Reaction of tetra-aquoethylenediaminechromium(III) with oxalate

Abstract

Spectrophotometric changes observed during the reaction of oxalate with tetra-aquoethylenediaminechromium(III) are consistent with the reaction sequence Cr(en)(H₂O)₄³⁺→ Cr(en)(C₂O₄)(H₂O)₂⁺→ Cr(enH)(C₂O₄)(H₂O)₃²⁺→ Cr(C₂O₄)(H₂O)₄⁺→ Cr(C₂O₄)₂(H₂O)₂^–, and the same sequence without intermediate formation of Cr(en)(C₂O₄)(H₂O)₂⁺(ca. 40% contribution, 45–60 °C), total oxalate in the range [C₂O₄^2–]_T= 1 × 10^–3 to 22 × 10^–1M, [H⁺]= 0·03–0·50M. Growth and decay curves have been plotted for the various species following ion-exchange separation of the products, and determination of the chromium(III) content by atomic absorption spectrometry. The kinetics of the complexing of the first oxalate have been investigated at 45–60 °C, I= 1·0M(NaClO₄), where formation of a unidentate oxalato-complex with the displacement of a water ligand is believed to berate determining and common to both reaction sequences. Second-order rate constants, k_obs, give a dependence on hydrogen-ion concentration 1/k_obs= 1/k₁+[H⁺]/k₁K₁ where k₁ is for the reaction of Cr(en)(H₂O)₄³⁺ with HC₂O₄^–, and K₁ is the first acid dissociation constant of oxalic acid. At 50 °C the rate constant K₁(=kK_IP, where K_IP is for ion-pair formation) is (4·02 ± 0·02)× 10^–2| mol^–1 s^–1 and activation parameters for k₁ are ΔH^‡= 23·4 ± 1·0. kcal mol^–1 and ΔS^‡= 7·4 ± 3·1 cal mol^–1 K^–1. Chelation of the oxalate to give Cr(en)(C₂O₄)(H₂O)₂⁺ and Cr(enH)(C₂O₄)(H₂O)₃²⁺ is rapid.