Easy access to stable pentavalent uranyl complexes

Reaction of UO2I2(THF)3 with 1 molar equivalent of KC5R5 (R = H, Me) in pyridine led to the uranyl(V) compound {[UO2(Py)5][KI2(Py)2]}(infinity), which is an infinite 1D polymer in its crystalline form; the UO2X(THF)n (X = I, OSO2CF3) complexes were obtained by reduction of their U(VI) parents with TlC5H5 or KC5R5 in THF.     

New insights into the acid mediated disproportionation of pentavalent uranyl

The reaction of benzoic acid with the uranyl(V) complex [(UO(2)Py(5))(KI(2)Py(2))] in pyridine leads to immediate disproportionation with formation of a hexanuclear U(IV) benzoate cluster, a bis-benzoate complex of uranyl(VI) and water.     

Uranyl peroxide pyrophosphate cage clusters with oxalate and nitrate bridges

Two complex cage clusters built from uranyl hexagonal bipyramids and multiple types of bridges between uranyl ions, U(30)Py(10)Ox(5) and U(38)Py(10)Nt(4), were crystallized from aqueous solution under ambient conditions. These are built from 30 uranyl hexagonal bipyramids, 10 pyrophosphate groups, and five oxalate bridges in one case, and 38 uranyl hexagonal bipyramids, 10 pyrophosphate groups, and four nitrate groups in the other. The crystal compositions are (H(3)O)(10)Li(18)K(22)[(UO(2))(30)(O(2))(30)(P(2)O(7))(10)(C(2)O(4))(5)](H(2)O)(22) and Li(24)K(36)[(UO(2))(38)(O(2))(40)(OH)(8)(P(2)O(7))(10)(NO(3))(4)](NO(3))(4)(H(2)O)(n) for U(30)Py(10)Ox(5) and U(38)Py(10)Nt(4), respectively. Cluster U(30)Py(10)Ox(5) crystallizes over a narrow range of solution pH that encourages incorporation of both oxalate and pyrophosphate, with incorporation of oxalate only being favored under more acidic conditions, and pyrophosphate only under more alkaline conditions. Cluster U(38)Py(10)Nt(4) contains two identical lobes consisting of uranyl polyhedra and pyrophosphate groups, with these lobes linked into the larger cluster through four nitrate groups. The synthesis conditions appear to have prevented closure of these lobes, and a relatively high nitrate concentration in solution favored formation of the larger cluster.     

A Raman and infrared spectroscopic study of the uranyl silicates--weeksite, soddyite and haiweeite

Raman spectroscopy has been used to study the molecular structure of a series of selected uranyl silicate minerals, including weeksite K2[(UO2)2(Si5O13)].H2O, soddyite [(UO2)2SiO4.2H2O] and haiweeite Ca[(UO2)2(Si5O12(OH)2](H2O)3 with UO2(2+)/SiO2 molar ratio 2:1 or 2:5. Raman spectra clearly show well resolved bands in the 750-800 cm-1 region and in the 950-1000 cm-1 region assigned to the nu1 modes of the (UO2)2+ units and to the (SiO4)4- tetrahedra. For example, soddyite is characterized by Raman bands at 828.0, 808.6 and 801.8 cm-1 (UO2)2+ (nu1), 909.6 and 898.0 cm-1 (UO2)2+ (nu3), 268.2, 257.8 and 246.9 cm-1 are assigned to the nu2 (delta) (UO2)2+. Coincidences of the nu1 (UO2)2+ and the nu1 (SiO4)4- is expected. Bands at 1082.2, 1071.2, 1036.3, 995.1 and 966.3 cm-1 are attributed to the nu3 (SiO4)4-. Sets of Raman bands in the 200-300 cm-1 region are assigned to nu2 (delta) (UO2)2+ and UO ligand vibrations. Multiple bands indicate the non-equivalence of the UO bonds and the lifting of the degeneracy of nu2 (delta) (UO2)2+ vibrations. The (SiO4)4- tetrahedral are characterized by bands in the 470-550 cm-1 and in the 390-420 cm-1 region. These bands are attributed to the nu4 and nu2 (SiO4)4- bending modes. The minerals show characteristic OH stretching bands in the 2900-3500 cm-1 and 3600-3700 cm-1.     

UO22+·nH2O和PuO22+·nH2O(n=2,4,5,6)密度泛函理论研究

首先用密度泛函理论(DFT)方法研究了铀酰和钚酰离子的几何与电子结构,计算结果与实验基本符合,表明DFT方法也能用于含铀和钚重原子的化合物计算.然后对铀酰和钚酰水合离子的几何构型、Mulliken集居数分布以及铀酰(钚酰)与配体水分子的结合能进行计算,计算结果表明UO22+·5H2O和PuO22+·5H2O分别为铀酰和钚酰系列水合离子中最稳定的配合物.     

