Salt effects upon the S2O82? + Ru(NH3)5pz2+ electron transfer reaction

Binding of Ru(NH3)5pz2+ to 4-sulfocalix[4]arene sodium salt. Effects of the host-guest interaction on electron transfer processes

The reactions [Ru(NH3)5pz]2++S2O8(2-) and [Ru(NH3)5pz]2++[Co(C2O4)3]3- have been studied in solutions of 4-sulfocalix[4]arene sodium salt. Results show a binding of the ruthenium complex to the calixarene with a 2:1 stoichiometry; that is, a ruthenium molecule binds to two calixarene molecules. This stoichiometry changes when NaCl is added to the medium. Thus, a mixture of 1:1 and 2:1 adducts is found in the presence of 0.1 mol dm(-3) NaCl and only 1:1 adducts when the salt concentration is increased up to 0.3 mol dm(-3). Results show that the binding of the ruthenium complex to the calixarene is due to electrostatic and nonelectrostatic interactions. Kinetic data are interpreted by using the pseudophase model and taking into account the stoichiometry of the ruthenium binding to calixarene. The presence of a supporting electrolyte in the medium produces ion pair formation which exerts an influence on the kinetic rate constants.

Salt and Solvent Effects on the Kinetics of the Oxidation of the Excited State of the [Ru(bpy)3]2+ Complex by S2O82-

The title reaction was studied in different reaction media: aqueous salt solutions (NaNO3) and water-cosolvent (methanol) mixtures. The observed rate constants, k(obs), show normal behavior in the solutions containing the electrolyte, that is, a negative salt effect. However, the solvent effect is abnormal, because a decrease of the rate constant is observed when the dielectric constant of the reaction medium decreases. These effects (the normal and the abnormal) can be explained using the Marcus-Hush treatment for electron transfer reactions. To apply this treatment, the true, unimolecular, electron-transfer rate constants, k(et), have been obtained from k(obs) after calculation of the rate constants corresponding to the formation of the encounter complex from the separate reactants, k(D), and the dissociation of this complex, k(-D). This calculation has been carried out using an exponential mean spherical approach (EMSA).