Kinetics of the solvolysis of the cis-dichlorobis(1,2-diaminoethane)cobalt(III) ion in water and in water–t-butyl alcohol mixtures

Abstract

The cis-[Coen₂Cl₂]⁺ cation is known to solvolyse with a transition state closely corresponding to the full dissociation of a Cl^– ion leaving a pentacoordinated Co^III cation. The kinetics of this solvolysis have been investigated in water with additions of t-butyl alcohol, which, as well as changing the dielectric constant of the mixtures, produces gradual changes in the solvent structure in a water-rich mixture. Curved plots of log(rate constant)vs. the reciprocal of the dielectric constant of the water–cosolvent mixtures suggest that changes in solvent structure have a differential effect on the initial and transition states for the solvolysis. The variations of both the enthalpy and the entropy of activation with solvent composition in water–t-butyl alcohol mixtures exhibit extrema which correlate well with the extrema in physical properties of the mixture influenced by changes in solvent structure. The application of a free-energy cycle to the loss of the Cl^– ion for the solvolysis at 25 °C in the mixtures of water with a series of cosolvents shows that changes in solvent structure have a greater influence on the pentacoordinated Co^III cation in the transition state than on the hexacoordinated Co^III cation in the initial state.