Kinetics of Ligand Exchange Reactions for Uranyl(2+) Fluoride Complexes in Aqueous Solution Zolta ´ n Szabo ´ ,* ,† Julius Glaser, and Ingmar Grenthe* Inorganic Chemistry, Department of Chemistry, Royal Institute of Technology (KTH), S-100 44 Stockholm, Sweden ReceiVed August 29, 1995 X Rates and mechanisms of fluoride exchange reactions between various uranyl fluoro complexes {UO 2 (H 2 O) 5-n F n 2-n }, and HF/F - have been studied in aqueous solution using 19 F and 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 2 F + + UO 2 *F 2 h UO 2 F*F + UO 2 F + (k 1,2 ), and fluoride exchange between a uranyl complex and F - /HF, e.g., UO 2 F + + H*F h UO 2 *F + + HF (k 1,HF ). The exchange between UO 2 2+ and UO 2 F + takes place mainly according to UO 2 2+ + HF h UO 2 F + + H + (forward, k′ 0,HF ; reverse, k 1,HF ). Most of these reactions have rate constants, k m,n ≈ 5 × 10 4 M -1 s -1 , at -5 °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 2 F n 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. Introduction The linear dioxoactinoid(VI) ions, e.g., UO 2 2+ , have all their exchangeable ligands in a plane perpendicular to the linear axis. The “-yl“ oxygens are substitution inert, 1 except in the case when the ion is excited by UV light. 2-4 This unusual coordina- tion geometry indicates that the pathway for ligand substitution reactions might be located in, or close to, this plane, a very different situation from those encountered in most other coordination geometries. There are comparatively few inves- tigations of the mechanisms for ligand substitutions in uranium- (VI) complexes. Most studies have been made in nonaqueous systems, and these have been reviewed by Lincoln, 5 and Tomiyasu and Fukutomi. 6 Ligand exchange reactions in aque- ous systems have been studied by Glaser et al. 7,8 and Tomiyasu et al. 9 In a previous paper 10 we have described the possibilities offered by 1D and 2D 19 F NMR methods for the study of the dynamics of fluoride complexes. Additional insight into the dynamics in the uranium(VI) fluoride system has been obtained from studies of luminescence lifetimes. 11,12 Substitution mechanisms have been discussed, 5,6 and the experimental evidence seems to favor dissociative (D) or dissociative interchange (I d ) mechanisms. An exception is a recent study by Tomiyasu et al., 9 interpreted in terms of an associative or associative interchange mechanism. The main indicators for this mechanism are a strongly negative activation entropy and that the rate of fluoride exchange is not influenced by irradiation at 488 nm, which is expected to increase the lability of the in-plane ligands. From the available literature information it is not possible to draw any clearcut mechanistic conclusions, and we have made the present study to try to resolve the conflicting mechanistic evidence. We have inves- tigated the exchange reactions in the U(VI)-F - system over a very broad concentration range in order to obtain the rates and mechanisms of the exchange reactions between the various complexes UO 2 F n 2-n , n ) 1-5, and between these and free F - and HF(aq). The experiments have been made in a 1.00 M NaClO 4 ionic medium, using the equilibrium constants previ- ously determined by Ahrland and Kullberg, 13 the known analytical total concentrations of U(VI) and fluoride, and the measured hydrogen ion concentration to calculate the species distribution of the various test solutions investigated. The pH of the solutions was varied in the range 0 > pH > 6, where no hydroxo or mixed fluoro/hydroxo complexes are present. Experimental Section Solutions. An uranium(VI) perchlorate stock solution prepared by a method described earlier 14 and a NaF stock solution (from recrystal- lized NaF) were used to prepare the investigated solutions. The ionic medium was kept constant by NaClO 4 ([ClO4 - ] ) 1 M). The free hydrogen ion concentration (-log[H + ]) was measured by a HF-resistant combined glass electrode (Ingold, HF-405-60-57/120; the inner solution * Author to whom correspondence should be addressed. † On leave from Alkaloida Chemical Co. Ltd., Tiszavasva ´ri, H-4440, Hungary. X Abstract published in AdVance ACS Abstracts, February 15, 1996. (1) Gordon, G.; Taube, H. J. Inorg. Nucl. Chem. 1961, 16, 272. (2) Jung, W.; Ikeda, Y.; Tomiyasu, H.; Fukutomi, H. Bull. Chem. Soc. Jpn. 1984, 57, 2317. (3) Kato, Y.; Fukutomi, H. J. Inorg. Nucl. Chem. 1976, 38, 1323. (4) Okuyama, K.; Ishikawa, Y.; Kato, Y.; Fukutomi, H. Bull. Res. Lab. Nucl. React. 1978, 3, 39. (5) Lincoln, S. F. Pure Appl. Chem. 1979, 51, 2059. (6) Tomiyasu, H.; Fukutomi, H. Bull. Res. Lab. Nucl. React. 1982, 7, 57. (7) Bru ¨cher, E.; Glaser, J.; To ´th, I. Inorg. Chem. 1991, 30, 2239. (8) Ba ´nyai, I.; Glaser, J.; Micskei, K.; To ´th, I.; Zeka ´ny, L. Inorg. Chem. 1995, 34, 3785. (9) Harada, M.; Fujii, Y.; Sakamaki, S.; Tomiyasu, H. Bull. Chem. Soc. Jpn. 1992, 65, 3022. (10) Szabo ´, Z.; Glaser, J. Magn. Reson. Chem. 1995, 33, 20. (11) Park, Y.; Sakai, Y.; Abe, R.; Ishii, T.; Harada, M.; Kojima, T.; Tomiyasu, H. J. Chem. Soc., Faraday Trans. 1990, 86, 55. (12) Moriyasu, M.; Yokoyama, Y.; Ikeda, S. J. Inorg. Nucl. Chem. 1977, 39, 2199. (13) Ahrland, S.; Kullberg, L. Acta Chem. Scand. 1971, 25, 3457. (14) Ciavatta, L.; Ferri, D.; Grenthe, I.; Salvatore, F. Inorg. Chem. 1981, 20, 463. 2036 Inorg. Chem. 1996, 35, 2036-2044 0020-1669/96/1335-2036$12.00/0 © 1996 American Chemical Society