Kinetics and mechanism of the reduction of the molybdatopentaamminecobalt(III) ion by aqueous sulfite and aqueous potassium hexacyanoferrate(II)

A detailed investigation on the oxidation of aqueous sulfite and aqueous potassium hexacyanoferrate(II) by the title complex ion has been carried out using the stopped-flow technique over the ranges, 0.01≤[S(IV)]T≤0.05 mol dm−3, 4.47≤pH≤5.12, and 24.9≤θ≤37.6 °C and at ionic strength 1.0 mol dm−3 (NaNO3) for aqueous sulfite and 0.01≤[Fe(CN)6 4−]≤0.11 mol dm−3, 4.54≤pH≤5.63, and 25.0≤θ≤35.3 °C and at ionic strength 1.0 or 3.0 mol dm−3 (NaNO3) for the hexacyanoferrate(II) ion. Both redox processes are dependent on pH and reductant concentration in a complex manner, that is, for the reaction with aqueous sulfite, kobs={(k1K1K2K3+k2K1K4[H+])[S(IV)]T]/([H+]2+K1[H+]+K1K2) and for the hexacyanoferrate(II) ion, kobs={(k1K3K4K5+k2K3K6[H+])[Fe(CN)6 4−]T)/([H+]2+K3[H+]+K3K4). At 25.0 °C, the value of k1′ (the composite of k1K3) is 0.77±0.07 mol−1 dm3 s−1, while the value of k2′ (the composite of k2K4) is (3.78±0.17)×10−2 mol−1 dm3 s−1 for aqueous sulfite. For the hexacyanoferrate(II) ion, k1′ (the composite of k1K5) is 1.13±0.01 mol−1 dm3 s−1, while the value of k2′ (the composite of k2K6) is 2.36±0.05 mol−1 dm3 s−1 at 25.0 °C. In both cases there was reduction of the cobalt(III) centre to cobalt(II), but there was no reduction of the molybdenum(VI) centre. k22, the self-exchange rate constant, for aqueous sulfite (as SO3 2−) was calculated to be 5.37×10−12 mol−1 dm3 s−1, while for Fe(CN)6 4−, it was calculated to be 1.10×109 mol−1 dm3 s−1 from the Marcus equations.