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Mixed-Valence Cyanopyridine-Bridged Complexes of Pentacyanoferrate and Pentaammineruthenium:  Electronic Structure, Stability, and Redox Reactivity

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journal contribution
posted on 18.12.1996, 00:00 by Alejandra E. Almaraz, Luis A. Gentil, Luis M. Baraldo, José A. Olabe
The mixed-valence binuclear complexes, [(NC)5Fe(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 complexes, R, with peroxydisulfate. By analysis of IR spectra, the solids were observed to be a mixture of the predominant electronic isomers with a FeII, RuIII distribution, with minor amounts of the FeIII, RuII isomers. In aqueous solution, R was oxidized with peroxydisulfate to M, the mixed-valence complex, and to Ox, the fully oxidized complex. The M complex shows an intervalence band at 938 nm; by application of the Hush model, it is described as a valence-trapped RuII, FeIII complex; the latter electronic distribution is supported by UV−visible, electrochemical, and kinetic data, but a minor amount of the isomer with a FeII, RuIII distribution is also present in the equilibrium. The M complex is unstable toward dissociation and further outer-sphere reactions, leading to hydrolyzed products in the time scale of minutes. Hydrolysis is also the main decomposition route of the Ox complex. In the reactions with excess peroxydisulfate, the analysis of successive spectra allows the elucidation of the rate constants for the one-electron 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 overall kinetic scheme, and the rate constant values for oxidation agree with predictions based on Marcus LFER, in accord with data published for related complexes.