Influence of solvation on the initial and transition states for anation reactions of transition-metal complexes in mixed aqueous solvents

Abstract

A free-energy cycle has been applied to the rate-determining step in mixed aqueous solvents for the anation of cations having all aquo-ligands or only one aquo-ligand together with a variety of other ligands. Combining rate data with activities of water and calculated values for the equilibrium constants for outer-sphere complexes of the incoming ligand with the cation in the mixture, the relative effects of changes of solvation in the initial and transition states can be assessed. At mole fractions of co-solvent less than those where sharp structural changes occur in the mixture, changes of solvation in either the initial or in the transition state may dominate, depending on the identity of the incoming ligand and of the complex cation; but, in general, at mole fractions of co-solvent higher than those where the sharp changes in solvent structure occur, changes in solvation in the initial state have a dominant effect on the rates.