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 CoIII 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 CoIII cation in the transition state than on the hexacoordinated CoIII cation in the initial state.