Yl. Shi et al., MECHANISTIC INFORMATION ON LIGAND-SUBSTITUTION REACTIONS OF EUROPIUM(III) IN AQUEOUS-SOLUTION FROM HIGH-PRESSURE STOPPED-FLOW EXPERIMENTS, Journal of the Chemical Society. Dalton transactions, (10), 1996, pp. 2127-2134
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.