Mixed-Valence Cyano-Bridged Complexes Containing MII(CN)6 4- (M = Fe, Ru, Os) and RuIII(edta) - : Synthesis, Spectroscopic and Kinetic Characterizaion

P. FORLANO; F. D. CUKIERNIK; O. POIZAT; J. A. OLABE

JOURNAL OF THE CHEMICAL SOCIETY, DALTON TRANSACTIONS

Royal Society of Chemistry

Año: 1997 p. 1595 - 1599

0300-9246

The mixed-valence, cyanide-bridged complexes K5[(NC)5MII–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 MII to RuIII, 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 MIII/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 kf and kd showing values nearly independent of MII (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.