MODERN APPROACH TO INORGANIC CHEMISTRY (B Sc.

Crystal Field Splitting in Tetragonal and Square Planar Complexes

The splitting of d-orbitals in tetragonal and square planar complexés can be easily understood from
the known splitting of d-orbitals in.octahed ral complexes. This is because tetragonal and square
planar geometriescan be understood by twithdrawing twotransligandsfrom an octahedral comoleT
process is cailed elongation. Generally, we consider the removal of trans ligands alorng the Z-ax5
Astheligands lying on the Z-axis are moved away, the ligands in the XY planetendto approach
the central ion mote closely. As a result of increase in metal ligands bond along
Z-axis, the repulsions from the lígands to electrons in da orbital decreases and therefore, the enerzy
of d, orbital is decreased relative to that in octahedral field. At the sarne time, the metal ligand bond
along X and Yare shortened so that the d-orbital in XYplane,dexperiences greaterrepuision from
the ligands and therefore, its energy is raised. Similarly, the d,, and d, orbitals are lowered in enery
e of decrease in repulsion effects along the Z-axis while the energy ofd orbital is raised. The
resulting splitting patterm is shown in Fig. 18.Thisstate representstetragonaly distorted octahedral
structure.

ASp
Q
2
Q
bitalvn. '. OCO.
Oxy
phuutal tield
Preeiom ougomd Octahedral
Structuree
dyz dx
Tetragonal dy x
structure
Square Planar

Fig 18. Crystal field splitting in octahedral, tetragonal and square planar complexes
As the trans ligands lying along Z-axis are completely removed, a square planar compes
rise in the energies o
formed. In the square planar complex, there is further

*****.

Square planar
(Trans ligand are
Tetragonal completely removed)
(two trans ligands are
at large distance))

Fig. 19. Change of octahedral geometry to tetragonal and finally to square plariar geometry by removing
two trans ligands.
|METAL LIGAND BONDING IN TRANSITION METAL COMPLEXES
25
s md a further fall in the energies of d , d,, and d orbital as s o a
t insquare planar complex, the enerey ofd 3 iTbitalfalls even below the y
ctaheral complexl otetragonally distorled octahedron and firally to sqla
arrangement is shown
Fig. 19 in

e Temembered that tetragonally distorted structure can also beobtaineg when

tansigands are brought the metal ion. This is called flattening oran oa
closer to
odron. the
shown ig 20
in
where Fig. 20(a) shows elongation of octahedron.In flattening e or

rans
energies l
M-bonds are shortened while other four M-L bonds become large. The sP
i - o r b i t a l s w i l l b e r e v e r s e o fw h a t h a s b e e n o b s e r v e d in the case of elongation ( r ' 8 10

Metal-Ligand
bond a > b
Metal-Ligand
bond a < b

a) Elongation (b) Flattening

Fig. 20. Tetragonally distorted octahedron (a) elongation (b) flattening
ample Caleuiate CFSEfor the following ions in octahedral complexes
(i d' strong field octahedral (ii) d weak field
strongfield (Pb. U. 2010)
Solution :(i) d*strong field

Fletronicconfiguration: tze"
FSE - 4 x ( 4Dq) + P= - 16 Dq + P

)d weak field

11X1
Electronic configuration:t2
(-4 2 -4
FSE- 4 Dq) + x
(6 Dq) =
Dq
ii) strong field

(t

P a i p i n g alstady

Electronicconfiguration:,
CrSF 6 (-4 Dq) +1 (6 Dg) +P=-18 Dg + P x
ACTORS DETERMINING THE MAGNITUDEOF CRYSTAL FIELD SPLITTING
The following factors influence the
magnitude of crystal field splitting:
A.
Nature of the ligand (spectrochemical series)
The crystal field theory depends upon the nature of the ligands. The greater the ease with which
theligandscan approach the metal ion, the greater will be the crystal field splitting caused byit. The
ligandswhich cause only a small degree of crystal field splitting are called weak field ligands while
thosewhich cause alarge splhittingarecalled strong field ligands. Thespectrochemical series isan
experimentally determined series. It is very difficult to explain, the order because it incorporates the
efects of both a and n
bonding. In general, the variation in the splitting is caused by a numbèr of
factors such as
a) Small ligandscan cause greatercrystal field splitting because they can approach the metal
Br ions.
1onclosely. For example, F ion produces more A value than large CF and
30 MODERN APPROACH TO INORGANIC CHEMISTRY
(B.Sc. I)
(b) The ligands containing easily polarizable electron pair will
be drawn more closely to the
metal ion.
(c) The ligands wlhich have greater tendency to form multiple bonds such as CN and N0
cause greater crystal field splitting. The very high crystal field splitting produced by CN
ligandis about double that for weak field ligands like halide ions, This is attributed to the
Tbonding in metal. The metal donates electrons from a filled t, orbital into the vacant orbital
on the ligand.
(d) The following pattern of increasingo donation is observe
halide donors < O donors < N donors < C donors
The effect of crystal field splitting (A) on the ligands attached to Cr(III) is shown below
Complex Ligand Donor atom 4,(cm )

ICCI C CI 13640

ICr(HO)P HO O 17830

[CrNH,* NH N 21680

ICr(CN), CN 26280

The common ligands can be arranged in an increasing order of crystal field splitting 4. This
regular order is called spectrochemical series. The order remains almost same for different metal
ons.

h e spectrochemical series in the increasing order of crystal field splitting is given below
Weak field ligands
I B r < CI< NO," < F < OH < ox <H,O < EDTA < py =NH < en < dipy < o-phen
NO <CN<
eld ligands
Strong field ligands
ncreasing Crystal Field

