Kinetics and mechanism of the replacement of carbonate by sulphite in the tris(carbonato)cobaltate(III) ion in solution

Abstract

Sulphite ligands become co-ordinated to the ion [Co(CO₃)₃]^3– in two distinct steps, without reducing the carbonate complex. The initial step is first order in the concentration of each reactant. The second substitution is not amenable to kinetic study because of slight precipitate formation, but filtration allowed the progress of the reaction to be monitored; this step is inferred to have an order of <1 in [SO₃^2–]. At pH <9 the rate constant for the first step is independent of pH, but increases at pH 9.2. The spectra of the two products indicate that they are the mono- and bis-(sulphito)-complexes of the carbonato-species. Sulphite and oxygen have been shown to oxidise cobalt(II) in carbonate solution to [Co(CO₃)₃]^3–.