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

Abstract

The kinetics of the solvolysis of the cation cis-[Co(en)₂SCNCl]⁺ have been investigated in water–t-butyl alcohol mixtures (en = 1,2-diaminoethane). The linear variation of log (rate constant) with the reciprocal of the dielectric constant expected when solvent structure has little influence on the kinetics is not found, and the enthalpy and entropy of activation both show extrema in the same regions of solvent composition where the physical properties of the mixtures influenced by changes in solvent structure also show extrema. By applying a free-energy cycle to the solvolysis in water and in the mixture, it is concluded that the effect of changes in solvent structure have a greater influence on the pentacoordinated cobalt(III) cation in the transition state than on the hexacoordinated cation in the initial state. By using data for the free energy of transfer of [Co(en)₂SCNCl]⁺ between water and the solvent mixtures, free energies of transfer for [Co(en)₂SCN]²⁺ have been calculated from the free-energy cycle. These results are compared with those for the solvolysis of the same ion in water–propan-2-ol mixtures and for the solvolysis of a range of coordinated cobalt(III) ions in mixtures of water with a range of alcohols.