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-methylethylenediamine) cobalt(III) ion have been measured over a wide range of solvent compositions and temperatures in water-propan-2-ol and water-acetonitrile mixtures. The rate of solvolysis is faster in the former mixtures rather than the latter. Plots of log(rate constant) versus the reciprocal of the dielectric constant of the co-solvent, and also versus the Grunwald-Winstein Y-values are non-linear for both co-solvents; this non-linearity is derived from a large differential effect of solvent structure between the initial and transition states. However, extrema in the variation of enthalpy Delta H double dagger and entropy Delta S double dagger of activation correlate well with the extrema in physical properties of the mixtures which are related to changes in solvent structure. Linear plots of Delta H double dagger versus Delta S double dagger were obtained and the isokinetic temperature indicates that the reaction is entropy controlled. The application of a free-energy cycle shows that changes in solvent structure affect the pentacoordinated cobalt(III) ion in the transition state more than the hexacoordinated cobalt(III) ion in the initial state. In addition, the stabilizing influence of changes in solvent structure is greater in propan-2-ol-water mixtures than in acetonitrile-water mixtures, and the difference becomes greater as the mole fraction, x(2) of the organic cosolvent increases.