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. 0.01less than or equal to [S(IV)](1) less than or equal to 0.05 mol dm (3). 4.47 less than or equal to pH less than or equal to 5.12, and 24.9 less than or equal to theta less than or equal to 37.6 degreesC and at ionic strength 1.0 mol dm (3) (NaNO3) for aqueous sulfite and 0.01 less than or equal to [Fe(CN)(6)(4-)] less than or equal to 0.11 mol dm (-3), 4.54 less than or equal to pH less than or equal to 5.63, and 25.0 less than or equal to theta 35,3 degreesC 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) = {(k(1)K(1)K(2)K(3) + k(2)K(1)jK(4)[H+])[S(IV)(r)]/([H+](2) + K-1[H+] + K1K2) and for the hexacyanoferrate(II) ion, k(obs) = {(k(1)K(3)K(4)K(5) + k(2)K(3)K(6)[H+])[Fe(CN)(6)(4)-](T))/([H+](2) + K-3[H+]+K3K4). At 25.0 degreesC. the value of k(1)' (the composite of k(1)K(3)) is 0.77 + 0.07 mol (1) dm(3) s (-1), while the value of V, (the composite of k,K,) is (3.78 + 0.17) x 10(2) mol (1) dm(3) s (i) for aqueous sulfite. For the hexacyanoferrate(II) ion k, (the composite of k,K,) is 1.13+/-0.01 mol (-1) dm(3) s(-1). while the value of k'(2) (the composite of k(2)K(6),) is 2.36+/-0.05 mol(-1) dm(3) s(-1) at 25.0 degreesC. In both cases there was reduction of the cobalt(III) centre to cobalt(II), but there was no reduction of the molybdenum(VI) centre. k(22) the self-exchange rate constant, for aqueous sulfite (as SO32-) was calculated to be 5.37 x 10 (12) mol (1) dm(3) s(-1), while for Fe(CN)(6)(4), it was calculated to be 1.10 x 10(9) mot (1) dm(3) s(-1) from the Marcus equations. (C) 2002 Elsevier Science B.V. All rights reserved.