The Cr2+ and V2+ reduction of µ-carboxylato-dicobalt(III) ammine complexes. Part V. The mechanism of reduction of µ-maleato- and µ-fumarato-complexes

Abstract

The Cr²⁺ and V²⁺ reductions of binuclear cobalt(III) complexes [(NH₃)₄Co·µ(NH₂, maleato)·Co(NH₃)₄]⁴⁺, [(NH₃)₃Co·µ(OH,OH, maleato)·Co(NH₃)₃]³⁺, and [(NH₃)₃Co·µ(OH,OH,fumarato)·Co(NH₃)₃]³⁺, referred to as µ(am), µ(hhm), and µ(hhf) respectively, proceed by a slow/fast reaction sequence. Kinetic data have been obtained for the first stages with Cr²⁺ as reductant and at 25 °C, I= 1.0M(LiClO₄), second-order rate constants (l mol^–1 s^–1), enthalpies of activation (kcal mol^–1), and entropies of activation (cal K^–1 mol^–1), are respectively for µ(am), 0.59, 5.5, and –41.3; µ(hhm), 1.87, 4.3, and –43.0; and µ(hhf), 2.64, 2.8, and –47.1. Rate constants are independent of [H⁺] in the range 0.02–1.00M. Product analyses are consistent with inner-sphere mechanisms involving remote attack. With V²⁺ as reductant kinetic data for the first stage are respectively for µ(am), 1.46, 9.8, and –24.9; µ(hhm), 0.164, 5.8, and –42.3; and µ(hhf), 0.099, 75. and –38.0. No dependence on [H⁺] was observed except in the V²⁺ reduction of µ(hhm), when significant contributions were obtained from an [H⁺]^–1 term with kinetic parameters, 0.45 (s^–1), 10.6, and –29.3.