Reaction Mechanism of Transition Metal Complexe

Inert and Labile Complexes :- 

The metal complexes in which the rate of ligand displacement reactions is very fast and hence show high reactivity are called as labile Complexes and this property is termed as lability.On the other hand, the metal complexes in which the rate of ligand displacement reaction is very slow and hence show less reactivity are called as inert complexes and this property is termed as inertness.

Labile and Inert Complexes on the Basis of Valence Bond Theory :- 

According to the valence bond theory of chemical bonding, octahedral metal-complexes can be divided into two types.Outer orbital complexes: These complexes have sp3d2 hybridization and are generally labile in nature. Valence bond theory proposed that the bonds in sp3d 2 hybridization are generally weaker than that of (n-1)d 2 sp3 orbitals and therefore they show labile character.

For example, octahedral complexes of Mn2+, Fe2+, Cr2+ complexes show fast ligand displacement.

Inner orbital complexes: Since d2sp3 hybrid orbitals are filled with six electron pairs donated by the ligands, dn electron of metal will occupy dxy, dyz and dxz orbitals.These d 2 sp3 hybrid orbitals can form both inert or labile complexes.

In order to show lability, one orbital out of dxy, dyz, dxz must be empty so that it can accept another electron pair and can form seven coordinated intermediate which is necessary step for the associative pathway of ligand displacement.

On the other hand, if all the dxy, dyz, dxz orbitals contain at least one electron, it will not be able to accept electron pair from the incoming ligand and hence is expected to show inert character.

Labile and Inert Complexes on the Basis of Crystal Field Theory :- 

Octahedral complexes react either by SN1 or SN2 mechanism in which the intermediate 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 crystal field stabilization (CFSE) value also changes.

The cases for lability and inertness are:

Labile complexes-

If the CFSE value for the five or seven-membered intermediate complex is greater than that of the reactant, the complex will be of labile nature as there is zero activation energy barrier.

Inert complexes-

If the CFSE value for the five or seven-membered intermediate complex is less than that of the reactant, the metal complex will be of inert nature as loss of CFSE will become the activation energy barrier.

Hence, the gain of crystal field stabilization energy will make complex labile while the loss of CFSE will make complex inert.

Factors Affecting the Kinetic Stability or Lability of Non Transition Metal Complexes :- 

Charge on the central metal ion: 

The Lability of a complex decreases with the increasing charge on the central metal ion. For example, lability order of the following complexes greatly depends upon the radius of the metal center in a complex.

As the radius of metal ion decreases, the lability of its complex decreases.This can be attributed to smaller metal–ligand bond, which in turn, results in stronger attraction between the metal and ligands involved.In other words, the lability of the complex is proportional to the radius of the cation

Charge to ionic size ratio:- 

It has been observed that for a series of octahedral complexes having the same ligandligands, the lability of the complexes decreases with the increase of charge to ionic size ratio.

For instance, consider the replacement of H2O16 by H2O18 or simply H2O*

The order of lability is:

This trend can be rationalized in terms of the highest charge to ionic-size ratio for hexaaquo complex of trivalent aluminum (6.0), lowest ionic-size ratio for hexaaquo complex of trivalent sodium (1.05). Similarly,

[AlF6]3- > [SiF6]2- > [PF6]

Geometry of the complex:

Four-coordinated complexes, tetrahedral as well as square planar, reacts more rapidly than that of six-coordinated complexes.This can be explained in terms of lesser steric repulsion and the availability of moresites to the incoming ligand.