MIXED-VALENCE CYANO-BRIDGED COMPLEXES CONTAINING [M-II(CN)(6)](4-) (M=FE, RN OR OS) AND [RU-III(EDTA)](-) (EDTA EQUALS ETHYLENEDINITRILOTETRAACETATE) - SYNTHESIS, SPECTROSCOPIC AND KINETIC CHARACTERIZATION

The mixed-valence, cyanide-bridged complexes K-5[(NC)(5)M-II-CN-Ru-III (edta)] (M = Fe, Ru or Os; edta = ethylenedinitriloteraacetate) were p repared 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 600-1 000 nm region (dependent on M), associated with an intervalence (i.v.) transition from M-II to Ru-III, as confirmed by electrochemical measu rements 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) x 10(3) dm(3) mol(-1). These compl exes were the predominant species in excess of free hexacyanide, but t ri- and poly-nuclear species were obtained by increasing the concentra tion of [Ru(edta)(H2O)](-), as demonstrated through the electronic spe ctral 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 expos ed nitrogen ends of cyanides in the precursor binuclear complexes. The kinetics of formation and dissociation of the binuclear complexes wer e 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 x 10(-3) s(-1) respectively, at 25.0 degrees C, I=0.1 mol dm(-3)), suggesting the onset of associa tive mechanisms. The Hush model showed the three complexes to adopt a valence-trapped behaviour, with a moderate coupling between the metal centres.