KINETICS OF THE SOLVOLYSIS OF TRANS-DICHLOROTETRA(4-T-BUTYLPYRIDINE)-COBALT(III) IONS IN MIXTURES OF WATER WITH METHANOL AND WITH ETHANOL

For the solvolysis of Co(4-t-Bupy)4Cl2- ions in water + methanol and w ater + ethanol, log (rate constant) does not vary linearly with the re ciprocal of the dielectric constant. The Gibbs free energy, the enthal py, and the entropy of activation are insensitive to changes in the so lvent composition in these mixtures, although a slight broad maximum i n DELTAH and DELTAS* probably exists at mole fractions of about 0.2 i n water + ethanol. This contrasts with the extrema in DELTAH and DELT AS found with more hydrophobic alcohols used as cosolvents. However, the application of a Gibbs energy cycle to the solvolysis in water and in the mixtures shows that there is a differential effect of changes in solvent structure on the emergent solvated Co(III) cation in the tr ansition state and on Co(4-t-Bupy)4Cl2+ in the initial state. The stab ility of the former increases relative to that of the latter as the co solvent content of the mixture rises. (C) 1995 John Wiley & Sons, Inc.