KINETICS OF THE SOLVOLYSIS OF TRANS-DICHLOROTETRA-(4-TERT-BUTYLPYRIDINE)-COBALT(III) IONS IN WATER + TERT-BUTYL ALCOHOL MIXTURES

Rates of solvolysis of the complex cation [Co(4tBupy)4Cl2]+ have been determined in mixtures of water with the hydrophobic solvent, t-butyl alcohol. The solvent composition at which the extremum is found in the variation of the enthalpy ΔH* and the entropy ΔS* of activation correlates well with the extremum in the variation of the relative partial molar volume of t-butyl alcohol in the mixture and the straight line found for the variation of ΔH* with ΔS* is coincident with the same plot for water + 2-propanol mixtures. A free energy cycle is applied to the process initial state (Cn+) going to the transition state [M(n+1)+...Cl-] in water and in the mixture using free energies of transfer of the individual ionic species, ΔGto(i), from water into the mixture. Values for ΔGto(i) are derived from the solvent sorting method and from the TATB/TPTB method: using data from either method, changes in solvent structure on going from water into the mixture are found to stabilize the cation in the transition state, M(n+1)+, more than in the initial state, Cn+. This is compared with the application of the free energy cycle to the solvolysis of complexes [Co(Rpy)4Cl2]+ and [Coen2LCl]+ in mixtures of water with methanol, 2-propanol or t-butyl alcohol: the above conclusion regarding the relative stabilization of the cations holds for all these complexes in their solvolyses in water+alcohol mixtures using values of ΔGto(Cl-) from either source. © 1990 Plenum Publishing Corporation.