SUBSTITUTION REACTTON IN OGTAHEDRALCOMPLEXES

Substitution reactions involve the

replacement of one ligand in coordination sphere by another
without anychange in coordination number and oxidation state of metalcation, For example, octahedral
complex containing a metal cation or atom bound to tive inert ligands (sayL) and one labile
X) which is to be replaced by an incoming ligand (Y). The overall reactior1 is shown ligand (say
below:

Since
[ML5X]+Y ML5Y+X
ligands behave as nucleophile, therefore, these reactions are called nucleophilic substitution
reactions or ligand substitution reactions.
 Mechanism in Octahedral Complexes
reactions involved in a reaction.
The mechanism of a reaction is the sequence of elementary three
either of the following
Substitution reactions in octahedral complexes take place through
mechanisms
1. Dissociative (D) Mechanism
2. Associative (A) Mechanism
3. Interchange () Mechanism

in which an
Mechanism: In the dissóciative mechanism, there is step
a
1. Dissociative (D)
intermediate of reluced coordination number is formed ie., the M-X
bond is fully broken before the
M-Y bond begins to form.

LsMX] o[L5M]
-X fast
[LsMY
Intermediate
C.N.=5

M OR M-L
L
Squarepyramidal L
Trigonal bipyramidal
where L is an inert ligand, X is labile (leaving ligand) and Y is the entering ligand.
The rate determining step is the slowest elementary reaction.Therateofoverallsubstitution reaction
depends only on the concentration ofthe original complex, [ML,X] and is independent of the
concentration of the inconing ligand Y.
Rate k[ML 5X]|
This reaction is offirst order in [MLsX]and [ML5X] gets dissociated is rate determining step. Thus,
this reaction is also called dissociative SN mechanish (substitution, nucleophilic first order).
Most substitution reactions in. octahedral complexes takes place by dissociative mechanism in
which an intermediate of coordination mumber = 5 (most probablesquare pyramid).is formed

2. Associative (4) Mechanism An associative mechanism involves a step in which an

interrediate is formed with a higher coordination number than the original complex i.e., the incoming
ligand Y directly attacks the original complex to form an intermediate with a coordination number = 7 in
the rate determining step. The rate determining step is slow.
L5MX]+y sow [LgMY)

Intermediate
C.N. 7
 XY
L L
OR
M,
M
L
L YL
Monocapped Pentagonal bipyramid
Octahedron
which the X and Y
This intermediate might be expected a monocapped octahedral structure in
structure. The second step is the
ligands share-one of the octahedral sites or a pentagonal bipramidal
concentration of bc th
dissociation of ligand X to give the product. The rate of reaction depends on the
ML5X and Y Therefore,
Rate =k[M5X][Y
This reaction is of second order and this associative mechanism is also called SN (Substituticn,
nucleophilic, second order) mechanism.
3. Interchange () Mechanism: This mechanism takes placein onestep without forminga fain ly
stable intemediate, instead the leaving and entering ligands exchange in a single step forming an
activated complex. The interchange mechan1sm is common for many reactions
of octahedral complexes.
little stabilization energy and
The activated complex (also called as transition state) has very or no

immediately passes on to products or reverts to the reactants.

