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

Abstract

The mixed-valence, cyanide-bridged complexes K ₅ [(NC) ₅ M ^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)(H ₂ O)] ^- . 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) × 10 ³ dm ³ 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)(H ₂ O)] ^- , 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 dm ³ 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.