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.