Kinetic studies on the interconversion of quadridentate and quinquedentate chromium(III)-EDTA (aquo) complexes and attempts to identify the sexidentate complex
The quinquedentate complex [CrIIIYH(H2O)](ethylenediaminetetra-acetate or EDTA ≡ Y4–), which is present as [CrIIIY(OH)]2– at pH >8, reacts in 0·1–1·0M NaOH to give the quadridentate complex [CrIIIY(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 [CrIIIYH2(H2O)2]+(k1= 1·4 s–1), [CrIIIYH(H2O)2](k2∼ 130 s–1)[CrIIIY(H2O)2]–(k3= 330 s–1), [CrIIIY(H2O)(OH)]2–(k4= 30 s–1) and [CrIIIY(OH)2]3–(k5= 0·004 s–1). The products are the quinquedentate species [CrIIIYH(H2O)], [CrIIIY(H2O)]–, and [CrIIIY(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 SN2 character. Attempts to identify the sexidentate complex [CrIIIY]– were unsuccessful.