Kinetics of a slow step in the chelation of ethylenediaminetetra-acetic acid to chromium(III)

Abstract

The kinetics of a slow step in the reaction of [Cr^IIIYH₃(Cl)₂(H₂O)] to give [Cr^IIIYH(H₂O)] and [Cr^IIIY(H₂O)]-(ethylenediaminetetra-acetate, EDTA = Y^4–), and in the analogous reaction of [Cr^IIIYH₃(Br)₂(H₂O)], have been studied in aqueous perchlorate media [µ 1·0M(LiClO₄)] at temperatures in the range 1–22°. The reactions are believed to occur in two steps, the first of which is slow and rate determining. First-order kinetics are observed for this step (k_obs) and the hydrogen-ion dependence is given by k_obs(K+[H⁺])=k₁[H⁺]+k₂K, where k₁ is the rate constant for the reaction of a protonated form of the complex (x is probably 2 in [Cr^IIIYH_x(X)₂(H₂O)]^x–3), k₂ is the rate constant for the reaction of the complex with one less proton, and K is the acid dissociation constant which relates the two species. At 20° values of k₁ and k₂ are 0·15 × 10^–2 and 1·43 × 10^–2 sec.^–1 respectively for the chloro-complex, and 1·72 × 10^–2 and 12·2 × 10^–2 sec.^–1 for the bromo-complex. Enthalpies of activation for the reactions whose rate constants are k₁ and k₂ are within the range 16·6–17·5 kcal. mole^–1, and the corresponding entropies of activation fall between –9·2 and –12·2 e.u.