Kinetics and mechanism of the interaction of N donor ligands with iodomethyl (aquo) cobaloxime

Abstract

The equilibrium constants, rate constants and activation parameters for axial ligation of alkyl(aquo)cobaloximes by N donor ligands in the presence of 0.2 <i>M </i>buffer (to maintain <i>p</i>H)

and 1.0 <i>M </i>KCl (to maintain ionic strength) have been determined spectrophotometrically at 442 nm. The rates and activation parameters (Δ<i>H</i>^≠ and Δ<i>S</i>^≠)<i> </i>for the reactions have been estimated from the effect of concentration of ligand and temperature on the kinetics of these reactions. From the results of experimentally determined rates and activation parameters of cobaloximes, it can be concluded that the reactivity of metal complexes is greatly influenced by the ligand field effect of the co-ordinated groups and the presence of pi acceptor ligand, which provides a low energy pathway for electron transfer from a pi donor ligand anion

leading to rate enhancement. The small negative Δ<i>S</i>^≠and low Δ<i>H</i>^≠values suggest the operation of a dissociative interchange (Id)<i> </i>substitution mechanism at the Co(III) center.