LsMX +Y [Y.... L5M....X} L3MY]+X

Activated complex

In this mechanism the M-X bond begins to break and starts to move away from the metal and the
stable
M-Y bonds begins to foim simultaneously and Y moves into the coordination sphere and no
intemediatc is formed.
Interchange mechanism is subdevided into tiwo categories
(i) Interchange Dissociative (a)
i) Interchange Associative (1)
(i) Interchange Dissociative Mechanism(la): The M-Y bond begins to form befo:e the M-X
bond is fully broken but the M-X bond þreaks preferentially and the interchange is closer to a
dissociative than to an associative niechanism, and no detectable intermediate appears.
a) Interchange Associative Mechanism (): The M-Xxbond begins to break before the M-Y
bond is fully formed but the M-Y bond forms preferentially and ihe interchange is closer to associative
and no detectable intermediate appears.
 (6) Trans Effect: Ligands other than the entering and leaving groups may affect the rate of
substitution reaction. A ligand which is not lost in the reaction is called a spectator ligand.
The efect of a spectator ligand coordinzted to metal caticn upon the rate of substitution of ligands
trans to it is called trans effect.
Chatt-et-al have suggested that the. rans effect ofa spectator ligand or group attached to a metalcation
18the terdency ofthat group to direct anincoming
directs an
group to occupy theposition transto that group.
The spectator ligand
called
or group which incoming ligand to occupy the position trans to it is
atrans directing group or tran. director.
Now consider the substitution re actions in square planar complex, trans MLX3 in which L is a
spectator (or non-leaving) group and Ktrans to L is a leaving group.
If the ligand X trans to
ligandLi replaced
said to have large labilizing effect th
rapidly by another ligand Y to give MLX2¥, then L is
labile effect. Since the
or ans ligand
L makes the ligand Xlahile
trans
to it so the trans effectis alsocalled s labilizing effect.
L L X
+Y
X
-X
X
M Y
The labilizing effect or trans effect of some ligands follow the order
(High end) CN",CO, NO, C2H4 > PR3, As R3, H"> CHi,SC(NH2)2 > C6H3, NOZ,I",
SCN> Br>CI> py >RNH2, NH3 > F" >OH
>H20(low end)
The series given above is called trans
directing series.
The ligands like CN, CO, NO,C2H4 etc.
lying on the high end are T-acceptors and are strong trans
directors. On the other hand, the ligands like OH,
H20lying on weak end are very pocr trans directors.
A strong trans director has the
ability to promotes more rapid substitut.on of the ligand trans to it
self than it does of the igand cis to itself.
The trans effect is a kinetic
phenomenon because trans effect of a ligand L promotes the re pid
substitution of ligand (X) trans to itself.
The irans effect (i.e., kinetic trans
is a thermodynamic phenomenon, i.e.,
effect) is differentiated from trans infuence. The trans influence
ligands can influence the ground state properties of ligands to
which they are trans such as the trans M-L bond distance or the vibrational frequency. For exampie: A
 goodtrans- directing ligand weaken the bond between the metal and the trans ligand. The trans
influenceis shown in the following complexes
238 pm 233 pm 232 pm

Cl CI Cl Cl Cl ,CI
P CH2 Pt
EtP CI Cl C
CH2
(a) (b) (c)
From the complexes (a), (b) and (c) it is observed that the orderoftrans influence oftrans ligands is:
EtP> CH =
CH2 > CI
Applications of Trans Effect
Most of the work on trans effect has been done on square planar complexes of Pt(lI).
The trans effect is very useful in the synthesis of large number of square planar complexes of Pt([I).
1. Synthesis of cis-
and trans- [Pt(NH3)2Cl2]
The synthesis of cis-[Pt{NH3}2CI2] is carried out by treatment ofthe[PtCl41 ion with ammonia.

NH
CI Pt
C CI2- +NHCI
NH3+NH3 Ci Pt
C. C.-Cc
-

CI
CI NH3
cis-[Pt (NHa)2 Cl2]
In the second step of this reacticn ammonia occupies the cis position because the trans effect ofCt
is greater thanthat of ammonia.
It means that the least reactive CI in [Pt{NH3 )2 Cl2] is trans to ammonia.
The synthesis oftrans [P(NH3)2 Cl2]is carried out by the treating (P:(NH3)4]* with Cl.
NH, 12+
HN N +C H3N CI
+CI
HN ,CI
HN Pt(
NH, - NH3 HN Pt NH3 - NH3 C PtNH
NH3 CI
trans-[Pt (NH3 )2 Cl2]
In the above reaction CI replaces most labile NH3 in [Pt(NH3)2 Cl2]' which is trans to Cl group
and thus results in the formation oftrans [Pt{NH3 )2 Cl2]
 5. Synthesis of Isomer of[Pt(NH3)(Py)CIBr]:
Br> CI> Py> NH and the
.he order of trans-directing ability of NH3, Py, CI and Br is,
facts govern the synthesis of
order of bond strength is Pt-NH3 >PE-Py>Pt- Br> Pt-CLJThese two
three isomers o [Pt(NH, (Py)CIBr].

C NH3 Cl NH3 +py NH

+NH3 +Br Pt P
Pt -CI
Pt
CI C Br- CI Br
C
When Br is added to [Pt{NH3 )Cl3l, one ofthe ligands trans to CI is replaced rapidly and the
bond
product so formed is [Pt(NH3 )BrCl2] rather than [PtBrCl3]because of greater Pt-NH
strength. Now when Py is added to cis-{Pt{NH3 )BrCl2], the Cl trans to Br is replaced by Py
becaue of greater trans effect of Br".

The second isomer of[Pt(NH3 )(Py)CIBr] is prepared as follows using the concept of trans effect.

