The structure, chemical bonding, and thermodynamics of alkali ions in M[12-crown-4](+), M[15-crown-5](+), and M[18-crown-6](+), M[UO2(O-2)(OH2)(2)](4,5)(+), and M[UO2(O-2)(OH)(OH2)]n(1)n (n = 4, 5) complexes have been explored by using quantum chemical (QC) calculations at the ab initio level. The chemical bonding has been studied in the gas phase in order to eliminate solvent effects. QTAIM analysis demonstrates features that are very similar in all complexes and typical for electrostatic M-O bonds, but with the M-O bonds in the uranyl peroxide systems about 20 kJ mol(-1) stronger than in the corresponding crown ether complexes. The regular decrease in bond strength with increasing M-O bond distance is consistent with predominantly electrostatic contributions. Energy decomposition of the reaction energies in the gas phase and solvent demonstrates that the predominant component of the total attractive (Delta E-elec + Delta E-orb) energy contribution is the electrostatic component. There are no steric constraints for coordination of large cations to small rings, because the M+ ions are located outside the ring plane, [O-n], formed by the oxygen donors in the ligands; coordination of ions smaller than the ligand cavity results in longer than normal MO distances or in a change in the number of bonds, both resulting in weaker complexes. The Gibbs energies, enthalpies, and entropies of reaction calculated using the conductor-like screening model, COSMO, to account for solvent effects deviate significantly from experimental values in water, while those in acetonitrile are in much better agreement. Factors that might affect the selectivity are discussed, but our conclusion is that present QC methods are not accurate enough to describe the rather small differences in selectivity, which only amount to 510 kJ mol(-1). We can, however, conclude on the basis of QC and experimental data that M[crown ether](+) complexes in the strongly coordinating water solvent are of outer-sphere type, [M(OH2)n(+)][crown ether], while those in weakly coordinating acetonitrile are of inner-sphere type, [M-crown ether](+). The observation that the M[UO2(O-2)(OH)(OH2)]n(+)n complexes are more stable in solution than those of M[crown ether](+) is an effect of the different charges of the rings.

The alkali metal ions Li+, Na+ and K+ have a profound influence on the stoichiometry of the complexes formed in uranyl(VI)-peroxide-hydroxide systems, presumably as a result of a templating effect, resulting in the formation of two complexes, M[(UO2)(O-2)(OH)](2)(-) where the uranyl units are linked by one peroxide bridge, mu-eta(2)-eta(2), with the second peroxide coordinated "end-on", eta(2), to one of the uranyl groups, and M[(UO2)(O-2)(OH)](4)(3-), with a four-membered ring of uranyl ions linked by mu-eta(2)-eta(2) peroxide bridges. The stoichiometry and equilibrium constants for the reactions: M+ + 2UO(2)(2+) + 2HO(2)(-) + 2H(2)O -> M[(UO2)(O-2)(OH)] 2 - + 4H(+) (1) and M+ + 4UO(2)(2+) + 4HO(2)(-) + 4H(2)O -> M[(UO2)(O-2)(OH)](4)(3-) + 8H(+) (2) have been measured at 25 degrees C in 0.10 M (tetramethyl ammonium/M+)NO3 ionic media using reaction calorimetry. Both reactions are strongly enthalpy driven with large negative entropies of reaction; the observation that Delta H(2) approximate to 2 Delta H(1) suggests that the enthalpy of reaction is approximately the same when peroxide is added in bridging and "end-on" positions. The thermodynamic driving force in the reactions is the formation of strong peroxide bridges and the role of M+ cations is to provide a pathway with a low activation barrier between the reactants and in this way "guide" them to form peroxide bridged complexes; they play a similar role as in the synthesis of crown-ethers. Quantum chemical (QC) methods were used to determine the structure of the complexes, and to demonstrate how the size of the M+-ions affects their coordination geometry. There are several isomers of Na[(UO2)(O-2)(OH)](2)(-) and QC energy calculations show that the ones with a peroxide bridge are substantially more stable than the ones with hydroxide bridges. There are isomers with different coordination sites for Na+ and the one with coordination to the peroxide bridge and two uranyl oxygen atoms is the most stable one.

