Kinetic studies of the complexation of oxalate to hexa-aquairidium(III)

Abstract

A kinetic study of the oxalate anation of hexa-aquairidium(III) perchlorate in HClO4–NaClO4 solution, I= 1.5 mol dm–3(NaClO4), is reported. The acidity dependence ([H+]= 0.14 – 1.00 mol dm–3) is consistent with the existence of two parallel pathways involving anation by both H2C2O4 and HC2O4–. Rate constants (25 °C) and activation parameters for the two paths are: H2C2O4, k1=(1.3 ± 0.2)× 10–5 dm3 mol–1 s–1, ΔH1‡= 73.1 ± 4.6 kJ mol–1, ΔS1‡=–94.1 ± 13.9 J K–1 mol–1; and HC2O4–, k2=(7.2 ± 1.2)× 10–6 s–1, ΔH2‡= 108 ± 7 kJ mol–1, ΔS2‡=+ 20.3 ± 22.4 J K–1 mol–1. Ion-pair association of HC2O4– is small as expected and a temperature-independent value of 1.0 dm3 mol–1 has been assumed for its association constant in evaluating the k2 path, although this does not affect the overall conclusions reached. Widely differing activation parameters are seen for the two paths suggesting that different mechanisms may operate. Anation by HC2O4– is discussed in terms of an ion-pair interchange mechanism. However, anation by H2C2O4 is suggested to involve a concerted mechanism in which replacement of the aqua ligands is achieved in the rate-determining step without Ir–OH2 bond cleavage, similar to that thought to operate in the corresponding anation reaction of [Co(NH3)5(OH2)]3+. The results highlight the first successfully monitored anation reaction of [Ir(OH2)6]3+ and are discussed with respect to anation of [Rh(OH2)6]3+ and in the context of anation by both H2C2O4 and HC2O4– of trivalent aqua metal ions in general.