Cl CI72 Py CI\D,/ Py +NH3 HN Py

P
+py B+Br Pt Pt
CI - CI -CI
C C CI Br CI Br

The third isomer of[Pt{NH3 )(Py)CIBr] is prepared as follows

[Cl +2py CC Py +NH3 HN Py Br HN Br

Pt Pt pt -Py Pt P
C CI -2C1 C
Py -CI C Py CI

In the second step of this reaction the substitution is controlled >y the lability of chloride whereas in
the third step it is controlled by higher trans effect of Cl.
In this reaction, the
second step is seen to be againstthe trans eifect but it is not so. Thetranseffect
does not predict that any ligand trans to Clshould be replaced more rapidly than any ligand trans to Py
buta C trans to CI will be replaced more rapidly than a CI trans to Py if there is an ambigity
(or choice) but here, there is no choice. In the last step if we know that Br is the enteringand Py is the
leaving grOup..then transefiect nredicts the renlacement.oftha P tranadn
To Distinguish between cls- and trans-lsomers of [PtA2X2]Complexes
The trans effect has also been used by Russian chemists to
distinguish between cis- and trans
ISomers of
the [PtA2X2] type complexes. (where A =NH3 or amine group and X an anion like
CI, Br etc.) For example cis-{Pt{NH3)2 Cl2] reacts with
thiourea(tu), H2NCSNH2 to give
[P(tu)4]t whereas under the same condition the trans{Pt{NH3)2 Cl2]gives [Pt(tu)2Cl2].
This test is known as the Kurnakov test. This test works for
{Pt(NH3)2X2] complexes
balides) because the trans effect oftu is greater than that of amine and halide ion.

Cl NH3 +2tu +2tu Tu Tu

72
Pt P
C
C NH
NH 2CI -2NH3
Tu TTu
u's

H3N CI + tu HN. ,Tu +tu HN Tu 12+

Pt -Cl
Pt -CI
CI NH3 CI
NH Tu NH3
trans-[Pt (NHhClh] Irans-(Pt (NH )2Clal
In the cis-complex, the ammonia inolecules
labilized by the trans-halide ions and, therefore,
are
ammonia molecuels are replaced by two thiourea molecules. Now the halide ions trans
to thiourea
molecules are labilized and these halides are also replaçed by the thiourea
molecules. Hence all the
positions are occupied by thiourea. In the trans- complex, the halide ions labilize each other and,
therefore, only these ligands are replaced by thiourea molecules.
Similar results have been reported for reactions of
thiosulphate (S20;") ion instead of tu, cis- and
trans- [P(NH3 )2 Cl2] reacts with excess of S20 ion to form [P{S203)4 ]and
and
[PL(NH3)2(S203)2]respectively.
 REDOXREACTIONS OR
CTRONTRANSEERR
In substitution reactions (either in octahedral or square planar complexes) there were some chemical
changes but the oxidation states of the metals were the same i.e., there was no electron transfer from one
complex to another. Electron transfer reactions of transition metal complexes are the
oxidation-reduction (redox) reactions which involve the transfer of an electron from one complex to
another. The electron transfer reactions may occur either with or without chemical changes.
The complex which loses electron, the oxidation state of metal increases and it is said to be oxidized
and is reducing agent. On the other hand, the complex which gains electron, the
this complex called
oxidation state of metal decreases and it is said to be reduced and this complex is called the oxidizing
agent. An example ofanelectron transfer reaction is givenbelow
Fe(CN1+[Fe(C)a1
Reducing agent Oxidizing agent
[Fe(CM)s+[Fe(CN)%1
In this reaction [Fe(CN)6]" is oxidized by [Fe(CN)61and [Fe(CN),1* is reduced by

Fe(CN)61
The electron transfer reactions involving transition metal complex are believed to occur by the
following two mechanisms
() Outer Sphere Mechanism e" oviy
(1)Inner Sphere Mechanism and mwe Aaa
0) Outer Sphere Mechanism
In this type of reactions, bothcomplexes participating in the reactiorn undergo substitution reactions
more slowly than the rate of electron transfer. The oxidant and the reductant come as close to each other
as possible and the coordination
spheres stay intact and transfer of an electron takes place from reductant
to oxidant. Thus, an outer
sphere mechanism involves electron transfer from reductant to oxidant when
the intact coordination spheres are in contact at their outer
edges i.e., the distance between two metals is
min mum
An outer sphere electron transfer may occur in the
following elementary steps
In the first step the oxidant and reductant come closer
an
fom a precursor complex
Ox + Red Ox | | Red
Precursor Complex
In the second step, there is activation of the precursór complex which includes
solvent molecules and changes in M-L bond lengths which occurs before electron reorganization of the
electron transfer takes place.
transfer. Then,

