MECHANISTIC INFORMATION ON LIGAND-SUBSTITUTION REACTIONS OF GADOLINIUM(III) IN AQUEOUS-SOLUTION FROM HIGH-PRESSURE STOPPED-FLOW EXPERIMENTS

Substitution reactions of arsenazo III phenylazo)-4,5-dihydroxynaphtha lene-2,7-disulfonic acid] complexes of Gd(III) with the polyaminocarbo xylates ethylenediaminetetraacetate and diethylenetriaminepentaacetate were studied as a function of ligand concentration, pH, temperature a nd pressure. Under all experimental conditions two consecutive reactio n steps were observed, for which the observed first-order rate constan ts are independent of aminocarboxylate concentration and show signific ant acid catalysis. The observed kinetic behaviour and the activation parameters determined under limiting conditions, i.e. low and high [H], can be accounted for in terms of dechelation of the arsenazo III li gand being the rate-determining step. The existence of 1:1 and 1:2 Gd( III):arsenazo III complexes is suggested to account for the fast and s low substitution reactions, respectively. The observed volumes of acti vation can mainly be ascribed to large solvational changes associated with the acid-catalysed dechelation process.