The constitution and equilibrium constants of ternary uranyl(VI) peroxide carbonate complexes [(UO2)(p)(O-2)(q)(CO3)(r)](2(p-q-r)) have been determined at 0 degrees C in 0.50 M MNO3, M = Li, K, and TMA (tetramethyl ammonium), ionic media using potentiometric and spectrophotometric data; O-17 NMR data were used to determine the number of complexes present. The formation of cyclic oligomers, "[(UO2)(O-2)(CO3)](n)", n = 4, 5, 6, with different stoichiometries depending on the ionic medium used, suggests that Li+, Na+, K+ and TMA ions act as templates for the formation of uranyl peroxide rings where the uranyl-units are linked by mu-eta(2)-eta(2) bridged peroxide-ions. The templating effect is due to the coordination of the M+-ions to the uranyl oxygen atoms, where the coordination of Li+ results in the formation of Li[(UO2)(O-2)(CO3)](4)(7-), Na+ and K+ in the formation of Na/K[(UO2)(O-2)(CO3)](5)(9-) complexes, while the large tetramethyl ammonium ion promotes the formation of two oligomers, TMA[(UO2)(O-2)(CO3)] 5 9-and TMA[(UO2)(O-2)(CO3)](6)(11-). The NMR spectra demonstrate that the coordination of Na+ in the five-and six-membered oligomers is significantly stronger than that of TMA(+); these observations suggest that the templating effect is similar to the one observed in the synthesis of crown-ethers. The NMR experiments also demonstrate that the exchange between TMA[(UO2)(O-2)(CO3)](5)(9-) and TMA[(UO2)(O-2)(CO3)](6)(11-) is slow on the O-17 chemical shift time-scale, while the exchange between TMA[(UO2)(O-2)(CO3)](6)(11-)and Na[(UO2)(O-2)(CO3)](6)(11-) is fast. There was no indication of the presence of large clusters of the type identified by Burns and Nyman (M. Nyman and P. C. Burns, Chem. Soc. Rev., 2012, 41, 7314-7367) and possible reasons for this and the implications for the synthesis of large clusters are briefly discussed.

The enthalpies of reaction for the formation of uranyl(VI) hydroxide {[(UO2)(2)(OH)(2)](2+), [(UO2)(3)(OH)(4)](2+), [(UO2)(3)(OH)(5)](+), [(UO2)(3)(OH)(6)]((aq)), [(UO2)(3)(OH)(7)](-), [(UO2)(3)(OH)(8)](2-), [(UO2)(OH)(3)](-), [(UO2)(OH)(4)](2-)} and peroxide complexes {[UO2(O-2)(OH)](-) and [(UO2)(2)(O-2)(2)(OH)](-)} have been determined from calorimetric titrations at 25 degrees C in a 0.100 M tetramethyl ammonium nitrate ionic medium. The hydroxide data have been used to test the consistency of the extensive thermodynamic database published by the Nuclear Energy Agency (I. Grenthe, J. Fuger, R. J. M. Konings, R. J. Lemire, A. B. Mueller, C. Nguyen-Trung and H. Wanner, Chemical Thermodynamics of Uranium, North-Holland, Amsterdam, 1992 and R. Guillaumont, T. Fanghanel, J. Fuger, I. Grenthe, V. Neck, D. J. Palmer and M. R. Rand, Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium, Elsevier, Amsterdam, 2003). A brief discussion is given about a possible structural relationship between the trinuclear complexes [(UO2)(3)(OH)(n)](6-n), n = 4-8.

