KINETICS OF THE SOLVOLYSIS OF CHLOROPENTA-AMMINECOBALT(III) IONS IN MIXTURES OF WATER WITH T-BUTYL ALCOHOL OR WITH 2-METHOXYETHANOL

As the transition state for the solvolysis of [Co(NH3)5Cl]2+ ions is k nown to have Cl- ions in a situation. closely similar to that in the b ulk solvent, the kinetics of this solvolysis have been investigated fo r comparison in H2O with added cosolvents of low and high hydrophobici ties. A linear variation of log(rate constant) with the reciprocal of the dielectric constant is found with the former, but not with the lat ter cosolvent. Maxima in the enthalpies and entropies of activation fo und using the more hydrophobic cosolvent appear at solvent composition s where extrema occur in the physical properties influenced by structu ral changes in the solvent. The application of a free energy cycle to the solvolysis in H2O and in the mixtures shows that the emergent solv ated cobalt(III) ion in the transition state is more stabilised in the latter than [Co(NH3)5Cl]2+ with both cosolvents. The application of s uch a cycle to cases where the initial state is destabilised in the mi xture is discussed.