Mixed-valence cyano-bridged complexes containing [M^II(CN)₆]^4- (M = Fe, Ru or Os) and [Ru^III(edta)]^- (edta = ethylenedinitrilotetraacetate): synthesis, spectroscopic and kinetic characterization

Abstract

The mixed-valence, cyanide-bridged complexes K₅[(NC)₅M^II–CN–Ru ^III(edta)] (M = Fe, Ru or Os; edta = ethylenedinitrilotetraacetate) were prepared by mixing the hexacyanide species with a solution of [Ru(edta)(H₂O)]^-. The complexes were characterized by chemical analysis, IR, Raman and VIS/NIR spectroscopy. They show an intense band in the 600–1000 nm region (dependent on M), associated with an intervalence (i.v.) transition from M^II to Ru^III, as confirmed by electrochemical measurements of the redox potentials at both metal centres. The equilibrium constants for the formation of the binuclear species were similar for the three complexes, (1.5 ± 0.1) × 10³ dm³ mol^-1. These complexes were the predominant species in excess of free hexacyanide, but tri- and poly-nuclear species were obtained by increasing the concentration of [Ru(edta)(H₂O)]^-, as demonstrated through the electronic spectral shifts of the i.v. band and the redox potentials of the M^III/II couples, which were indicative of Ru(edta) co-ordination to the exposed nitrogen ends of cyanides in the precursor binuclear complexes. The kinetics of formation and dissociation of the binuclear complexes were also measured, with k_f and k_d showing values nearly independent of M^II (ca. 10 dm³ mol^-1 s^-1 and 8 × 10^-3 s^-1 respectively, at 25.0 °C, I = 0.1 mol dm^-3), suggesting the onset of associative mechanisms. The Hush model showed the three complexes to adopt a valence-trapped behaviour, with a moderate coupling between the metal centres.

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