Kinetics of the solvolysis of [Co(3Rpy)4Cl2]+ ions (R = Me or Et) in water and in water + methanol mixtures

Rates of solvolysis of ions [Co(3Rpy)4Cl2]+ with R = Me and Et have been measured over a range of temperatures for a series of water‐rich water + methanol mixtures to investigate the effect of changes in solvent structure on the solvolysis of complexes presenting a largely hydrophobic surface to the solvent. The variation of the enthalpies and entropies of activation with solvent composition has been determined. A free energy cycle relating the free energy of activation in water to that in water + methanol is applied using free energies of transfer of individual ionic species from water into water + methanol. Data for the free energy of transfer of chloride ions ΔG t∘(Cl−) from both the spectrophotometric solvent sorting method and the TATB method for separating ΔG t∘(salt) into ΔG t∘(i) for individual ions are used: irrespective of the source of ΔG t∘(Cl−), in general, −ΔG t∘(Co(Rpy)4Cl2+) > −ΔG t∘(Co(Rpy)4Cl2+), where Rpy = py, 4Mepy, 4Etpy, 3Etpy, and 3Mepy, showing that changes in solvent structure in water‐rich water + methanol mixtures generally stabilize the cation in the transition state more than the cation in the initial state for this type of complex ion. A similar result is found when the free energy cycle is applied to the solvolysis of the dichloro (2,2′,2″‐triaminotriethylamine)cobalt(III) ion. The introduction of a Me or Et group on the pyridine ring in [Co(Rpy)4Cl2]+ has little influence on the difference {ΔG t∘(Co(Rpy)4Cl2+)−ΔG t∘(Co(Rpy)4Cl2+)} in water + methanol with the mol fraction of methanol < 0.20.