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 (λ_max= 320 nm, ε= 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 k_o and k_N 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–.