Sulfite ion substitution of trans-dichlorobis(ethylenediamine)cobalt(III):: a classic autocatalytic reaction

The trans-[Co(en)(2)Cl-2](+) complex yields cis-Co(en)(2)(SO3)Cl cleanly and rapidly via substitution by sulfite ion in water in a rare example of a direct anion substitution process. The kinetics are reproducible and reveal an autocatalytic process involving a slow initial reduction by sulfite followed by a rapid inner sphere e(-) transfer process involving the catalyst Co-II(en)(2)(SO3) There is also some triggering of the autocatalysis by a trace Co-II impurity in the starting material. Rate constants for several of the steps in the detail mechanism have been determined, and the linear dependence of these on [sulfite] seem to require both bridging chloride and bridging sulfite in the reactive species. This account also accommodates the previously observed anomaly of sulfite's preferentially displacement of the poorer leaver group Y from the ions cis-[Co(en)(2)ClY](+) where Y = NCS-, N-3(-) or NO2-. (C) 2003 Elsevier Science B.V. All rights reserved.