The kinetics of solvolysis of trans-[Co(3,4-dimethylpyridine)4Cl2]ClO4 was measured over a wide range of solvent compositions (0-60% v/v) and temperatures (40-55 °C) in water, water-ethanol and water-propan-1-ol. Plots of log(rate constant) versus the reciprocal of the relative permittivity of the co-solvent 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 ∆H# and entropy ∆S# of activation correlate well with the extrema in physical properties of the mixtures which are related to changes in solvent structures. Linear plots of ∆H# versus ∆S# were obtained and the isokinetic temperature indicates that the reaction is entropy controlled. The Gibbs free energy cycle relating the Gibbs energy of activation in water and in the binary aqueous mixtures to Gibbs energies of transfer of individual ionic species between water and the mixtures was applied.
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