Kinetics of solvolysis of the trans- dichlorotetra(4-methylpyridine)cobalt(III) ion in water and in water–propan-2-ol mixtures

Abstract

The solvolysis of trans-[Co(4-Mepy)₄Cl₂]⁺ followed spectrophotometrically in water and in water–propan-2-ol is always first order in the complex, showing no isosbestic points. The logarithm of the rate constant shows no linear correlation with the reciprocal of the dielectric constant or with the Grunwald–Winstein Y factor. However, extrema in the variation of the enthalpy and entropy of activation correlate well with the extrema in the physical properties of the mixture which are related to changes in solvent structure. Correlation of the free energies of activation in the mixture with that in water using Gibbs free energies of transfer of individual ionic species suggests that the effect of changes in solvent structure on the complex ion in the transition state dominates over that on the complex ion in the initial state. These findings are compared with similar results for the solvolysis of other Co^III complexes in water–propan-2-ol and in mixtures of water with other co-solvents.