Quantum chemical (QM) methods have been used to calculate the changes in free energy, G(o), for adduct formations such as Ln(TTA)(3)(OH2)(2)(org) + TMP(org) Ln(TTA)(3)(TMP)(OH2)(org) + H2O (1) in CHCl3 and two phase equilibria such as Ln(3+)(aq) + 3HTTA(org) Ln(TTA)(3)(org) + 3H(+)(aq) (2). In these reactions TTA and TMP represent the chelating acidic extractants thenoyltrifluoroacetone and the trimethyl phosphate synergist. The use of theory requires approximations of both of the theoretical methods and the chemical models used. Different models have been compared and tested against experimental data, demonstrating that theoretical methods provide accurate information on structures and on some features of chemical bonding, while thermodynamic data have much larger uncertainty. However, by suitable choice of chemical models the latter can be decreased significantly, allowing reasonably good estimates of trends in reaction energies throughout the Ln(III) and An(III) series.

The focus of this study is on the relationship between uranyl(VI) poly-peroxo clusters in the solid state and their possible precursors in solution. For this purpose, the complex formation in the ternary U(VI)-H2O2-F-system has been studied by potentiometric titrations, measuring p[H+] and p[F-], revealing that significant amounts of ternary uranyl(VI)-peroxide-fluoride complexes are formed. Based on the analysis of these data we find that there are two models consistent with structure data and previous speciation in the uranyl(VI)peroxide- carbonate system (Dalton. Trans., 2012, 41, 11635-11641). One model contains ternary complexes (UO2)(4)(O-2)(4)F- and (UO2)(4)(O-2)(4)F-2(2-) and the other (UO2)(4)(O-2) (F-)(4) and (UO2)(5)(O-2)(5)F-3(3-); we have chosen the second model as the one most consistent with available information. We suggest that (UO2)(4)(O-2)(4)(F-) is a building block in the U-24 cluster, [Na-6(OH2)(8)]@[UO2(O-2)F](24)(18-) identified in a single-crystal X-ray diffraction study of the solid phase that slowly precipitates from the slightly acidic test solutions. At p[H+] approximate to 9.5, a new solid phase is formed that contains the cluster [Na-6(OH2)(8)]@[UO2(O-2)OH](24)(18-), also identified from an X-ray structure. Both structures contain. 2-. 2 bridging peroxide and. 2 bridging fluoride or hydroxide ions, respectively. As fluoride bridges are unknown in solution coordination chemistry, it is unlikely that the U-24 fluoride cluster is formed in solution. We suggest that both the solid state fluoride and hydroxide clusters are formed in the crystallization from smaller precursors identified in solution. The study illustrates the importance of accurate control of the solution chemistry when preparing poly-peroxo-metallate clusters and also that the mechanism of their formation is still an open field of research.

The composition and equilibrium constants of the complexes formed in the binary U(VI)-hydroxide and the ternary U(VI)-hydroxide-peroxide systems have been studied using potentiometric and spectrophotometric data at 25 degrees C in a 0.100 M tetramethylammonium nitrate medium. The data for the binary U(VI) hydroxide complexes were in good agreement with previous studies. In the ternary system two complexes were identified, [UO2(OH)(O-2)](-) and [(UO2)(2)(OH)(O-2)(2)](-). Under our experimental conditions the former is predominant over a broad p[H+] region from 9.5 to 11.5, while the second is found in significant amounts at p[H+] < 10.5. The formation of the ternary peroxide complexes results in a strong increase in the molar absorptivity of the test solutions. The absorption spectrum for [(UO2)(2)(OH) (O-2)(2)](-) was resolved into two components with peaks at 353 and 308 nm with molar absorptivity of 16200 and 20300 M-1 cm(-1), respectively, suggesting that the electronic transitions are dipole allowed. The molar absorptivity of [(UO2)(OH)(O-2)](-) at the same wave lengths are significantly lower, but still about one to two orders of magnitude larger than the values for UO22+(aq) and the binary uranyl(VI) hydroxide complexes. It is of interest to note that [(UO2)(OH)(O-2)](-) might be the building block in cluster compounds such as [UO2(OH)(O-2)](60)(60-) studied by Burns et al. (P. C. Burns, K. A. Kubatko, G. Sigmon, B. J. Fryer, J. E. Gagnon, M. R. Antonio and L. Soderholm, Angew. Chem. 2005, 117, 2173-2177). Speciation calculations using the known equilibrium constants for the U(VI) hydroxide and peroxide complexes show that the latter are important in alkaline solutions even at very low total concentrations of peroxide, suggesting that they may be involved when the uranium minerals Studtite and meta-Studtite are formed by alpha-radiolysis of water. Radiolysis will be much larger in repositories for spent nuclear fuel where hydrogen peroxide might contribute both to the corrosion of the fuel and to transport of uranium in a ground water system.

