SOLVENT EFFECTS ON THE KINETICS OF SOLVOLYSIS OF TRANS-DICHLOROBIS (N-METHYLETHYLENEDIAMINE)COBALT(III) ION IN WATER-PROPAN-2-OL AND WATER-ACETONITRILE MIXTURES

First order solvolysis rates of the trans-dichlorobis (N-methylethylen ediamine) cobalt(III) ion have been measured over a wide range of solv ent compositions and temperatures in water-propan-2-ol and water-aceto nitrile mixtures. The rate of solvolysis is faster in the former mixtu res rather than the latter. Plots of log(rate constant) versus the rec iprocal of the dielectric constant of the co-solvent, and also versus the Grunwald-Winstein Y-values are non-linear for both co-solvents; th is non-linearity is derived from a large differential effect of solven t structure between the initial and transition states. However, extrem a in the variation of enthalpy Delta H double dagger and entropy Delta S double dagger of activation correlate well with the extrema in phys ical properties of the mixtures which are related to changes in solven t structure. Linear plots of Delta H double dagger versus Delta S doub le dagger were obtained and the isokinetic temperature indicates that the reaction is entropy controlled. The application of a free-energy c ycle shows that changes in solvent structure affect the pentacoordinat ed cobalt(III) ion in the transition state more than the hexacoordinat ed cobalt(III) ion in the initial state. In addition, the stabilizing influence of changes in solvent structure is greater in propan-2-ol-wa ter mixtures than in acetonitrile-water mixtures, and the difference b ecomes greater as the mole fraction, x(2) of the organic cosolvent inc reases.