Kinetics of the solvolysis of the trans-dichlorotetrapyridinecobalt(III) ion in water + dimethyl sulphoxide mixtures

Abstract

The kinetics of the solvolysis of 1,6-[Copy₄Cl₂]⁺ have been followed over a range of temperatures in mixtures of water with DMSO up to a mole fraction of DMSO(x₂)≈ 0.2. The logarithm of the first-order rate constant varies approximately linearly with the reciprocal of the dielectric constant at the same temperature, contrary to findings for the solvolysis of this complex in mixtures of water with alcohols. Extrema in the enthalpy, ΔH^*, and entropy, ΔS^*, of activation for solvolysis of the complex in the latter mixtures occur at low x₂ of the cosolvent and correlate well with the extrema found in the relative partial molar volume, V₂–V°₂, of the cosolvent. With additions of DMSO to water, sharp changes in V₂–V°₂ do not occur until x₂≈ 0.2, and extrema for ΔH^* and ΔS^* also do not occur until x₂≈ 0.2 is reached. The application of a free-energy cycle by combining free energies of activation, ΔG^*, with free energies of transfer, ΔG°₂, shows that the effect of solvent structure is greater on the pentacoordinated cation in the transition state than on the hexacoordinated cation in the initial state.