Kinetic studies of the complexation of oxalate to the t2g 6 hexaaqua ions of ruthenium(II) and rhodium(III)

Abstract

A kinetic study on the reactions of oxalate with the isoelectronic t2g6 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 (kobs) 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.