The Cr2+and V2+reduction of µ-carboxylato dicobalt(III) ammine complexes. Part VIII. The mechanism of reduction of the di-µ-hydroxo-µ-oxalato-bis[triamminecobalt(III)] complex

Abstract

The kinetics of the Cr²⁺and V²⁺reductions of the dicobalt(III) complex [(NH₃)₃Co·µ(OH,OH,C₂O₄)·Co(NH₃)₃]²⁺ have been studied, I= 1.0M(LiClO₄). Reduction of the first cobalt(III) is rate determining and the only observed step. With Cr²⁺the products are equivalent amounts of [Cr(H₂O)₆]³⁺and [Cr(H₂O)₄C₂O₄]+. It is concluded that reduction of the first cobalt (III) occurs with Cr²⁺attack at the remote end of the oxalate. The constant for protonation of the µ-oxalato-ligand has been determined by spectrophotometric method and at 25 °C is 19.9 l mol^–1. Rate constants (l mol^–1S^–1), enthalpies (kcal mol^–1) and entropies of activation (cal K^–1mol^–1) are for the protonated complex 4.7.9.0, –25.1, and for the unprotonated complex 1.31 × 10², 6.3, –27.9 respectively at 25 °C. The second stage differs from that observed in the Cr²⁺reduction of the [(NH₃)₄CO·µ(NH_2′C₂O₄)·CO(NH₃)₄]³⁺ complex which yields a binuclear chromium(III) product in a slower second stage (consecutive treatment required). Rate constants (l mol^–1s^–1) at 25 °C for the V²⁺reduction of [(NH₃)₃Co·µ(OH,OH,C₂O₄)·Co(NH₃)₃]²⁺ are 1.55 for the protonated and 27.8 for the unprotonated complex.