This document discusses substitution reactions and their mechanisms in coordination complexes. There are two main types of substitution reactions: electrophilic and nucleophilic. Nucleophilic substitution can occur through acid hydrolysis, base hydrolysis, and ligand exchange reactions. The main mechanisms of substitution are associative, dissociative, and interchange mechanisms. Lability and inertness of complexes depends on both kinetic and thermodynamic factors. According to valence bond theory, the electronic configuration of the metal center determines if a complex is labile or inert. Crystal field theory also aims to explain lability and inertness based on crystal field stabilization energies. Square planar complexes can exhibit substitution governed by trans influence effects.

2. Substitution reactions
•Electrophilic substitution reactions
•Nucleophilic substitution reactions

3. •Nucleophilic substitution reactions
*acid hydrolysis
*base hydrolysis
*anation
*ligand exchange etc…

4. Mechanisms of substitution:
• Associative mechanism(A)
• Dissociative mechanism(D)
• Interchange mechanism(I)
* Interchange Associative(Ia)
* Interchange Dissociative(Id)

5. slow +Y(fast)
[L5MX] [L5M] [L5MY]
5-coordinate complex
slow X -X(fast)
[L5MX]+Y [L5M ] [L5MY] + X
Y
7-coordinate complex
slow X -X(fast)
[L5MX]+Y [L5M ] [L5MY] + X
Y
7-coordinate transition state
Associative :
Dissociative :
Interchange :

6. Note :
There will be some cases in which it is difficult to find which
mechanism is followed to give the product i.e., in cases when there is
* solvent interaction
*ion pair interaction etc…

7. STOP THE DRIP TO
SAVE THE DROP
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9. DEFINITIONS
The ability of a complex to engage in reaction that results in
replacing one or more ligands in it’s coordination sphere
(by other ligands) is called lability and the complexes 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 labile complexes.

10. > H.TAUBE has described the complexes as labile if they have
half life(t1/2) of reaction under 30 sec while the reactions having
half life greater than 30 sec are termed as inert.
t1/2 < 30 sec LABILE complex
t1/2 >= 30 sec INERT complex

11. The terms inert and labile are kinetic terms because they reflect the rate with
which the reaction proceeds and these kinetic terms should not be confused
With thermodynamic terms stable and unstable which refer to tendency of
species to exist(governed by equilibrium constants.)
Consider the reaction: M + nL ⇌ MLn ; βn =[MLn]/[M][L]n
where βn is formation constant of the complex.
The higher values of βn indicate it’s higher thermodynamic stability of the
complex. Thus it gives measure of the extent to which the reaction proceeds
but it cannot say anything about the speed with which equilibrium is attained.

12. Examples:
* [Hg(CN)4]-2 + 4 14CN- ⇌ [Hg14(CN)4]-2 + 4CN-
log βn =42 ; t1/2= very small
*[Cr(CN)6]-3 + 6 14CN- ⇌ [Cr14(CN)6]-3 + 6 CN-
log βn =37 ; t1/2= 24 days
Thermodynamically
Stable and kinetically
Unstable
Thermodynamically
Stable and kinetically
stable

13. * The inertness or lability depends upon the activation energy i.e., high
Activation energy imparts inertness while low activation energy
imparts lability.Thus inertness or lability is determined by ∆G‡
(free energy of activation).
∆G‡ = ∆H‡ - T ∆S‡
The stability of a complex is determined by free energy change(∆G0)in a
reaction.
∆G0 = ∆H0- T ∆S0
• ∆G‡ depends on reaction pathway while ∆G0 depends upon the difference
in free energy of reactants and products.

14. VBT interpretation of lability and
inertness in octahedral complexes :
• Outer and inner orbital complexes and their stability
• Kinetic behavior of outer orbital complexes
• Kinetic behavior of inner orbital complexes

15. d0,d1,d2,d7,d8,d9 highly labile
d3,d4 less labile than d0,d1,d2
d5,d6 highly inert
For outer orbital complexes:

16. [V(NH3)6]+3:
[MnCl6]-3:
[Co(CN)6]-3:
(n-1)d
(n-1)d
(n-1)d
ns
ns
ns
np
np
np nd
d2sp3 hybridisation
vacancy for entering nucleophile

17. Limitations of VBT interpretation

18. CFT interpretation of lability and
inertness in octahedral complexes :
CFAE = loss of cfse in forming the activated complex
= cfse of the starting complex – cfse of the activated complex
cfse of activated complex ???

21. Generalisations:
• Both the high spin and low spin complexes of d0,d1,d2,d7,d9,d10 and
high spin complexes of d4,d5,d6 are generally labile.
• Both high spin and low spin complexes of d3,d8 and low spin
complexes of d4,d5,d6 are generally inert.

22. Limitations of CFT interpretation
.

23. INTERESTING MECHANISM

24. Substitution in square planar complexes is
governed by TRANS EFFECT
TRANS EFFECT???

25. 4 days
TRANS EFFECT
BY

26. References:
CSIR MATERIAL-
INORGANIC
CHEMISTRY

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