Effect of solvent on the reactions of coordination complexes. Part 2.—Kinetics of solvolysis of cis-(chloro)(imidazole)bis(ethylenediamine)-cobalt(III) and cis-(chloro)(benzimidazole)bis(ethylenediamine)cobalt(III) in methanol–water and ethylene glycol–water media

Abstract

The kinetics of solvolysis of cis-(chloro)(imidazole)bis(ethylenediamine)-cobalt(III) and cis-(chloro)(benzimidazole)bis(ethylenediamine)cobalt(III) have been investigated in aqueous methanol (MeOH) and aqueous ethylene glycol (EG) media (0–80% by weight of MeOH or EG) at 45–64.7 °C. The logarithm of the pseudo-first-order rate constants for MeOH–water media exhibits linear dependence with the reciprocal of the bulk dielectric constant (D^–1_s), the mole fraction of MeOH (X_MeOH) and the solvent ionizing power Y(Y_1-AdCl) as determined by the solvolysis rates of 1-adamantyl chloride. Similar plots (log k^s_obsvs.x_EG or D^–1_s) for EG–water media are non-linear. It is evident that the solvation phenomenon plays dominant role and the rate of solvolysis is mediated by the dual solvent vectors, the overall acidity and basicity of the solvent mixtures. The relative transfer free-energy calculations indicate that the mixed solvent media exert more destabilizing effect on the transition state as compared to the initial state. The activation enthalpy and entropy vs.X_org(where X_org is the mole fraction of the organic solvent component) plots display maxima and minima indicating that the solvent structural changes play significant role in the activation process. The activation free energy at a given temperature, however, increases only marginally and linearly with increasing X_org. The mutual compensatory effect of activation enthalpy and entropy on the activation free energy is in keeping with the fact that the perturbations of the reaction zone and the solvent network remain approximately proportional to each other with increasing X_org so that the isodelphic and the lyodelphic components of ΔH^± and ΔS^± correlate well with each other.