Base hydrolysis of the oxalatobis(ethylenediamine)cobalt(III) ion

Abstract

The kinetics and the steric course of the replacement by hydroxide, of the co-ordinated oxalate in the [Co(en)₂(C₂O₄)]⁺ ion (where en is ethylenediamine) have been studied in solutions of hydroxide up to 4·0M. The reaction consists of two stages : the first, which produces the intermediate [Co(en)₂(C₂O₄)OH], is second order. The subsequent loss of oxalate is also second order at low base concentrations and at temperatures near 70°. The product of the reaction is the racemic cis-[Co(en)₂(OH)₂]⁺ ion. Under conditions where the hydrolysis is slower than the isomerisation of the dihydroxo-product, the equilibrium mixture of the cis- and trans-[Co(en)₂(OH)₂]⁺ cations is observed. At high base concentrations and low temperatures the reaction becomes greater than first order in hydroxide concentration, and under these conditions a 1 : 1 mixture of the cis- and trans-dihydroxo-cations is observed even when the cis-isomer is expected. The change in steric course in concentrated base is discussed in terms of the five-co-ordinate intermediates in an S_N1 CB mechanism, where the change in the medium alters either (a) the distribution of the various intermediates or (b) the position of the group entering the intermediates.