The focus of this study is on the identification of precursors in solution that might act as building blocks when solid uranyl(VI) poly-peroxometallate clusters containing peroxide and hydroxide bridges are formed. The precursors could be identified by using carbonate as an auxiliary ligand that prevented the formation of large clusters, such as the ones found in solids of fullerene type. Using data from potentiometric and NMR (O-17 and C-13) experiments we identified the following complexes and determined their equilibrium constants: (UO2)(2)(O-2)(CO3)(4)(6-), UO2(O-2)CO32-, UO2(O-2)(CO3)(2)(4-), (UO2)(2)(O-2)(CO3)(2)(2-), (UO2)(2)(O-2)(2)(CO3)(2-) and [UO2(O-2)(CO3)(5)(10-). The NMR spectra of the pentamer show that all uranyl and carbonate sites are equivalent, which is only consistent with a ring structure built from uranyl units linked by peroxide bridges with the carbonate coordinated "outside" the ring; this proposed structure is very similar to [UO2(O-2)(oxalate)](5)(10-) identified by Burns et al. (J. Am. Chem. Soc., 2009, 131, 16648; Inorg. Chem., 2012, 51, 2403) in K-10[UO2(O-2)(oxalate)](5)center dot(H2O)(13); similar ring structures where oxalate or carbonate has been replaced by hydroxide are important structure elements in solid poly-peroxometallate complexes. The equivalent uranyl sites in (UO2)(2)(O-2)(2)(CO3)(2-) suggest that the uranyl-units are linked by the carbonate ion and not by peroxide.

To assess the nature of chemical bonds in uranyl(VI) complexes with Lewis base ligands, such as F-, Cl-, OH-, CO 3 2-, and O2 2-, we have used quantum chemical observables, such as the bond distances, the internal symmetric/asymmetric uranyl stretch frequencies, and the electron density with its topology analyzed using the quantum theory of atoms-in-molecules. This analysis confirms that complex formation induces a weakening of the uranium-axial oxygen bond, reflected by the longer U-Oyl bond distance and reduced uranyl-stretching frequencies. The strength of the ligand-induced effect increases in the order H2O &lt; Cl- &lt; F- &lt; OH- &lt; CO3 2- &lt; O2 2-. In-depth analysis reveals that the trend across the series does not always reflect an increasing covalent character of the uranyl-ligand bond. By using a point-charge model for the uranyl tetra-fluoride and tetra-chloride complexes, we show that a significant part of the uranyl bond destabilization arises from purely electrostatic interactions, the remaining part corresponding either to charge-transfer from the negatively charged ligands to the uranyl unit or a covalent interaction. The charge-transfer and the covalent interaction are qualitatively different due to the absence of a charge build up in the uranyl-halide bond region in the latter case. In all the charged complexes, the uranyl-ligand bond is best described as an ionic interaction. However, there are covalent contributions in the very stable peroxide complex and, to some extent, also in the carbonate complex. This study demonstrates that it is possible to describe the nature of chemical bond by observables rather than by ad hoc quantities such as atomic populations or molecular orbitals.