Kinetics of the solvolysis of [Co(3Rpy)4Cl2]+ (R = Me or Et) in water + propan‐2‐ol mixtures

Abstract

AbstractRates of solvolysis of complex ions [Co(3Rpy)4Cl2]+ with R = Me and Et have been determined in mixtures of water with the hydrophobic co‐solvent propan‐;2‐ol for a range of temperatures. The variations of the enthalpies and entropies of activation with the solvent composition have been correlated with the physical properties of the solvent and it has been shown that the linear plot found for ΔH* against ΔS* in water + propan‐2‐ol is coincident with that found for water + methanol mixtures. A free energy cycle has been applied to the dissociative‐type process of initial state (Cn+) going to the transition state (M(n+1)+…‥ Cl−) using values for the free energy of transfer of the chloride ion from water into the mixture, ΔG(Cl−), derived from the solvent sorting technique or from the TATB/TPTB method. Irrespective of the source of ΔG(Cl−), it is found that ΔG(Co(3Rpy)4Cl2+*) ca. ΔG(Co(3Rpy)4Cl2+) (where * indicates a species in the transition state) for mol fractions of propan‐2‐ol x2 ≳ 0.04–0.05 with ΔG(Co(3Rpy)4Cl2+*) becoming increasingly more negative than ΔG(Co(3Rpy)4Cl2+) as x2 increases above 0.05. These variations are compared with those for the solvolysis of the same ions in water + methanol mixtures and of a range of other ions in water + propan‐2‐ol mixtures. It is concluded that, using values for ΔG(Cl−) from either source, the stabilizing influence of changes in solvent structure is greater on the cation in the transition state than on the cation in the initial state for the solvolyses of complexes [Co(Rpy)4Cl2+] and [Coen2LCl]n+.