reaction.
nodentate and one is bidentate.116 VIII. Et2Pt(PPh3)2 is much less stable than the metallocycle
VLB. Yoshikawa and co-workers292 have shown that phos- (CH2)4Pt(PPh3)2. This difference can be rationalized in terms of
phorus ligand steric effects dominate the product distribution the steric constraints of the ring, which prevent the 0° Pt-C-C-H
in the nickel catalyzed 4 + 2 cycloaddition of norbornadiene to dihedral angle necessary for /3-hydride elimination.295 The more
acrylonitrile, giving VI and VII. Larger ligands favor VII. In the 2 flexible (CH2)6Pt(PPh3)2 complex is only marginally more stable
+ 2 cyclodimerization of norbornadiene, on the other hand, than the dimethyl.
electronic factors dominate.293 The isolation of M[SbAr3]4 complexes (M = Ni, Pd, or Pt; Ar
=
Ph or para-substituted phenyl) which do not dissociate ex-
tensively in solution296 indicates that SbAr3 ligands have smaller
effective cone angles than the corresponding PAr3, whose ML4
complexes are completely dissociated in solution at ambient
temperature.58
A study of the coordination mode of BH4~ in the series
Cp2LnBH4THF (Ln = lanthanide element) shows that decreasing
the effective ionic radius from 1.09 A (samarium) to 0.98 (yt-
terbium) causes the BH4~ to go from tridentate to biden-
Heimbach and co-workers294 are using the idea of the Steric tate.297
and Electronic Map to fit data on product distributions in a variety Kinetically stable MR2 (M = Ge, Sn, Pb) complexes have been
of homogeneous catalytic reactions. By expressing the surfaces prepared using the very bulky R = CH(SiMe3)2 or N(Si-
as polynomial expansions in v and 9 and optimizing the coeffi- Me3)2.298

XI. Appendixes
APPENDIX A. Values9 of the Electronic Parameter v

Type of Phosphorus Ligand

PX3 P(OR)3 PR3 Other v, cm

P(f-Bu)3 2056.1
PCy3 2056.4
P(o-C6H4OMe)3 2058.3
P('-Pr) 3 2059.2
PBu3 2060.3
PEt3 2061.7
P(NMe2)3 2061.9'
PEt2Ph 2063.7
PPh(pip)2 2064.0
PMe3 2064.1
PMe2Ph 2065.3
P(p-C6H40Me)3 PPh2(o-C6H4OMe) 2066.1
PBz3 2066.4'
P(o-Tol)a 2066.6
P(p-Tol)3 PEtPh2 2066.7
PMePh2, PPh2pip 2067.0
P(m-Tol)2 2067.2
PPh2NMe2 2067.3'
PPh2(2,4,6-C6H2Me3) 2067.4
PPhBz2 2067.6
PPh2(p-C6H4OMe) 2068.2
PPh2Bz 2068.4
PPh3 2068.9
PPh2(CH=CH2) 2069.3
P(CH=CH2)3 PPh2(p-C6H4F) 2069.5
PPh2(m-C6H4F) 2070.0
PPh2CH2CH2CI 2070.8
P(p-C6H4F)3 2071.3
P(OEt)Ph2 2071.6
P(OMe)Ph2 2072.0
P(0-/-Pr)2Ph 2072.2
P(p-C6H4CI)3 2072.8
PHPh2 2073.3
P(OBu)2Ph 2073.4
P(m-C6H4F)3 2074.1
P(OEt)2Ph 2074.2
P(OPh)Ph2 2074.6
PPh2C6F5 2074.8
P(0-/-Pr)3 2075.9
PPh2(O-o-06H4CI) 2076.1
P(OEt)3 2076.3
PH2Ph 2077.0
344 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

Appendix A (Continued)

Type of Phosphorus Ligand

PX3 P(OR)3 PR3 Other v, cm

1

P(CH2CH2CN)3 2077.9
P(OCH2)2Ph 2078.7
P(OCH2CH2OMe)3 2079.3
P(OMe)3 2079.5
P(OPh)2Ph 2079.8
PCIPh2 2080.7
PMe2CF3 2080.9
ph3 P(0-2,4-C6H3Me2)3 2083.2°
P(OCH2CH2CI)3 2084.0
P(0-p-Tol)3, P(0-p-C6H40Me)31 2084 1
P(0-o-Tol)3 )
P(0-o-C6H4-/-Pr)3 2084.6°
P(0-o-C6H4Ph)3 2085.0°
P(OPh)3 2085.3
P(0-o-C6H4-f-Bu)3 2086.1°
P(OCH2)2OPh 2086.5
P(OCH2)3CPr, P(OCH2)3CEt 2086.8
P(OCH2)3CMe 2087.3
P(OCH2CH2CN)3 2087.6
P(0-o-Tol-p-CI)3 2088.2
P(0-p-c6H4CI)3 2089.3
P(C6F5)3 2090.9
P(OCH2CCI3)3 2091.7
PCI2Ph 2092.1
P(0-P-C6H4CN)3 2092.8
PCI3 2097.0
pf3 2110.8

