Separation of the effect of solvent structure on the kinetics of substitution reactions into contributions to the initial and transition states using free energies of transfer. Kinetics of the solvolysis of 1,2-chlorothiocyanatobis(1,2-diaminoethane)-cobalt(III) ions in water and water + propan-2-ol mixtures

Abstract

Rates of the first-order solvolysis of 1,2-chlorothiocyanatobis(1,2-diaminoethane)cobalt(III) ions have been measured for a range of temperatures in water and in water + propan-2-ol. A plot of log (rate constant) against the Y factor is linear, but a plot of log (rate constant) against the reciprocal of the dielectric constant is curved. From a comparison of the variation of the enthalpy and entropy of activation with the physical properties of the solvent mixture it is concluded that solvent structure is an important factor determining solvolytic reactivity. The evidence currently available concerning the nature of the transition state in substitution reactions and, in particular, the solvolytic transition states of Co^III complexes is assessed. The influence of values for the free energy of transfer of ions on the assignment of the contributions of solvent structure to the initial and transition states for substitution reactions is discussed. As a result of the application of a free-energy cycle relating the process initial state → transition state in water to that in the mixture, it is concluded that the influence of solvent structure predominates on the pentacoordinated cation in the transition state over its influence on the hexacoordinated cation in the initial state.