Kinetics of the solvolysis of trans-dichlorotetra(4-t-butylpyridine)cobalt(III) ions in water and in water–propan-2-ol mixtures

Abstract

[Co(4Bu^tpy)₄Cl₂]Cl has been prepared to investigate the solvolysis kinetics of a complex which presents a largely hydrocarbon or hydrophobic surface to the solution in water and in water-rich mixtures of water with a co-solvent, propan-2-ol, which has a hydrophobic effect in water. Non-linear plots for log k(k= first-order rate constant) against the reciprocal of the dielectric constant and the Grunwald–Winstein Y function are found. Although the free energy of activation changes little with solvent composition, the enthalpy and entropy of activation each have a compensating broad extremum at a mole fraction of propan-;2-ol x₂≈ 0.04–0.15. From the application of a free-energy cycle to the free energy of activation, it is found by comparison with data for other complexes that [ΔG^°_t(cation in the transition state)–ΔG^°_t(cation in the initial state)] is largely independent of the ‘hydrocarbon content’ of the coordination shell (ΔG^°_t= free energy of transfer water → mixture) for any particular co-solvent, but does vary with the identity of the co-solvent.