REACTIVITY OF METAL COMPLEXES

Lability &Inertness Of Complexes In order to understand the Thermodynamic Terms: Stable &Unstable and the Kinetic Terms: Labile & Inert, consider the following cyano complexes [Ni(CN)4]-2, [Mn(CN)6]-3, [Cr(CN)6]-3 i)All these complexes are extremely stable from thermodynamic point of view but kinetically, these are quite different. ii) if the rate of exchange of radio carbon labelled cyanide is measured,it is found that despite the thermodynamic stability the following observations are made A) [Ni(CN)4]-2 complex exchanges cyanide ligands very rapidly(t1/2=30 seconds) and is labile [Ni(CN)4]-2 +4 14 CN-(labelled)-------- [Ni(14CN)4]-2+ 4CN- (t1/2=30s ) B) [Mn(CN)6]-3,complex exchanges cyanide ligands moderately (t1/2= 1hour) and is moderately labile.

[Mn(CN)6]-3 + 6 14 CN-(labelled)-------- [Mn(14CN)6]-3 + 6CN- (t1/2= 1 hr ) iii) [Cr(CN)6]-3 complex exchanges cyanide ligands extremely slowly(t1/2=24 days) and is considered to be inert. [Cr(CN)6]-3 + 6 14 CN- (labelled)-------- [Cr(14CN)6]-+ 6CN- (t1/2= 24 days ). Rule: Taube proposed that at 25oC, in 0.1 M solution of complexes,if the substitution takes place in less than one minute,the complexes are designated as labile complexes.

The classic example of kinetically inert complex,which is thermodynamically unstable,is the [Co(NH3)6]+3 in acid solution.its decomposition is expected as [Co(NH3)6]+3 +6H3O+-------- [Co(H20)6]+3 +6NH4+ Acidification of a solution of [Co(NH3)6]+3 results in no immediate change and at room temperature ,several days are required for the degradation of despite of favorable thermodynamics. the [Co(NH3)6]+3 ion is unstable in acid but is stable with respect to water substitution in neutral solution. ThusThermodynamic stability is strictly a function of the change in free energy (ΔG), which is a state function, meaning that its value is determined exclusively by the difference between the initial state and the final state (or the free energy of the initial products and the final products). In other words, thermodynamic stability is totally independent of the pathway between reactants and products. Kinetic lability or kinetic inertness is dependent on the activaion energy,i.e. the difference in energy between the reactants and activated complex,a low activation energy results in a fast reaction.

Labile and inert complexes Labile Complexes: these are complexes which permit quick exchange of one or more ligands from their coordination sphere by other ligands are called as labile complexes. Inert Complexes: these are complexes which either do not permit exchange of ligands or in which exchange of ligands is slow. On the basis of the Rate Of Exchange Of Ligands exhibited by a complex,the complexes are designated as labile &inert .this tendency is known as kinetic stability

The labile and the inert nature of a metal complex depends on the following i)The electronic configuration of the central metal ion of the complex ii) The three dimensional geometry of the complex iii) The coordination number exhibited by the metal ion iv) The CFSE values of the reactant complex and the activated state complex 4- co-ordinated Cu+2complexes are labile 6-co-ordinated Rh+3complexes are inert. (labile)- [Cu(H2O)4]+2 + 4NH3---- [Cu(NH3)4]+2+ 4H2O Inert )-[RhCl6]-3+C2O42- [Rh(C2O4)3]-3 +6Cl-