3
Substituent Contributions to v for PX1X2X3: v =
2056.1 + £
/=i
x/

Substituent X,- x/ Substituent X/ Xi

f-Bu 0.0 p-C6H4OMe 3.4

Cy 0.1 o-Tol, p-Tol, Bz 3.5
o-C6H4OMe 0.9 m-Tol 3.7
i-P r 1.0 Ph 4.3
/-Bu 1.2 est CH=CH2 4.5
Bu 1.4 p-c6h4f 5.0
Et 1.8 p-CeH4CI 5.6
NMe2 1.9 m-C6H4F 6.0
piperidyl 2.0 CH2CH2CI 6.1°
Me 2.6 0-/-Pr 6.3
2,4,6-C6H2Me3 2.7 OBu 6.5
OEt 6.8 (OCH2)2/2 9.8
CH2CH2CN 7.3 0-o-C6H4-f-Bu 10.0°
OMe, OCH2CH=CH2, 7.7 OCH2CH2CN 10.5
OCH2CH2OMe O-o-Tol-p-CI 11.1
H 8.3 c6f6 11.2
0-2,4-C6H3Me2 9.0 0-o-CgH4CI 11.4
OCH2CH2CI, O-o-Tol, 9.3 OCH2CCI3 11.9
O-p-Tol, 0-p-C6H40Me 0-p-C6H4CN 12.2
0-o-C6H4-/-Pr 9.5° Cl 14.8
0-o-C6H4-f-Bu 9.6° F 18.2
OPh 9.7 cf3 19.6
b c
This value for PH3 estimated
3
"co(A1) of Ni(CO)3L in CH2CI2 from ref 1 unless noted otherwise, pip =
piperidine. Previously unpublished value. was
from extrapolation of values for PH3_„Phn (n =
1 to 3).

APPENDIX B. Values*’ of the Ligand Cone Angle 0

Type of Phosphorus Ligand

PX3 P(OR)3 pr3 Other 0, deg

PH3 87
P(OCH2)3CR' PH2Ph°c 101
PF3 104
P(OCH)3(CH2)3 106
P(OMe)3 Me2PCH2CH2PMe2° 107
Sterlc Effects of Phosphorus Ligands Chemical Reviews, 1977, Vol. 77, No. 3 345

APPENDIX B (Continued)

Type of Phosphorus Ligand

PX3 P(OR)3 PR3 Other 0, deg

P(NCH2CH2)3 108
P(OEt)3 109
P(OCH2CH2CI)3 110
P(CH20)3CR'9 114
9,9
P(OCH2CCI2)3e Et2PCH2CH2PEt2
P(OMe)2Ph, P(OMe)2Et 115
P(OEt)2Ph9c 116
PMe3 118
Ph2PCH2PPh29'9 121
PMe2Ph9c 122
PCI3 PMe2CF39 124
9,9 125
Ph2PCH2CH2PPh2
9i 9 127
Ph2P(CH2)3PPh2
6
P(OPh)3, P(0-p-Tol)3' PHPh2 128
P(0-/-Pr)31 130
PBr3 131
PEt3, PBu3,9 PPr3,1
POMePhor 132
P(CH2CH2CN)3 J

POEtPh29 133
PMePh2/PEt2Ph9 136
P(CF3)3 137
PEtPh29 140
P(0-o-ToI)3' 141
Cy2PCH2CH2PCy299 142
P(/-Bu)39 143
PPh3, P(p-Tol)3, 145
P(m-C6H4F)3
P(0-o-C6H4-/-Pr)3e 148
PPh2(/-Pr)9 150
P(0-o-C6H4Ph)3e 152
P(NMe2)3 PPh2(f-Bu)9 157
PPh2C6F69 158
P(/-Pr)3, P(sec-Bu)39 160
PBz3® 165
PCy3h PPh((-Bu)29 170
P(0-f-Bu)3 172
h
P(0-o-C6H4'f-Bu)3 175
P(neopentyl) ~180
P(f-Bu)3 182
P(C6F5)3 184
P(0-2,6-C6H3Me2)3® 190
P(o-Tol)3 194
P(mesityl)39 212
a From model measurements in ref 2 unless noted otherwise. b Previously unpublished value. 9 Based on a “sideways” phenyl ring with 8</2 = 65°.
9 Values
given are for half of the chelate assuming PMP angles in M[R2P(CH2)nPR2] of 74, 85, or 90'“for n = 1, 2, or 3, respectively. e Reference 250.
' Reference 9. 9 Increased 2° from
the value in ref 2 because PBu3 should not be smaller than PEt3. 9 Value based in part on the degree of substitution
of CO from Ni(CO)4 (Figure 9).

