A study of some redox reactions of the chromium(II)–ethylenediamine-tetra-acetate complex. Evidence for outer-sphere mechanisms

Abstract

Reactions of the chromium(II)–EDTA complex with CrF²⁺, CrCl²⁺, CrBr²⁺, Co(NH₃)₅Cl²⁺, Cl₂, Br₂, I₂, and IrCl₆^2– have been studied in perchlorate reaction media pH 2·7–5·0, and at temperatures in the range 0–20°. The reactions are all rapid and final spectra were recorded within 3 min. of mixing. The products are [Cr^IIIYH(H₂O)] and the related conjugate-base form [Cr^IIIY(H₂O)]^–(ethylenediaminetetra-acetate, EDTA = Y^4–). Complexes of the type [Cr^IIIYH(X)]^–, where X^– is a halide ion, have not as yet been prepared. If it is assumed that the aquation of such complexes is slow then the chromium(II)–EDTA reactions are probably of the outer-sphere type. Other possible mechanisms are considered. Reactions with H₂O₂, O₂, and (H₂O)₄Cr(OH)₂Cr(H₂O)₄⁴⁺ are also rapid, the same products are obtained, and there is no evidence for the formation of binuclear chromium(III)–EDTA complexes. Again outer-sphere mechanisms are a strong possibility.