THE KINETICS OF THE SOLVOLYSIS OF CHLOROPENTACYANOCOBALTATE(III) IONSIN WATER CONTAINING 2-METHOXYETHANOL OR DIETHYLENE GLYCOL - A CONTRAST WITH THE EFFECT OF MORE HYDROPHOBIC COSOLVENTS

The kinetics of the solvolysis of [Co(CN)5Cl]3- have been investigated in water + 2-methoxyethanol and water + diethylene glycol mixtures. A lthough the addition of these linear hydrophilic cosolvent molecules t o water produces curvature in the variation of log(rate constant) with the reciprocal of the dielectric constant, their effect on the enthal py and entropy of activation is minimal, unlike the effect of hydropho bic cosolvents. The application of a Gibbs energy cycle to the solvoly sis in water and in the mixtures using either solvent-sorting or TATB values for the Gibbs energy of transfer of the chloride ion between wa ter and the mixture shows that the relative stability of the emergent solvated Co(III) ion in the transition state compared to that of Co(CN )5Cl3- in the initial state increases with increasing content of cosol vent in the mixture. By comparing the effects of other cosolvents on t he solvolysis, this differential increase in the relative stabilities of the two species increases with the degree of hydrophobicity of the cosolvent.