The Cr2+ and V2+ reductions of µ-carboxylato-dicobalt(III) ammine complexes. Part IV. The mechanism of reduction of the di-µ-hydroxo-µ-phthalato-bis[triamminecobalt(III)] and penta-amminephthalatocobalt(III) complexes

Abstract

The kinetics of the Cr²⁺ and V²⁺ reductions of di-µ-hydroxo-µ-phthalato-bis[triamminecobalt(III)] complex, [(NH₃)₃Co·µ(OH,OH,O₂CC₆H₄CO₂H)·Co(NH₃)₃]³⁺, have been studied at temperatures in the range 9.7–40 °C, I= 1.0M(LiClO₄). Comparisons are made with the Cr²⁺ reduction of penta-amminephthalatocobalt(III), [Co(NH₃)₅(O₂CC₆H₄CO₂H)]²⁺. Second-order rate constants for the latter exhibit an [H⁺]-dependence, a+b[H⁺]^–1, where it has now been demonstrated using ion-exchange techniques that a yields the unidentate product [Cr(H₂O)₅(O₂CC₆H₄CO₂H)]²⁺, and b the chelated product [Cr(H₂O)₄(O₂CC₆H₄CO₂)]⁺. With the binuclear µ-phthalato-complex reduction of the first cobalt(III) is rate determining, and that of the second relatively rapid. Second-order rate constants conform to the [H⁺]-dependence, k_a+k_b[H⁺]^–1, where in this instance k_a corresponds to an outer-sphere process, and k_b involves some form of remote attack. Evidence for the latter is that phthalate is incorporated into a binuclear chromium(III) product. Full product analyses have been carried out. With an excess of Cr²⁺ a second stage involving Cr²⁺-catalysed dissociation of primary chromium(III) products becomes increasingly important at the lower [H⁺] values.