Kinetics of the solvolysis of a cationic and of an anionic chlorocobalt(III) complex in water–glucose mixtures: anomalous effects on the enthalpy and entropy of activation

Abstract

The kinetics of the solvolysis of the ions [Co(4-Bu^tpy)₄Cl₂]⁺ and [Co(CN)₅Cl]^3– have been investigated in a range of water-rich mixtures of water with hydrophilic glucose over a range of temperatures. The variations of the enthalpies and entropies of activation with solvent composition show extrema such as found in mixtures of water with hydrophobic co-solvents even though the extrema found in the physical properties of the latter mixtures are not present in those of the mixtures of water with glucose. The application of a Gibbs energy cycle to the process of the initial state going to the final state is compared for both complexes with the results from the same cycle applied using both hydrophilic and hydrophobic solvents mixed with water. Although there are differences between the behaviour of the two complexes with varying co-solvent, in all cases the emergent solvated cobalt ion in the transition state is more stable in the mixtures than the cobalt ion in the initial state.