Kinetics of the solvolysis of chloropenta-amminecobalt(III) ions in mixtures of water and glucose

Abstract

The kinetics of the solvolysis of [Co(NH₃)₅Cl]²⁺ ions have been investigated in mixtures of water with hydrophilic glucose to make a comparison with the kinetics in mixtures of water with hydrophobic co-solvents. Broad compensating extrema in the enthalpy and entropy of activation are found. Consideration of volumes of activation shows that the Co³⁺⋯ Cl^– bond is sufficiently extended in the transition state for the Cl^– ion there to be regarded as identical to a Cl^– ion in the bulk solvent. This allows the application of a free-energy cycle to the process initial state going to a transition state for this solvolysis in water and in water–co-solvent mixtures using free energies of transfer for ions, ΔG^°_t(i), from water into the mixture. It is concluded that ΔG^°_t[Co(NH₃)^3+*₅]≈ΔG^°_t[Co(NH₃)₅Cl²⁺] in water–glucose mixtures, with the former becoming more negative than the latter as the hydrophobicity of the co-solvent molecules increases.