The first part includes a definition of labile and inert. lability and inertness on the basis of VB theory and CFT and also factors affecting inertness and lability of the complexes. And also the second part includes Substitution Reactions in Octahedral Complexes like mechanisms and their evidence.

1. LABILE & INERT
AND
SUBSTITUTION
REACTIONS IN
OCTAHEDRAL
COMPLEXES
Presented by:
K. Muthu Kannan

2. Contents:
1 . Labile and inert
1.1 Definition
1.2 Lability and inertness on the basis of VB theory
1.3 Lability and inertness on the basis of CFT
1.4 Factors affecting lability inertness of complexes
(i) Size of the central metal ion
(ii) Charge on the central metal ion
(iii) d-electron configuration
2 . Substitution reactions in octahedral complexes
(i) Dissociative Mechanism
(ii) Associative Mechanism
(iii) Interchange Mechanism
2.1 Evidence for dissociative Mechanism (Water exchange)

3. LABILE AND INERT:
Definition:
The ability of a complex to engage in reaction that results
in replacing one or more ligands it’s co-ordination sphere is called
lability and the complex in which the ligands are rapidly replaced by
others are called labile complexes.
The inability of a complex to engage in such reaction is
termed as inertness and the complexes which exhibit such property are
called inert complexes.

4. The complexes as labile if they have half life (t1/2) of
reaction under one minute while the reactions having half life greater
than one minute are termed as inert.
t1/2 < 1 minute [ labile complexes ]
t1/2 > 1 minute [ inert complexes ]
Ex:
[ Thermodynamically stable and kinetically unstable ]
[Cr(CN)6]-3 + 6 C*N- [Cr(C*N)6]-3 + 6 CN- [ t1/2 = 24 days ]
[ Thermodynamically stable and kinetically stable ]
[Hg(CN)4]-2 + 4 C*N- [Hg(C*N)4]-2 + 4 CN- [ t1/2 = very small ]

5. Lability and inertness on the basis of VB theory:
According to valance bond theory, if the transition metal
complexes undergoing substitution reactions through dissociation
mechanism, then all outer orbital complexes are labile and inner orbital
complexes are inert.
Outer orbital complexes (Sp3d2) - Outer d-orbitals - Labile complexes
Inner orbital complexes (d2sp3) - Inner d-orbitals - Inert complexes
(M-L bond weak)
(M-L bond strong)

6. Ex:1
[Cr(CN)6]4- - Cr2+ - d4
d4
3d 4s 4p d2sp3hybridisation
complex
3d 4s 4p
There is no empty d-orbital. It is inert complex.
↑ ↑ ↑ ↑
↑ ↓ ↑ ↑ ×
×
×
×
×
×
×
×
×
×
×
×

7. Ex: 2
[V(NH3)6]3+ - V3+ - d2
d2
3d 4s 4p d2sp3hybridisation
complex
3d 4s 4p
There is one empty d-orbital. It is also labile complex. It does not
explain by VB theory.
↑ ↑
↑ ↑ ×
×
×
×
×× ×
×
×
×
×
×

8. Lability and inertness on the basis of CFT:
According to crystal field theory, octahedral
complexes react either by SN
1 or SN
2 mechanism in which the
intermediates are five and seven coordinated species, respectively.
In both cases, the symmetry of the complex is lowered
down and due to this change in crystal field symmetry, the CFSE
value also changes. The cases for lability and inertness are;
If the CFSE value for the five and seven membered
intermediate complex is greater than that of the reactant, the complex
will be of labile nature.

9. If the CFSE value for the five or seven membered intermediate
complex is less than that of the reactant, the complex will be of inert
nature.
For isoelectronic metal cations, the inertness increases with
increase of charge on the metal. Because high charge strengthen the M-L
bond.
Ex:
Cr3+ (d3) more inert than V2+ (d3).
Crystal field
activation energy
(CFAE)
= CFSE of intermediate – CFSE of reactant

10. Factors affecting lability and inertness of complexes:
(i) Size of the central metal ion:
Smaller the size of the metal ion, greater will be the
inertness because the ligands are held tightly by the metal ion.
Ex:
[Cs(H2O)6]+ < [Rb(H2O)6]+ < [K(H2O)6]+ < [Na(H2O)6]+
(ii) Charge on the central metal ion:
Greater the charge on the metal ion, greater will be the
inertness of the complex. Since the M-L bonds are stronger.
Ex:
[AlF6]3- < [SiF6]2- < [PF6]- < [SF6]

11. (iii) d-electron configuration:
If electrons are present in the anti-bonding eg orbitals, the
complex will be labile. The ligands will be weakly bonded to the metal
and hence can be substituted easily.
Complexes with empty t2g orbitals, will be labile because
ligands can approach easily without much repulsion.
In short, if the complex contains less than three d-electrons, it
will be labile. If one or more eg electrons are present, it will be labile
and half filled or more than half filled t2g orbitals are inert.

