Kinetic studies on the interconversion of quadridentate and quinquedentate chromium(III)-EDTA (aquo) complexes and attempts to identify the sexidentate complex

Abstract

The quinquedentate complex [Cr^IIIYH(H₂O)](ethylenediaminetetra-acetate or EDTA ≡ Y^4–), which is present as [Cr^IIIY(OH)]^2– at pH >8, reacts in 0·1–1·0M NaOH to give the quadridentate complex [Cr^IIIY(OH)₂]^3–. The rate (k_–4) is first order in complex and independent of [H⁺]. At 25°k_–4= 4·8 × 10^–3 s^–1, ΔH‡_–4= 17·4 ± 0·6 kcal mol^–1 and ΔS‡_–4=–6·4 ± 2·2 e.u. The reverse reaction has been studied over the pH range 0–12. At 25° rate constants for the reactions of five quadridentate species are [Cr^IIIYH₂(H₂O)₂]⁺(k₁= 1·4 s^–1), [Cr^IIIYH(H₂O)₂](k₂∼ 130 s^–1)[Cr^IIIY(H₂O)₂]^–(k₃= 330 s^–1), [Cr^IIIY(H₂O)(OH)]^2–(k₄= 30 s^–1) and [Cr^IIIY(OH)₂]^3–(k₅= 0·004 s^–1). The products are the quinquedentate species [Cr^IIIYH(H₂O)], [Cr^IIIY(H₂O)]^–, and [Cr^IIIY(OH)]^2– depending on the pH; protonation reactions are assumed to be rapid. Consecutive acid dissociation constants relating the five quadridentate species (25°, mol l^–1) are ca. 3 × 10^–3(K₁), ca. 1 × 10^–3(K₂), 7 × 10^–7(K₃), and 1 × 10^–8(K₄). The five rate constants and four equilibrium constants were computed simultaneously by a method of non-linear least squares, by a procedure of function minimisation. At the extremities of the pH range values of k₁, k₂K₁, and k₄/K₄ were obtained by graphical method and are in good agreement with the above. Activation parameters have been obtained for k₁ and k₃. The fast rates of substitution are attributed to neighbouring-group effects which are most favourable when –CO₂^– is replacing an H₂O ligand. The reactions are believed to have a high degree of S_N2 character. Attempts to identify the sexidentate complex [Cr^IIIY]^– were unsuccessful.