Solvent dependence of kinetics and equilibria of thallium(I) cryptates in relation to the free energies of solvation of thallium(I)

Abstract

Stability constants and dissociation rate constants of thallium (I) cryptates have been measured in several solvents at 25 °C. The Tl+ cryptates are more stable and less sensitive to ligand cavity size than the corresponding complexes of the alkali-metal cations. The stability constants vary strongly with solvent, and the solvent dependence of the complex stabilities appears to reflect mainly changes in the solvation of Tl+. It is shown that free energies of transfer of the solvated Tl+ among non–aqueous solvents calculated on the assumption that the difference in the free energies of transfer of the Tl+ cryptates and the corresponding cryptands is zero are in good agreement with literature data. Changes in the stability constants with solvent and ligand are reflected in changes in both dissociation and formation rate constants, but more so in the former. Thus the solvation of the transition state, (Tl+⋯Cry)[graphic omitted], is rather closer to that of the reactants, and includes additional solvent interactions compared with the stable cryptate complex, TlCry+.