Kinetics of Multidentate Ligand Substitution Reaction. III. Substitution Reactions of BT with Cobalt(II) and Nickel(II) Chelates of N-(2-Hydroxyethyl)-ethylenediamine-N,N′,N′-triacetic Acid and 2,2′-Ethylenedioxybis[ethyliminodi(acetic acid)]

Abstract

BT can displace EDTA-OH and GEDTA from their cobalt(II) and nickel(II) chelate as in the case of the substitution reaction of BT with cobalt(II)-EDTA chelate or nickel(II)-EDTA chelate. In this paper, kinetics of the substitution reactions of BT with cobalt(II)-and nickel(II)-chelates of EDTA-OH and of GEDTA was studied spectrophotometrically.All substitution reactions were found to be first-order in BT and first-order in metal(II)-chelate of EDTA-OH or GEDTA, and to proceed through two simultaneous reaction paths analogous to those proposed for the substitution reaction of BT with cobalt(II)-EDTA chelate. Although EDTA-OH differs from EDTA by the presence of a hydroxyethyl group in place of one acetate group, its metal(II) chelate behaves in a nearly identical manner with EDTA chelate in the substitution reaction and the cleavage of the metal-nitrogen bond of the glycinate group is considered to be the probable rate-determining step. The substitution reactions of BT with GEDTA chelates are considered to be more favored thermodynamically over the reactions of EDTA-OH chelates, but were found to be less favored kinetically. This fact may suggest that the geometrical structure of metal(II)-GEDTA plays an important role in determining the rate of the substitution reaction of GEDTA.In the previous papers,1,2) kinetics of the substitution reactions of Eriochrom Black T (BT) with cobalt(II) and nickel(II) chelates of ethylenediaminetetraacetic acid (EDTA) and of nitrilotriacetic acid (NTA) were studied. Reactions were firstorder in BT and first-order in metal(II) chelate of 1 to 1 composition.BT can also displace N-(2-hydroxyethyl)-ethylenediamine-N,N',N'-triacetate (EDTA-OH) and 2,2'-ethylenedioxybis[ethyliminodi(acetate)] (GEDTA) from their chelates of cobalt(II) and nickel(II).In this paper, the kinetics of the substitution reactions of BT with EDTA-OH and GEDTA chelates of these metal ions were dealt with spectrophotometrically. From comparison of the rate constants of the substitution reactions of EDTA-OH chelates of cobalt(II) and nickel(II) with those of EDTA chelates, the reaction mechanism was assigned in detail. ExperimentalReagents. The preparation and the standardization of solutions ofcobalt(II) and nickel(II) perchlorates were described in the previous papers.1,2) Reagent grade EDTA-OH and GEDTA were recrystallized from water. The concentration of EDTA-OH solution was standardized against the standard copper(II) solution by a volumetric titration with Murexide as indicator.3) The standard solution of GEDTA was prepared by dissolving a known amount of GEDTA dried for two purification of BT was also described in the previous paper.4) Other reagents were of guranteed reagent grade and used without further purification.Apparatus and Procedure. The apparatus and the experimental procedures were the same as described previously.1) In this study, all measurements were conducted in solutions of ionic strength 0.30 (NaClO4), and no buffer reagent was used, because free EDTA-OH and GEDTA can have enough buffer cap...