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, k_obs={(k₁K₁K₂K₃+k₂K₁K₄H⁺])S(IV)]_T]/(H⁺]²+K₁H⁺]+K₁K₂) and for the hexacyanoferrate(II) ion, k_obs={(k₁K₃K₄K₅+k₂K₃K₆H⁺])Fe(CN)₆ ⁴⁻]_T)/(H⁺]²+K₃H⁺]+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.