Metal ion-sulfur(IV) chemistry. 2. Kinetic studies of the redox chemistry of copper(II)-sulfur(IV) complexes

The redox chemistry of Cu(II)-S(IV) complexes is examined, and the rate of reduction of Cu(II) is determined. The reduction of Cu(II) is shown to proceed via Cu^(II)_2SO_3^(2+) and Cu^(II)SO_3OH^+ intermediates. Copper(I), SO_4^(2-), and a mixed-valence compound, Cu^(II)SO_3Cu^l_2SO_3.2H_2O, are determined to be the principal products. The rate law is consistent with consecutive first-order reactions. Results are interpreted in terms of the initial formation of an inner-sphere complex, which is followed by a rate-limiting electron-transfer step. These results are discussed in light of previously accepted mechanisms for the trace metal catalysis of the autoxidation of SO_3^(2-).