Kinetics of ligand exchange reactions for uranyl(2+) fluoride complexes in aqueous solution

Journal Article · Wed Mar 27 04:00:00 EST 1996 · Inorganic Chemistry

DOI:https://doi.org/10.1021/ic951140q· OSTI ID:263753

Szabo, Z; Glaser, J; Grenthe, I ^[1]

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1. Royal Inst. of Technology (KTH), Stockholm (Sweden)

Rates and mechanisms of fluoride exchange reactions between various uranyl fluoro complexes UO{sub 2}(H{sub 2}O){sub 5-n}F{sub n}{sup 2-n}, and HF/F{sup {minus}} have been studied in aqueous solution using {sup 19}F and {sup 17}O NMR line broadening technique. A group of 15 different exchange pathways has been identified, and their rate laws and rate constants have been determined. All reactions are first order with regard to the uranyl complex and second order overall. Two pathways dominate: fluoride exchange between two uranyl complexes, presumably through the formation of a fluoride bridging intermediate/transition state, e.g., UO{sub 2}F{sup +} + UO{sub 2}{sup *}F{sub 2} {rightleftharpoons} Uo{sub 2}F{sup *}F + UO{sub 2}F{sup +} (k{sub 1,2}), and fluoride exchange between a uranyl complex and F{sup {minus}}/HF, e.g., UO{sub 2}F{sup +} + H{sup *}F {rightleftharpoons} UO{sub 2}{sup *}F{sup +} + HF (k{sub 1,HF}). The exchange between UO{sub 2}{sup 2+} and UO{sub 2}F{sup +} takes place mainly according to UO{sub 2}{sup 2+} + HF {rightleftharpoons} UO{sub 2}F{sup +} + H{sup +} (forward, k{sup {prime}}{sub 0,HF}; reverse, k{sub 1,HF}). Most of these reactions have rate constants, k{sub m,n} {approx} 5 x 10{sup 4} M{sup {minus}1} s{sup {minus}1}, at -5 {degrees}C. The exchange reactions seem to follow the Eigen-Wilkins mechanism, where the rate determining step is a ligand promoted dissociation of coordinated water. The exchanges involving UO{sub 2}F{sub n}{sup 2-n}, n=4 and 5, are much faster than the others, indicating mechanistic differences. The exchange rate was approximately 3 times faster for reactions involving DF than for HF. The activation parameters have been determined for two reaction pathways.

• OSTI ID: 263753
• Journal Information: Inorganic Chemistry, Journal Name: Inorganic Chemistry Journal Issue: 7 Vol. 35; ISSN 0020-1669; ISSN INOCAJ
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
AQUEOUS SOLUTIONS
CHEMICAL REACTION KINETICS
NUCLEAR MAGNETIC RESONANCE
URANYL COMPLEXES
URANYL FLUORIDES