NITROSYL-NITRITE INTERCONVERSION IN PENTACYANORUTHENATE(II) COMPLEXES

The nucleophilic addition of OH- to [Ru(CN)5(NO)]2- leads to the N-bonded pentacyanonitroruthenate(II) ion (lambda-max = 320 nm, epsilon = 3850 dm3 mol-1 cm-1). The stoichiometry is 2:1 ([OH-]:[Ru]) and the equilibrium formation constant is 4.4 x 10(6) dm6 mol-2 (25-degrees-C, I = 1 mol dm-3). A kinetic study of the forward reaction showed that it is first order in the concentration of each reactant, with k = 0.95 dm3 mol-1 s-1 (25-degrees-C, I = 1 mol dm-3), DELTA-H double-ended dagger = 57.3 +/- 3.3 kJ mol-1 and DELTA-S double-ended dagger = -54.0 +/- 4.5 J K-1 mol-1. The mechanism involves two consecutive attacks by OH-, the first being rate determining. The reaction product decays by an aquation process, leading to [Ru(CN)5(H2O)]3- and free NO2-. The rate constant for the dissociation reaction of [Ru(CN)5(NO2)]4- is k-N = 2.00 x 10(-4) s-1 (25-degrees-C, I = 1 mol dm-3). In the formation reaction, both nitrite (O-bound) and nitro (N-bound) linkage isomers are formed, with k(o) and k(N) being 0.23 and 0.15 dm3 mol-1 s-1 respectively (25-degrees-C, I = 1 mol dm-3). The O-bound isomer isomerizes slowly to the thermodynamically more stable N-bound isomer. The kinetic and thermodynamic parameters have been analysed by comparison with the chemistry of the complexes [Fe(CN)5(NO)]2- and [Fe(CN)5(NO2)]4-.