Rates of solvolysis of the 1,2-chloroazido-bis(ethylenediamine)cobalt(III) ion have been measured in water and in water + isopropanol in the composition range mole fraction of isopropanol 0–0.2. Although log (rate constant) varies linearly with the reciprocal of the dielectric constant, comparison with other co-solvents shows that the slope of these plots varies with the co-solvent used. The influence of solvent structure on the reaction is shown by comparing the variation with solvent composition of the transition-state parameters for the solvolysis with the physical properties of the solvent mixture. The application of a free-energy cycle to the loss of the Cl– in the transition state shows that changes in solvent structure have a much greater effect on the transition state than on the initial state.
