Kinetics and mechanisms of the oxidation of the octacyanoniobate(III)ion by oxyanions in alkaline aqueous media

Summary The kinetics and mechanisms of the oxidation of Nb(CN)_inf8^sup5–by the oxyanions S₂O_inf8^sup2–, BrO_inf3^sup–, and IO_inf4^sup–have been investigated in alkaline aqueous media (pH 12). The second-order rate constant for the electron transfer reaction between Nb(CN)_inf8^sup5–and S₂O_inf8^sup2–at 25.0 °C, I = 0.36m (K⁺), is 11.1± 0.3 m^–1s^–1 with H = 30 ± 2kJmol^–1 and S = - 125 + 7JK^–1 mol^–1. The rate constant for the oxidation of Nb(CN)_inf8^sup5–by BrO_inf3^sup–at 25.0 °C, I = 0.20m (Na⁺), is 2.39 ± 0.08m^–1s^–1 with H = 28 ± 2kJmol^–1 and S = -139 ± 7JK^–1mol^–1. The oxidation of Nb(CN)_inf8^sup5–by IO_inf4^sup–proceeds by two parallel pathways involving the monomeric IO_inf4^sup–ion and the hydrated dimer H₂I₂O_inf10^sup4–. The second-order rate constant for the oxidation of Nb(CN)_inf8^sup5–by monomeric IO_inf4^sup–at 5.0 °C, I = 0.050m (Na⁺), is (3.3 ± 0.6) × 10³m^–1s^–1 with H = 75 ± 6 kJ mol^–1 and S = 94 ± 15 J K^–1 mol^–1, while the rate constant for the oxidation by H₂I₂O_inf10^sup4–is (1.8 ± 0.1) × 10³m^–1s^–1 with H = 97 ± 5 kJ mol^–1 and S = 166 ± 16 J K^–1 mol^–1 under the same reaction conditions. The rate constants for each of the oxidants employed display specific cation catalysis with the order of increasing rate constants: Li⁺ < Na+ < NH_inf4^sup+< K⁺ < Rb⁺ < Cs⁺, in the same direction as the electronic polarizability of the cations. The results are discussed in terms of the outer-sphere electron-transfer processes and compared with the corresponding data and mechanisms reported for other metal-cyano reductants.