From the series, it is clear that CO, CN and NO are strong field ligands whereas
I, Br and Cl are weak field ligands.
a 2 Oxidation state of the metal ion
T h e magnitude of the crystal field splitting depends upon the oxidation state of the transibion
metal ion.The metal ion with higheroxidation state causeslargercrystalfield splittingthan is done
by the ion with lower oxidation state. For example,the crystal field splitting energy for
ICo(HO) complex in which the oxidation state of cobalt is +3, is 18,600 cm, whereas for the
complex jCo(H,Ó),P* in which the oxidation state of Co is +2, A, = 9,300 cm'. Similarly, for the
complexes[Fe(H,O),J and [Fe(HO),P* in which iron has oxidation state +3 and+2, respectively, the
values of A, are 13,700 cm' and 10,400 cm, respectively.
This is clear from the following data in which crystal field splitting energies (4,) for hexa aqua
complexes of M2* and M* ions are given:

Metal Oxidation Electronic Oxidation Blectronic

ion state configuration (cm-1) state configuration (cm-)

Cr 13900 III d 17830

Mn II d5 7800 III 21000
Fe II 10400 d 13700
9300 do 18600
Co
 METAL LIGAND BONDING IN
ptctuohemltol sthses 31
TRANSITION METAL COMPLEXES
3. Type of d-orbitals (transition series)
ALC O Crystaltield splitting for similar tomplexes of a metal
thesame OXIGa
in
increases by abOut 30 to 50% on eoine from 3d-series (first transition series) to 4d series (secona
transition series). The
increase is almost of the same amount (30--50%) on_going from 44-Sernes
(second transition series),to 5d-series (third transition
This series).
E explained on the basis that 4d-orbitals in comparison to 3d-orbitals_are_biggeT
in size ana extend farther into space, As Tésult, the
with the ligands and, 4d-orbitals can interucl
therefore, the crystal field splitting is more. Similarly, 5d-orbitals are
-

DBerhan 44-0rbitais and A, for the third transition series s more than that for second transluon
series. This is evident from the following data
Complex ion Electronic configuration (cm)

[Co(NH),J 3d 23,000
[Rh(NH),J 4d 34,000
[Ir(NH),JPA
5d 41,000
Geomefry 6rthecomplex T9
As mentioned earlier, the crystal field
spiitting energy of tetrahedrai complexes (A,) is nearly half
the value (A) for octahedral complexes (A, 4/9 A). In other words, the
value of splitting energy for
tetrahedral complexes, in general, is small as cornpared to the pairing
energy P. The tetrahedral
Complexes are, therefore, mostly high spin complexes
 'DL O.

CRYSTAL FIELD THEORY AND MAGNETIC PROPERTIES OF COMPLEXES

One of the important applications of crystal field theory has been in understanding themagneti
properties of coordination compounds. The primary object of studying magnetism is to know
whether a particular complex is paramagnetic or diamagnetic. Thesubstances which have all paired
te
electrons arecalled diamagneti while thesubstances whichcontain unpaired etectrorns are called
paramagnetic substances. The number of unpaired electrons, in a given complex of known geometry
 METAL LIGAND BONDING IN TRANSITION METAL COMPLEXES 7
high spin.
can be easily predicted provided we know whether the complex is low spin
or

also depend upon the magnitude of A, and pairing energy P. For example,
() IfP> A the electrons will not pair up and the complex will be high spin compie
the electrons will prefer to pair up and the complex will
be low P"
(1)f A>P,
complex. d those
Thus, the complexes with weak ligand field are high spin complexes (paramag
with strong ligand field are low spin complexes (diamagnetic
low magnetic.charat
or n
it has been
us illustrate this by considering the
complexes of cobalt (IID. From experiments,
Let while the complex [CoFJis paramage
okserved that the complex [Co(NH,),J* is diamagnetic

OO,
Large 4, value A,
A Small 4, value

001
[CoF Co(NH)

Low spin complex

High spin complex (Diamagnetic)
(Paramagrnetic)
and
and explanation for [CoF,
Fig. 21. Crystal field splitting
[Co(NH),* complexes.
It has been
and it has six d electrons.
and [CoF]3 is in +3 oxidatiorn state Therefore, A,, will be less than
Cobalt in [Co(NH)3*
ligand and therefore 4,Thesmall.
is
observed thatF ion is a weak field
complex will behigh spin.
As clear
far as possible. On the
electron will remain unpaired as offour unpaired electrons.
P and the due to the presence
paramagnetic a result, the
from Fig.21.(a), the complexs therefore, A, will be greater than P. As
other hand, NH,
is a strong field ligand and This happens in the case of
[Co(NH)., as
resuitsin a lou Spin complex. in the c a s e
electrons pair up and this is shown to be much higher
be noted that in Fig. 21., 4,
crystal field splitting energies (A) and
It may pairing
represented in Fig.21.(b). The
that in the case of [CoF 5.
of [Co(NH)J3* than a r e given
in Table
for s o m e complexes
energies energies (P) for some complexes.
and pairing
Table 5. CFSE (A)
A(cm)
P (cm-) High spin/low spin
Configuration
Complex 21000 high spin
13000
d6
CoF d
23000 21000 low spin
Co(NH).* 10400 17600 high spin
d6
Fe(HO)P 17600 low spin
32850
Fe(CN) J easily can be predicted if w e know
of unpaired electrons a given complex
in
number of unpaired electrons for
Thus, the number spin compiex. Ihe
is a high spin
or
a low Table 6. For
a i t r e r e n t configurations are given in
whether the complex complexes having
bctahedral and tetrahedral are discussed (because A, is always less
than P
high spin conigurations
setrahedral complexes, only
a r e not
known).
low spin complexes
nd