Kinetics of the solvolysis of trans-dichlorotetra(4-methylpyridine)-cobalt(III) ions in water + t-butyl alcohol mixtures

Abstract

The solvolysis of a cobalt complex presenting a largely hydrocarbon outer surface to the solvent, trans-[Co(4Me-py)₄Cl₂]⁺(4Me-py = 4-methylpyridine), has been investigated with solvents consisting of mixtures of water with the co-solvent Bu^tOH, which possesses a high structure-inducing capacity in water. Extrema found in the variation of the enthalpy and entropy of activation with solvent composition correlate very well with extrema in the variation of the physical properties of the mixture which relate to sharp changes in the solvent structure. Using a free-energy cycle and solubility measurements on salts of trans-[Co(4Me-py)₄Cl₂]⁺, values for the free energy of transfer of the cation in the transition state, ΔG_t^⊖[Co(4Me-py)₄Cl²⁺], have been obtained. These are compared with the free energies of transfer of this complex in the initial and transition states in water + PrⁱOH and for other cobalt complexes in the initial and transition states in mixtures of water with various alcohols.