Kinetic studies of solute-solvent interactions in the solvolysis of trans-[Co(4-tert-butylpyridine)4Cl2)ClO4 in organic-aqueous mixtures

The kinetics of solvolysis of trans-[Co(L)(4)Cl-2]ClO4 where (L = 4-tert-butylpyridine) were followed spectrophotometrically in water-dioxane and water dimethylsulfoxide media (0-60% v/v) over the temperature range (40-55 degrees C). Non-linear plots were found for the logarithm of the rate constant of the first-order reaction versus the reciprocal of the relative permittivity, epsilon(r), of the mixed solvent. This behaviour can be attributed to the differential solvation of the initial and transition states of the complex. The enthalpies and entropies of activation showed compensating extrema with the mole fraction of solvent, leading to a small variation in the free energy of activation. By applying a free-energy cycle, it was found that the difference between the values of the free energy of transfer of the cations in the transition and initial states were negative, indicating that the cation in the transition state is more solvated than that in the initial state.