Nitrosyl–nitrite interconversion in pentacyanoruthenate(II) complexes

Abstract

The nucleophilic addition of OH– to [Ru(CN)5(NO)]2– leads to the N-bonded pentacyanonitroruthenate(II) ion (λmax= 320 nm, Iµ= 3850 dm3 mol–1 cm–1). The stoichiometry is 2:1 ([OH–]:[Ru]) and the equilibrium formation constant is 4.4 × 106 dm6 mol–2(25 °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 °C, I= 1 mol dm–3), ΔH‡= 57.3 ± 3.3 kJ mol–1 and ΔS‡=–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 × 10–4 s–1(25 °C, I= 1 mol dm–3). In the formation reaction, both nitrite (O-bound) and nitro (N-bound) linkage isomers are formed, with ko and kN being 0.23 and 0.15 dm3 mol–1 s–1 respectively (25 °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–.