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.