Kinetics of formation, dissociation, and isomerization reactions in complexes of cyanopyridines and benzonitrile with pentacyanoruthenate(II)

Abstract

The complexes of pentacyanoruthenate(II) with 3- and 4-cyanopyridines and benzonitrile have been prepared in aqueous solution by mixing [Ru(CN)5(H2O)]3– with the appropriate ligand. With the cyanopyridines, the reaction proceeds in two stages: the first corresponds to the formation of a mixture of pyridine- and nitrile-bonded isomers, and the second to the linkage isomerization of the unstable (nitrile-bonded) isomer to the stable one (pyridine-bonded). The complexes exhibit metal-to- ligand and intra-ligand charge-transfer absorptions. The rate constants for the formation, kf, and dissociation, kd, of all the complexes have been determined (25 °C), as well as the isomerization rate constant, kiso, for the cyanopyridine complexes. Activation parameters were obtained for the formation reaction of the mixture of isomers, as well as for the dissociation reactions of the stable, pyridine-bonded complexes. The results for the formation and dissociation reactions are analysed in terms of dissociative mechanisms. From kf and kd, values of Kst, the stability constants of the different complexes, have been calculated. The discussion is supported by previous data obtained for related pentacyanoferrate(II) and penta-ammineruthenium(II) complexes. Thus, the influence of the metal centre, M, as well as that of the auxiliary ligands, X, on the spectral and substitution reactivity properties of MX5L systems is described.