Unusual Electron Transfer Rate Effects due to Pyridyl Ring Substituents and Trace Group VIII b Hexacyano Complexes as Added Salts

The rates of pseudo-self-exchange electron transfer reactions between [(NH3)5RuIIL]2+ and [(NH3)5RuIIIL’]3+ at reactant concentrations of 1.0 х 10-4 M (where L, L’ = substituted pyridines) and the reaction between [(NH3)5RuIIPy]2+ and [(NH3)5RuIII3FPy]2+ at different reactant concentrations in the presence of various salts added were studied by using the stopped-flow kinetic spectroscopy. Marcus theory rate vs. driving force plots yielded distinct families of lines depending on whether a 3- or 4- phenylpyridine ligand was present, and reactions of 4-phenylpyridine were in all cases the fastest. Both temperature dependent studies (stopped-flow and dynamic NMR measurements) indicate that the origin of the phenyl substituent effect is in the enthalpic portion of the free-energy of activation. One explanation is that the reorganizational barrier λ might be significantly lower for the 3-Phpy and 4-Phpy complexes, or it could be that energetically-favorable p-p stacking interactions (known to be common in compounds bearing the phenyl rings) may be helping to enhance bimolecular precursor complex formation. Salt effect studies showed that the apparent catalytic activity of the salts muconate, terephthalate and 1,4-DCCH decreased modestly as the reactant ion concentrations were increased. In agreement with prior NMR work (Yinshin,Q.;2011) we found that the superexchange ET catalysis by trace MII(CN)6-4 varied strongly in the order Ru \u3c Os \u3c F