Kinetics of the solvolysis of trans-dichlorotetra(4-methylpyridine)-cobalt(III) ions in water + t-butyl alcohol mixtures
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 ButOH, 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, ΔGt⊖[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.