12. d0 - t2g
0eg
0 - Labile
d1 - t2g
1eg
0 - Labile
d2 - t2g
2eg
0 - Labile
d3 - t2g
3eg
0 - Inert
d4 (high spin) - t2g
3eg
1 - Labile d4 (low spin) - t2g
4eg
0 - Inert
d5 (high spin) - t2g
3eg
2 - Labile d5 (low spin) - t2g
5eg
0 - Inert
d6 (high spin) - t2g
4eg
2 - Labile d6 (low spin) - t2g
6eg
0 - Inert
d7 - t2g
6eg
1 - Labile
d8 - t2g
6eg
2 - Labile
d9 - t2g
6eg
3 - Labile
d10 - t2g
6eg
4 - Labile

13. Note:
All the complexes of co-ordination number four are
labile.
They easily participate in substitution reaction which
involves form intermediate which involves associative mechanism.
We add more ligands to increase co-ordination number.
Complexes of 4d and 5d group metals are maximum inert.

14. Substitution reactions in octahedral
complexes:
Replacement of one ligand in co-ordination sphere
by another without changing co-ordination number and oxidation
state of metal cation.
[ML5X] + Y [ML5Y] + X
Mechanism:
Dissociative mechanism (D)
Associative mechanism (A)
Interchange mechanism (I)

15. (i) Dissociative Mechanism:
One of the ligands dissociates from the reactant, to form
a reaction intermediate with lower co-ordination number than
reactants.
s
l
o
w
-
X fast
+ Y
[ML5X]
C.N=6
[L5M]
C.N=5
[L5MY]
C.N=6
The dissociative mechanism predicts that rate of overall
substitution reaction depends on only the concentration of the
original complex [ML5X], and is independent of the concentration
of the incoming ligand [Y].

16. Rate = k1[ML5X].
It follows SN
1 mechanism. The intermediate can be
either square pyramidal (most probable) or trigonal bipyramidal.
M
L
L
L
L
L
T r i g o n a l b i p y r a m i d a l
M
L
L
L
L
L
S q u a r e
p y r a m i d a l

17. (ii) Associative Mechanism (A):
Associative of an extra ligand with the complex to
give an intermediate of higher co-ordination number: one of the
original ligands is then lost to restore the initial co-ordination
number.
[ML5X] + Y
s
l
o
w
M
Y
X
L5
C.N=7
C.N=6
fast
-X
[ML5Y]
C.N=6
Rate determining step is the collision between the original
complex ML5X and the incoming ligand Y to produce a seven co-
ordinated intermediate.

18. The second faster step is dissociation of the X ligand to
produce the desired product. The associative mechanism predicts that
the rate of reaction depends on the concentration of ML5X and Y.
It follows SN
2 mechanism. The formed intermediate can
be either mono-capped octahedron or pentagonal bipyramidal.
Rate = k1[ML5X][Y]
M
L
L
L
L
L
X Y
M o n o - c a p p e d
o c t a h e d r o n
M
L
L
L
L
L
X
Y
P e n t a g o n a l
p i p y r a m i d a l

19. (iii) Interchange Mechanism (I):
It is a continuous single step process. It takes place in
one step without forming stable intermediate. Two steps exist;
(i) Interchange associative (IA)
(Bond making is more important)
(ii) Interchange dissociative (ID)
(Bond breaking is more important)
ML5X + Y [Y-------ML5------X] [ML5Y] + X
(Activated complex)

20. It follows formation of activated complex or transition state.
As Y begins to bond X begins to leave. i.e. the bond making to Y and
bond breaking to X occur simultaneously.
The terms associative and dissociative are reversed for
situations where 7 and 5 co-ordinate intermediates have actually been
isolated and positively identified.
Evidences for dissociative (SN
1) mechanism:
1) Water Exchange:
Water molecule in the co-ordination sphere are exchanged
with isotopically labelled bulk water(H2O18).

21. [M(H2O)6]n+ + H2O18 [M(H2O)5(H2O18)]n+ + H2O
EX:
[Cr(H2O)6]2+ + 6 H2O18 [Cr(H2O18)6]2+ + 6 H2O
It depends on charge density. The water exchange
mechanism is directly proportional to charge density. Charge density
increases the metal and ligand bond strength also increases therefore
it difficult to break.
In group the size of the atom increases the charge density
will be decrease. So, the metal and ligand bond strength decreases.
Now substitution reactions happens in it.

22. Order of water exchange:
Cu2+ > Cr2+ > Zn2+ > Mn2+ > Fe2+ > Ni2+ > V2+
Ti3+ > Fe3+ > V3+ > Cr3+
Di-positive metal complexes are more water exchange
order compare to tri-positive metal complexes.
Alkali and alkaline earth metals are also very high water
exchange and Be2+, Mg2+ are exceptional cases.

23. References:
1) https://www.slideshare.netchemsantreactions-of-complexes.
2) https://www.slideshare.netsaikumardarsiniinert-and-labile-
complexes-and-substitution-reactions.
3) https://www.dalainstitute.com/books/a-textbook-of-inorganic-
chemistry-volume-1/inert-and-labile-complexes.
4) https://youtu.be/k4ilpQmGlHo.

24. THANKYOU