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, ka+kb[H+]–1, where in this instance ka corresponds to an outer-sphere process, and kb 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.