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

Substitution reactions of arsenate III phenylazo)-4,5-dihydroxynaphtha lene-2,7-disulfonic acid] complexes of Eu-III with the polyaminecarbox ylates dtpa (diethylenetriaminepentaacetate) and edta (ethylenediamine tetraacetate) were studied as a function of polyaminecarboxylate conce ntration, pH, temperature, pressure and ionic strength by stopped-flow spectrophotometry within the acidity range 3.61 < pH < 5.56 and betwe en 17.5 and 40 degrees C. Under all experimental conditions two consec utive steps (k(1) and k(2)) were observed over different time-scales. For the fast step (k(1)) the substitution rate of Eu(III)L(2)(H2O) (L = arsenazo III) by dtpa and edta (L') increases with increasing acidit y of the medium, k(1) = k(a) + k(b)[H+], and depends on the L' concent ration, i.e. k(1) = k(c) + k(d)[L]. For the slow step (k(2)) plots of k(2) versus [H+] are linear and exhibit no intercept and k(2) is indep endent of L' concentration. Ionic strength studies indicated no signif icant dependence of k(1) and k(2) on [NaClO4]. Rate constant k(1) incr eases with increasing pressure, and there is a very good linear relati onship between In k(1) and pressure at any temperature, ligand concent ration, pH and ionic strength. In contrast, k(2) exhibits almost no pr essure dependence for any temperature, pH and ligand concentration. It is suggested that this latter step involves acid-catalysed dechelatio n of arsenate III accompanied by chelation of L'. In terms of intrinsi c volume changes, bond formation and breakage contributions seem to ca ncel in order to account for the zero Delta V-2 double dagger values. The substitution of arsenate III by dtpa on Eu-III is much faster than the same process on Gd-III.