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) ^sup5−_inf8 by the oxyanions S₂O ^sup2−_inf8 , BrO ^sup−_inf3 , and IO ^sup−_inf4 have been investigated in alkaline aqueous media (pH 12). The second-order rate constant for the electron transfer reaction between Nb(CN) ^sup5−_inf8 and S₂O ^sup2−_inf8 at 25.0 °C, I = 0.36m (K⁺), is 11.1± 0.3 m ⁻¹ s ⁻¹ with ΔH ^‡ = 30 ± 2kJmol⁻¹ and ΔS ^‡ = - 125 + 7JK⁻¹ mol⁻¹. The rate constant for the oxidation of Nb(CN) ^sup5−_inf8 by BrO ^sup−_inf3 at 25.0 °C, I = 0.20m (Na⁺), is 2.39 ± 0.08m ⁻¹ s ⁻¹ with ΔH^‡ = 28 ± 2kJmol⁻¹ and ΔS ^‡ = -139 ± 7JK⁻¹mol⁻¹. The oxidation of Nb(CN) ^sup5−_inf8 by IO ^sup−_inf4 proceeds by two parallel pathways involving the monomeric IO ^sup−_inf4 ion and the hydrated dimer H₂I₂O ^sup4−_inf10 . The second-order rate constant for the oxidation of Nb(CN) ^sup5−_inf8 by monomeric IO ^sup−_inf4 at 5.0 °C, I = 0.050m (Na⁺), is (3.3 ± 0.6) × 10³ m ⁻¹ s ⁻¹ with ΔH ^‡ = 75 ± 6 kJ mol⁻¹ and ΔS ^‡ = 94 ± 15 J K⁻¹ mol⁻¹, while the rate constant for the oxidation by H₂I₂O ^sup4−_inf10 is (1.8 ± 0.1) × 10³ m ⁻¹ s ⁻¹ with ΔH ^‡ = 97 ± 5 kJ mol⁻¹ and ΔS ^‡ = 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 ^sup+_inf4 < 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.