Successfully reported this slideshow.
Activate your 14 day free trial to unlock unlimited reading.
Complexes in which exchange of one or
more ligands are rapidly exchanged are
called labile complexes.
If the rate of ligand exchange is slow then
the complex is said to be inert.
Lability is not related to the thermodynamic
stability of a complex.
A stable complex may be labile or inert , so
as the unstable complex .
is labile. Its aqueous
solution is blue in color.
When concentrated hydrochloric acid is
added to this solution, the blue solution
immediately turns green ,giving [CuCl4]2-
But when the complex is kept as such it
remains as such with out any decomposition
reacts slowly. When this complex
is treated with concentrated HCl, no reaction
takes place. Only when it is heated with 6M
HCl for many hours, one NH3 is substituted by
+ HCl [Co(NH3)5Cl]2+
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.
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
If electrons are present in the antibonding eg
the complex will be labile -the ligands will be weakly
bonded to the metal and hence can be substituted
Complexes with empty t2g orbitals, will be labile
because ligands can approach easily without much
In short, if the complex contains less than three d-
electrons, it will be labile. Or, if one or more eg
electrons are present, it will be labile
CFT assumes the splitting of d orbitals of metal.
Filling of e-
s in them results in different CFSE.
CFAE – Crystal Field Activation Energy
CFAE = CFSE of intermediate – CFSE of Reactant
Since the geometries of the reactant and intermediate
are different their splitting and CFSE are also different.
If the calculated CFAE is negative or zero or low
the reacting complex will require less energy to
form the intermediate, hence it will be labile.
If CFAE is a high positive value then the complex
will be inert.
It must be borne in mind that CFAE is only a part
of actual AE and other factors are also operative.
The geometry of the complex is assumed to
be Oh even if all the ligands are not identical.
The inter electronic repulsions are neglected.
The Dq values of the reactant and
intermediate are assumed to be same.
The Jahn-Teller effect is not affecting CFSE.
Because of the drastic assumptions made,
some of the CFAE values are –ive.
However when calculated with proper
attention to all effects, CFAE is always +ive.
CFAE can be small or zero but never –ive
By oversimplified approach the –ive values of
CFAE may be taken as zero.
Substitution of ligands
Reactions of coordinated ligands
Electron transfer reactions
Photo chemical reactions
• Ligand displacements are nucleophilic
• Rate is governed by ligand
The rate of attack on a complex by a
given ligand relative to the rate of attack
by a reference base.
Three types of ligands are present
– Entering Ligand: Y
– Leaving Ligand: X
– Spectator Ligand
• Species that neither enters nor leaves
• Particularly important when located in a Trans
position, designated T
Dissociative: One of the ligands
dissociates from the reactant, to form a
reaction intermediate with lower
coordination number than reactants
• Octahedral complexes and smaller metal centers
• Rates depend on leaving group
Associative: reaction intermediate is
formed by including the incoming
ligand in the coordination sphere
and has higher coordination
number than reactants or products
• Lower coordination number
• Rates depend on the entering group
It is a continuous single step
Two types exist
Interchange associative (IA ) –
Bond making more important
Interchange dissociative (ID) –
Bond breaking more important
Ammine complexes of Co(III) are the most
Water is the medium of reaction.
Usually replacement of NH3 derivatives is
very slow, so only other ligands are
+ H2O [Co(NH3)5(H2O)]3+
Rate = k. [Co(NH3)5X]2+
Rate = k’. [Co(NH3)5X]2+
Charge on the complex
Effect of leaving group
Effect of solvent
Presence of pi-donors and acceptors as
The increase in positive charge decreases the rate
of reaction following a dissociative mechanism
because the breaking the metal-ligand bond
For aquation of the Ru complexes the trend is as
2.1 x 10-6
Complex Rate constant S-1
Thus it is proved that M-X bond
breaking is very much important
in aquation reactions than bond
The rate of aquation of [Co(NH3)5X]2+
depends on the
stability of M-X bond.
If the M-X bond is more stable rate of reaction is
The order of reactivity is
This is the order of decreasing thermodynamic
stability of the complexes formed with these groups
Anation reactions do not depend very much on the
nature of the entering group, Y-
Instead, it is very much dependent on the nature of the
bond being broken.
Experimental data show that the rate is of the order 10-6
for the different entering groups (Y-
clearly indicating that the rate is independent of the
nature of the entering group
Another important experimental support for
this observation is that ligand exchange
reactions do not take place directly but
instead takes place through aquation and
This indicates that the Co-X bond breaking is very much
significant and then whatever species is present at a higher
concentration will add in anation reaction. Thus, nature of Y-
When the non-leaving ligands are bulky, they will be
crowding the central metal ion.
The incoming ligand will find it difficult to approach
the central metal ion slowing down the rate of
reaction taking place by associative mechanism.
Instead, if the reaction takes place by dissociative
mechanism, the rate of the reaction will increase
because the crowding around the metal ion is
• Steric crowding around the metal centre favors
• Dissociative activation relieves crowding around
• Steric crowding has been qualitatively and
– Tolman Cone Angle
Complex k x 104
Complex k x 104
Very fast [Co(NH3)5Cl]2+
AA k x 103
H2N-CH2-CH2-NH2 (en) 1.9
H2N-CH2-CH(CH3)-NH2 (pn) 3.7
H2N-CH(CH3)-CH(CH3)-NH2 (dl - bn) 8.8
H2N-CH(CH3)-CH(CH3)-NH2 (m – bn) 250