Ox || Red Ox | Red
In the final step, the ion pair is dissociated into
products
Ox || Red* Ox+Red*
Salilent Features of Outer Sphere Mechanism
(1) Both the reactants (.e., oxidant and reductant) should be kinetically inert.
However, ifong of the
complexes is labile, then the inert complex shoüd not possess a donor átom which can be used t
form the bridge with a5le compl x.
For example : Reduction
of[Fe(CN)61*by [Cr (H;0)615*
Ifboththe oxidantand thereducta.itarelabile and there is a possibility of electrontransferfrom n
ofreductant to the n of oxidant, then the reaction proceedsthrough outer sphere mechanism.
Forexample : [Fe(H20)61* +[Fe(li20)61**> [Fe(H,0)61* +[Fe(H;0)613*
(2) When both the reactants are inert with respect to ligand exchange e.g., [Fe(CN)6]*and
[Fe (CN)6]",aclose approack ofthe metal ion_ is impossible andtheclectrontransfer takes place
byatunneling orouterspheremechanism. Therateofelectrontransfer depends uponthe ability of
electronsto tunnel throughtheligands,
(3) The rate. constants of electron transfer by outer sphere mechanism are found to vary over a ivide
range as shown in the Table 7.5 and 7.6.
According to Freank-Condon principie,the electronic transition occur much more rapidl than
rearengement of atoms so that bond distances donotchange duringvery short timeof electronic
transition.
The electron transfer is very fastwhenboththe complexes arelow spinincludingthat theelect:on
transtertakes place from t2g (t") of reductant to thei2g (T)of oxidant. The first reason is that
energylevels ofthesetwo12g orbitals aresame. The 12g orbitalsare notshielded fromtheligands
and the electron transfer from and to is easier andho inputenergy1Srequired.Thesecond reason is
thatthere is no appreciable change in M-L bond lengthdue to n T electron transfer.
The rates ofelectrontransfer aremuchfaster betweenthe complexeswhichhave1-acceptorligamds
(like CN",phen, bpy etc) than for complexes ofthe same metal having purely o-donor ligands (like
H20, NH3,en etc). The a¢ceptor ligands have vacant orbitals that can accept electron beng
transferred, then pass them on to the receiving metal ion (i.e., Oxidant) whereas the o-donor ligands
do not have such tendency. Thus, outer sphere electron transfer is direct electron transfer from one
metal to another in case of complexes having -donor ligands. On the other hand, electron transíer is
indirect (i.e., from one metal to ligands to the another metal) if the complexes have 7-acceptor
ligands.
The electron transfer is slowifelectron transfer occurs from e, (or o ) orbital of reductant to e
(or a)ofthe oxidant because the e, orbitals are shielded from the surroundings (ie., ligands) and
the transfer of electron is sterically slow. Theizg /2 electron transferisfaster than eg
transfer.
A Electrontransferbetweeighspin)andlowspinkomplexgia alsslowkecauseelectontransferof
1akesplace from e, or a(called OMO)of the reductant to vacante or o (called LUMO)
theoxidant resultinginlargechanges in M=Lband length. This conditions arises between high spin
Co (d')and lowspin Co" (a")complexes.
A Outer sphere electron transfer reactions between
compiexes of different me:al cations i.e., cross
reactions are faster than that of self exchange reactions.
(5) Electron transfer occurr.ig between the
complexes of same metal cations are called self exchange
reactions. In self exchange reactions no net
chemical reaction actually occurs because the
are
indistinguishable from the reactant. Various
the other hand, the electron transfer self exchange reactions are shown in Tableproducts
7.5. On
occurring
called cross reactions. In cross reactions net between the complexes of different metal
cations are
chemical reaction occurs.
(nb) Mn(CN)1 +[M n(CN)6 [Min(CN)s1- +[Mn(CN)61
self exchange reaction)
(Fe(CN)61+ [Mo(CN)s1 [Fe(CN)6] [Mb(CN)s1*
i n outer sphere mechanis1ns bonds are neither broken (cross reaction)
nor made. For example
(Fe(CH)61 +[Fe(CN)6* [Fe(CN)s1 +[Fe(CN),1*
In outer sphere mechanism, the (spin
forbidden reaction are(slow)because these reactions require
more activation energy.
The outer sphere mechenism is
illustrated below by taking various
examples
(A) Fe(CN)61
Low spin
(Fe(CN)61 [Fe(CN)6]*+[Fe(CN)61*
Low spin

