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 [C2O42–]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, kobs, give a dependence on hydrogen-ion concentration 1/kobs= 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(=kKIP, where KIP 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.