Kinetics of complexing of oxalate to penta-ammineaquochromium(III)

Abstract

The complex penta-ammineaquochromium(III) reacts with oxalate ions to form tetra-ammineoxalatochromium(III) and there is no evidence for any significant build-up of the monodentate oxalato-intermediate [Cr(NH₃)₅(C₂O₄)]⁺. It is concluded that the step in which the oxalate ion chelates is rapid. The kinetics of the first stage of the reaction, involving co-ordination of oxalate ion to [Cr(NH₃)₅H₂O]³⁺, have been investigated at pH 3–4 and at temperatures in the range 40–60 °C, I= 1·0M(NaClO₄). The interpretation given is consistent with the formation of ion-pairs between the oxalate ions C₂O₄^2–(K′) and C₂O₄H^–(K″) and [Cr(NH₃)₅H₂O]⁸⁺, where, at 50 °C,K′= 4·5± 0·2 l mol^–1 and K″= 1·1 6± 0·06 l mol^–1. The rate constants K₁ and K₂ for the reactions of the ion-pairs [Cr(NH₃)₅H₂O]³⁺, C₂O₄H^– and [Cr(NH₃)₅H₂O]⁸⁺, C₈O₄H^– are equal (6·2 × 10^–4 s^–1 at 50 °C), with ΔH^‡= 26·4 ± 0·8 kcal mol^–1 and ΔS^‡= 8·6 ±0·4 cal K^–1 mol^–1. Rate constants do not differ significantly from values obtained for the water exchange of the ion [Cr(NH₃)₆H₂O]³⁺, which suggests that the reactions proceed by a dissociative-interchange mechanism.