Kinetic studies of the complexation of oxalate to the t2g6 hexaaqua ions of ruthenium(II) and rhodium(III)

Abstract

A kinetic study on the reactions of oxalate with the isoelectronic t_2g⁶ hexaaqua ions of ruthenium(II) and rhodium(III) reveals vastly differing mechanisms occurring for the two ions. For hexaaquaruthenium(II) equilibration kinetics is relevant giving a 1:1 oxalato complex, which is presumably chelated. For the forward reaction the dependence on [H⁺] indicates a rate-determining reaction of [Ru(OH₂)₆]²⁺ with the monoanion of oxalic acid. A comparison of rate constants and activation parameters for HC₂O₄^– with those obtained for H₂O, Cl^–, Br^– or I^– as incoming ligands on [Ru(OH₂)₆]²⁺ suggests the presence of an interchange reaction largely controlled by the rate of water exchange. For hexaaquarhodium(III) a two-stage process occurs ultimately giving rise to tris(oxalato) products. In the first stage, a single rate-determining anation reaction proceeds to completion giving mainly cis-[Rh(OH₂)₂(C₂O₄)₂]^–(≈90%) with a smaller amount (≈10%) of the trans product. Rate-determining entry of the first oxalate ligand is believed to be involved. The dependence of the observed rate constants (k_obs) for the first stage with both temperature and [H⁺] is discussed as indicating involvement of a mechanism where in interaction of H₂C₂O₄ occurs with both [Rh(OH₂)₆]³⁺ and [Rh(OH₂)₅(OH)]²⁺ in a process involving retention of the Rh–OH(OH₂) bond during the activation step. Rate constants are not only observed to be far in excess (>10³×) of those for water exchange on the two aqua rhodium(III) ions but also found to be extremely close to those re-evaluated with respect to the corresponding reaction on [Ir(OH₂)₆]³⁺ in support of the existence of a metal independent C–O bond-breaking process. Possible reasons behind the differing behaviour observed for Ru²⁺ and Rh³⁺ are considered.