Heterogeneous catalysis in solution. Part 16. Racemisation of (+)589-tris(ethylenediamine)cobalt(III) by a carbon black in alkaline and ethylenediamine solutions

Abstract

The rate of racemisation at 25 °C of aqueous (+)₅₈₉[Co(en)₃][ClO₄]₃ in the presence of Black Pearls 2 carbon increases sharply on the addition of sodium hydroxide. The catalytic reaction is first order in adsorbed [OH^–], and the mechanism probably involves a conjugate base leading to a five-co-ordinated intermediate. The racemisation rate is also greatly increased by adding ethylenediamine (en), mainly due to the hydroxide ions formed by its hydrolysis. In en solutions a second and even faster pathway to racemisation appears when [Co(en)₃]²⁺ ions are present. This redox pathway operates also on suitably preconditioned platinized platinum surfaces.