Inert Inert
Fe2 CN Fe3-CN
Bond length =
195 pm Bond
The rate of electron
length =
192 pm
transfer is fast for the following reasons
() Both the complexes
[Fe(CN)6] and [Fe(CN) 61*are inert and electron transfer
than cyanide
exchange for either reactant. is faster
Gi Electron transfer is spin allowed.
(ii) The electron transfer occurs from of [Fe(CN) 61* to
/zg t2g of [Fe(CN) 6]* complex. The
2g orbitals point between the ligands
and are not engaged in Fe CN
electron transfer becomes, easy. g-bonding. Therefore,
-

(iv) Since /2g orbitals point

between the ligands, therefore, M-L distance do not
appreciably. change
(v) The ligand CN is unsaturated and a
since the CN ligand is a ne-acceptor which facilitates the electron tunneling. AISo
1-acceptor it, therefore, stabilizes complexes in lower
by formation of back »bonding. oxidation state
(11) Inner Sphere Mechanism
n inner sphere electron trarsfer reaçtions, the Qxidant and reductant share a ligand in their
coordination sphere to form a bridged con1plex, the electron is then transferred through the bric ging
ligand.

Salient Features of Inner Sphere Mechanism

(1) One complex (the reductant) is labile and the other (i.e., the oxidant) is inert.
(2) The inert complex possesses atleast one ligand capable of bridging two metal ions to foria the
bridged intermediate and this bridged intermediate is called a precursor complex.

(3) Often, but not always, the bridging ligand isalsotransferred from oxidant toreductant. The
transferornon-transfer ofbridgingligand depends upon the relative stabilies of the product.
The ligand transfer is a good indicator that elecron transfer takes place by innersphere
mechanism. If thereis no bridging ligand, thenelectron transfer does not takeplace byinner
sphere mechanism. However it may täke place by outer sphere mechanism. Ifabridgingligand
(attachedto inert complex) is available but iot transfer, thenthe electron transfermay occur
either by an innr or outer sphere mechanism.

(4) Either eg (o*) or /2g (r*) orbitals of both the reactants participate in electron transfer by inner
sphere mecbanism. In general these orbitals may be HOMO of the reductant and LÜMo of the
oxidant. If both the reactants in an electron transfer reaction involve orbitals ofsame symmetry,
no or a little activation energy is required and electron transfer will be fast. If both the orbitals
are of different symmetries, greater activation energy. encompassing both structural
deformation and electron configuration change is required. Such reaction will be slower than
those requiring no electron configuration change. Electron transfer by inner sphere mechanism
is faster when electron transfer takes plàcebetween e (oro) orbitals ofoxidant and reductant.
15) Inner sphere electron transfer reactions are faster than similar reaction occurring through outer
sphere mechanism.
(6) The rate of electron transfer increases ifthe bridging ligand prossess unsaturation or extended
conjugation.
 WOECERONTRANSFER
The simultancous transfer of two or more electrons by an outer sphere mechanism has not been
established because it would involve yet
equalizing bond lengths species of
in
bonding electrons than the other and requires high1 activation energy. Thewhich
one contains two or
more
two electron transfer has
been observed to take place by
inner sphere mechanism.
One of the best two electron
transfer reactions, for example, is the [Pt(en) 2 1*" catalyzed exchange
of radioactive CI for chloride bound to trans-[Pt(en) 2Cl2]*.
Pt(en)2 2
trans-[Pt(en)2Cl,}2*+ "C trans-fPt(en)a' CICi2* +Cr
The rate law for this reaction is:

Rate =k[Pt"j[P:"i['ari
where Pt and Ptstand for [Pt(en)21* and (Pt(en)2 Cl21**
respectively. The mechanism
proposed involves rapid addition of radioactive chloride
( Ci) to the [Pt(en)2 ]f* to form a five
coordinate [Pt(en)2 CI]* which then forms a six
coordinate, inner sphere bridged binuclear complex
with [Pt(en)2 Cl21*. The transfer of
two a electrons accompanied by the transfer of a
chloride in the opposite direction between the bridging
Pt(I) and Pe(IV) complexes
readily occurs followed by
breaking of the bridged complex.
(Pt(en)+r P(en),cI*
CI CI 2+
/N N en
N N en
en en

N N N
CI
Ci 73+
3+
en N n Electron N N
en
N transfer en

CI
CI
en( /
N
en en NuNAen
N N
C
2+ CI
N IVN N
en
N
Pt en PUN en
CI

trans-Pt(en),CIcPlen), +Cr