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, k_f, and dissociation, k_d, of all the complexes have been determined (25 °C), as well as the isomerization rate constant, k_iso, 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 k_f and k_d, values of K_st, 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.