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Titolo:
Equilibria and dynamics in binary and ternary uranyl oxalate and acetate/fluoride complexes
Autore:
Aas, W; Szabo, Z; Grenthe, I;
Indirizzi:
Royal Inst Technol, Dept Chem, S-10044 Stockholm, Sweden Royal Inst Technol Stockholm Sweden S-10044 m, S-10044 Stockholm, Sweden
Titolo Testata:
JOURNAL OF THE CHEMICAL SOCIETY-DALTON TRANSACTIONS
fascicolo: 8, , anno: 1999,
pagine: 1311 - 1317
SICI:
0300-9246(19990421):8<1311:EADIBA>2.0.ZU;2-0
Fonte:
ISI
Lingua:
ENG
Soggetto:
NUCLEAR-MAGNETIC-RESONANCE; LIGAND-EXCHANGE REACTIONS; DIOXOURANIUM(VI) COMPLEXES; AQUEOUS-SOLUTION; KINETICS; ION;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
24
Recensione:
Indirizzi per estratti:
Indirizzo: Grenthe, I Royal Inst Technol, Dept Chem, S-10044 Stockholm, Sweden Royal Inst Technol Stockholm Sweden S-10044 Stockholm, Sweden
Citazione:
W. Aas et al., "Equilibria and dynamics in binary and ternary uranyl oxalate and acetate/fluoride complexes", J CHEM S DA, (8), 1999, pp. 1311-1317

Abstract

The formation of ternary UO2(ac)(p)F-q(2-p-q) (n = 1 or 2 and q = 1-3) complexes, and their equilibrium constants were investigated by potentiometrictitrations and F-19 NMR spectroscopy. The equilibrium constants have been determined from the emf data in a NaClO4 medium at constant sodium concentration, [Na+] = 1.00 M at 25 degrees C, except for the UO2(ac)F-3(2-) complex where F-19 NMR at -5 degrees C was used. The magnitude of the equilibriumconstant for the stepwise addition of fluoride indicates that prior co-ordination of acetate has only a small effect on the subsequent bonding of fluoride. The acetate exchange in the ternary UO2(ac)F-3(2-) complex was studied using F-19 NMR. Through magnetisation transfer experiments, it was possible to confirm the provisional mechanism from a previous study and also theconsistency of the rate constants for the five different exchange pathwaysrequired to describe the fluoride exchange. The exchange takes place via the intermediate UO2F3(H2O)(2)(-), indicating that the acetate exchange follows an interchange mechanism with solvent participation in the transition state. The rates and mechanisms of the ligand exchange reactions in UO2(ox)(2)(H2O)(2-) and UO2(ac)(2)(H2O) have been studied using C-13 NMR techniquesat -5 degrees C. The rate law is v = k[complex][ligand], and the second order rate constant and the activation parameters for both systems have been determined. The reactions most likely take place through an Eigen-Wilkins type of mechanism, where the first step is a pre-equilibrium of an outer-sphere complex followed by a rate determining exchange of water. The rate constants for the water exchange reactions are very similar to that in UO2(H2O)(5)(2+). The information from the binary oxalate system rules out the formation of UO2(ox)(2)(H2O)(2)(-) as an intermediate in the exchange reactions in the previously studied UO2(ox)(2)F3-, also in this case confirming a previously suggested exchange mechanism. The H+/D+ isotope effects and a linear free energy relationship suggest that the main catalytic effect of He on ligand exchange rates is due to the formation of a protonated precursor. Hence, the catalytic effect depends on the basicity of the ligand and the site for the proton attack.

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Documento generato il 19/09/20 alle ore 09:12:07