MIXED-VALENCE CYANOPYRIDINE-BRIDGED COMPLEXES OF PENTACYANOFERRATE AND PENTAAMMINERUTHENIUM - ELECTRONIC-STRUCTURE, STABILITY, AND REDOX REACTIVITY

The mixed-valence binuclear complexes, [(NC)(5)Fe(pyCN)Ru(NH3)(5)](n) (4- and 3-cyanopyridine isomers, with nitrile-N and pyridine-N binding to Ru and Fe, respectively) were prepared as solid compounds through stoichiometric oxidation of the fully reduced (II,II) binuclear comple xes, R, with peroxydisulfate. By analysis of IR spectra, the solids we re observed to be a mixture of the predominant electronic isomers with a Fe-II, Ru-III distribution, with minor amounts of the Fe-III, Ru-II isomers. In aqueous solution, R was oxidized with peroxydisulfate to M, the mixed-valence complex, and to Ox, the fully oxidized complex. T he M complex shows an intervalence band at 938 nm; by application of t he Hush model, it is described as a valence-trapped Ru-II, Fe-III comp lex: the latter electronic distribution is supported by UV-visible, el ectrochemical, and kinetic data, but a minor amount of the isomer with a Fe-II, Ru-III distribution is also present in the equilibrium. The M complex is unstable toward dissociation and further outer-sphere rea ctions, leading to hydrolyzed products in the time scale of minutes. H ydrolysis is also the main decomposition route of the Ox complex. In t he reactions with excess peroxydisulfate, the analysis of successive s pectra allows the elucidation of the rate constants for the one-electr on processes leading to M and Ox. The rate constants for the formation and dissociation of M, as well as for the hydrolysis of Ox, were also obtained. A kinetic control is operative in the oxidation reactions, with a preferential attack of peroxydisulfate on the more reactive Ru( II) center. The role of electronic isomerization is discussed in the o verall kinetic scheme, and the rate constant values for oxidation agre e with predictions based on Marcus LFER, in accord with data published for related complexes.