APPENDIX C. Cone Angles® for Ligands not Bound by a Group 5 Atom

Cone angle, Cone angle,

X9 R Other deg X9 R Other deg

H 75 Br Ph9 105
Me 90 I 107
F 92 /-Pr 114
CO, CN, N2, NO ~95c CH2CMe3® 120
coch3 100 f-Bu 126
Cl Et 102 Cp 136
CH2Ph 104
3 9
Based on a metal covalent radius of 1.32 A. Calculated using the covalent and van der Waals radii (A) of the CPK models: H (0.33, 1.00), F (0.57,
1.35), Cl (0.99, 1.80), Br (1.14, 1.95), and I (1.35, 2.15). c The covalent and van der Waals radii of the CPK -C=atom lead to a cone angle of 113°. This
must be an overestimate since group 6 M(CO)6 complexes are very stable. A 0 for CO slightly larger than 90° is suggested by the angles in Mn2(CO)10
9
(Table VIII) and HMn(CO)5 (structure 102). The maximum and minimum half angles are 65 and 40°. 9 Essentially the same cone angle was obtained
for CH2SiMe3, assuming a 0.96 A covalent radius for Si. ' An M-Cp distance of 2.03 A was assumed, as reported for CpWCI(CF3C2CF3)2 by J. L. Davidson,
M. Green, D. W. A. Sharp, F. G. A. Stone, and A. J. Welch, Chem. Commun., 706 (1974).

Acknowledgments. I am indebted to several people for helpful M. C. Baird, R. Baker, P. Binger, T. L. Brown, J. A. Connor, A. D.
discussions and for providing me with results prior to publication: English, L. W. Gosser, T. A. Herskovitz, C. S. Kraihanzel, M.
346 Chemical Reviews, 1977, Vol. 77, No. 3 Tolman

Laing, L. G. Marzilli, P. Meakin, A. Musco, J. H. Nelson, J. F. (1967).

(42) L. J. Vande Griend, J. G. Verkade, C. Jongsma, and F. Bickelhaupt,
Nixon, A. J. Poe, R. R. Schrock, B. L. Shaw, M. A. Weiner, and Phosphorus, in press.
J. G. Verkade. I would also like to acknowledge the great effort (43) C, Jongsma, J. P. De Kleijn, and F. Bickelhaupt, Tetrahedron, 30, 3465
of Ms. Kay Burton in typing the manuscript and the patience of (1974) .

(44) M. Y. Darensbourg and D. Daigle, Inorg. Chem., 14, 1217 (1975).

my wife Ann during its preparation. (45) Reference 37, p 183.
(46) J. G. Verkade, Coord. Chem. Rev., 9, 1 (1972), and private communica-
tion.
(47) V, S. Mastryukov, L. V, Vilkov, and P. A. Akishin, Acta Crystallogr., Suppl.,
XII. References 16, 128 (1963), reported an approximate angle of 104° in trivinyl phos-
phite by electron diffraction.
(1) C. A. Tolman, J. Am. Chem. Soc., 92, 2953 (1970). (48) See ref 37, Figure 2.2 on p 117, and the discussion beginning on p 80.
(2) C. A. Tolman, J. Am. Chem. Soc., 92, 2956 (1970). (49) D. G. Gorenstein, J. Am. Chem. Soc., 97, 898 (1975).
(3) CPK atomic models (scale 1.25 cm/A) are available from the Ealing Corp., (50) B. E, Mann, C. Masters, and B. L. Shaw, J. Chem. Soc. A, 1104
22 Pleasant St., South Natick, Mass. (1971).
(4) A. Pidcock, “Transition Metal Complexes of Phosphorus, Arsenic, and (51) P. E. Garrou, Inorg. Chem., 14, 1435 (1975).
Antimony Ligands”, C. A. McAuliffe, Ed., Wiley, New York, N.Y., 1973, (52) R. B. King and J. C. Cloyd, Jr., Inorg. Chem., 14, 1550 (1975).
p 25. (53) B. E. Mann, J. Chem. Soc., Perkin Trans. 2, 30 (1972).
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