Mechanism of Metal Ion Catalysis in the dissociation of cis-diaquobisoxalatochromate(III) ion

The metal ion (M²⁺) catalysed dissociation of cis-diaquobisoxalatochromate into the tetraaquomonooxalato complex in aqueous perchloric acid medium which follows the rate law — d(complex)/dt = {k_H[H⁺] + k_M[M²⁺]}[complex] has been studied. Based on k_M values the order of catalysing effect of the different metal ions studied is Cu²⁺ > Ni²⁺ > Co²⁺ > Mn²⁺, which is also the order of stabilities (K_MOx) of the monooxalato complexes of these metal ions; in fact the plot of log k_M vs. log K_MOx is linear. This together with the relative values of ΔH^≠ and ΔS^≠ for the H⁺ catalysed and M²⁺ catalysed paths is in agreement with a mechanism involving chelation of the catalysing cation through the free carbonyl oxygens of the oxalate ligand bound to Cr(III), followed by the dissociation of the Cr(III)? O bonds with simultaneous entry of two water molecules into the coordination sphere of Cr(III).