Kinetic and oxygen-18 tracer studies on the reaction of sulphite with the superoxo-complex (NH3)5CO·O2·CO(NH3)55+ in aqueous media

Abstract

The stoicheiometry and products of the reaction between the superoxo-complex, (NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺, and sodium sulphite in aqueous perchloric acid media are as in (i) with the total sulphite in large (>fifty-fold) excess (NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺+ 2SO₃^2–→ Co(NH₃)₅SO₄⁺+ Co²⁺+ SO₄^2–+ 5NH₃(i) With smaller sulphite concentrations there is only an 80% yield of the sulphatopenta-amminecobalt(III) complex, the discrepancy being accounted for by the formation of aquopenta-amminecobalt(III) and sulphate. Pseudo-first-order and second-order plots for the disappearance of complex d[(NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺]/dt=k_obs[(NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺][Sulphur(IV)](ii) are linear to at least 65% completion, the curvature in the latter stages being accounted for by loss of SO₂. From the hydrogen-ion dependence of k_obs it can be concluded that the reaction is between SO₃^2– and (NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺(k₁= 6·2 × 10⁴ l. mole^–1 sec.^–1 at 25° and µ 0·5M), or alternatively that reaction occurs within the ion-pair complex (NH₃)₅Co·O₂·Co(NH₃)₅⁵⁺,SO₃^2–(in which case k₁K_IP^SO₃= 6·2 × 10⁴ l. mole^–1sec.^–1) and that HSO₃^– and H₂SO₃ make negligible contribution to the reaction.

The extent of transfer of oxygen atoms from the oxidizing agent to the reducing agent has been studied using ¹⁸O as a tracer. In the subsequent analysis it was possible to distinguish between transfer of oxygen to (a) the complexed and (b) the free sulphate. The values obtained indicate significant retention of the bridging oxygen atoms in both sulphate fractions, there being greater retention in the complexed fraction. Possible mechanisms are considered.