Reactivity of the "yl"-bond in uranyl(VI) complexes. 1. Rates and mechanisms for the exchange between the trans-dioxo oxygen atoms in (UO2)2(OH)2 2+ and mononuclear UO2(OH)n 2-n complexes with solvent water

The stoichiometric mechanism, rate constant, and activation parameters for the exchange of the "yl"-oxygen atoms in the dioxo uranium(VI) ion with solvent water have been studied using 17O NMR spectroscopy. The experimental rate equation, (-->)v= k(2obs)[UO2(2+)]tot2/[H+]2, is consistent with a mechanism where the first step is a rapid equilibrium 2U(17)O2(2+) + 2H2O<==>(U(17)O2)2(OH)2(2+) + 2H+, followed by the rate-determining step (U(17)O2)2(OH)2(2+) + H2O<==>(UO2)2*(OH)2(2+) + H2(17)O, where the back reaction can be neglected because the (17)O enrichment in the water is much lower than in the uranyl ion. This mechanism results in the following rate equation (-->)v= d[(UO2)2(OH)2(2+)]/dt = k(2,2)[(UO2)2(OH)2(2+)] = k(2,2*)beta(2,2)[UO2(2+)]2/[H + ]2; with k(2,2) = (1.88 +/- 0.22) x 10(4) h(-1), corresponding to a half-life of 0.13 s, and the activation parameters DeltaH++ = 119 +/- 13 kJ mol-1 and DeltaS++ = 81 +/- 44 J mol(-1) K(-1). *Beta(2,)2 is the equilibrium constant for the reaction 2UO2(2+) + 2H2O<==>(UO2)2(OH)2(2+) + 2H+. The experimental data show that there is no measurable exchange of the "yl"-oxygen in UO2(2+), UO2(OH)+, and UO2(OH)4(2-)/ UO2(OH)5(3-), indicating that "yl"-exchange only takes place in polynuclear hydroxide complexes. There is no "yl"-exchange in the ternary complex (UO2)2(mu-OH)2(F)2(oxalate)2(4-), indicating that it is also necessary to have coordinated water in the first coordination sphere of the binuclear complex, for exchange to take place. The very large increase in lability of the "yl"-bonds in (UO2)2(OH)2(2+) as compared to those of the other species is presumably a result of proton transfer from coordinated water to the "yl"-oxygen, followed by a rapid exchange of the resulting OH group with the water solvent. "Yl"-exchange through photochemical mediation is well-known for the uranyl(VI) aquo ion. We noted that there was no photochemical exchange in UO2(CO3)3(4-), whereas there was a slow exchange or photo reduction in the UO2(OH)4(2-) / UO2(OH)5(3-) system that eventually led to the appearance of a black precipitate, presumably UO2.     

铀氧化物的氧化还原行为及其价态的X—射线光电子能谱的研究

应用Ｘ－射线光电子能谱对铀氧化物（ＵＯ２、ＵＯ３、Ｕ３Ｏ８）的化学态及其不同价态的相对含量（Ｕ＾４＋／Ｕ＾６＋）进行了研究，结果表明，常温下ＵＯ２在空气中可氧化形成ＵＯ２＋ｘ；ＵＯ３易与水结合形成水合物，在高温和高真空中易脱氧，部分Ｕ＾６＋转变成Ｕ＾４＋；Ｕ３Ｏ８中存在Ｕ＾４＋和Ｕ＾６＋两种价态，其相对量之比为１∶２。铀氧化物中Ｕ＾４＋和Ｕ＾６＋的Ｕ４ｆ７／２结合能相差１ｅＶ左右，借助于曲线拟合技     

UO 2 2+ sorption on bentonite

The capacity of bentonite and purified bentonite to remove UO ₂ ²⁺ ions from aqueous solutions has been investigated. The UO ₂ ²⁺ uptake in these clays was determined for 0.2 and 0.002M uranyl nitrate solutions. It was found that under these conditions (0.2M) the maximum UO ₂ ²⁺ uptake was 1.010±0.070 meq UO ₂ ²⁺ /g of bentonite and 0.787±0.020 meq UO ₂ ²⁺ /g of purified bentonite. In purified bentonite UO ₂ ²⁺ sorption is irreversible up to 50 hours as no desorption was observed. Such is not the case in the natural bentonite. X-ray diffraction, thermal analyses, and transmission electron microscopy were used to characterize the solids. The uranium content was determined by neutron activation analysis.     

Rates and mechanisms of water exchange of UO2(2+)(aq) and UO2(oxalate)F(H2O)2-: a variable-temperature 17O and 19F NMR study

This study consists of two parts: The first part comprised an experimental determination of the kinetic parameters for the exchange of water between UO2(H2O)5(2+) and bulk water, including an ab initio study at the SCF and MP2 levels of the geometry of UO2(H2O)5(2+), UO2(H2O)4(2+), and UO2(H2O)6(2+) and the thermodynamics of their reactions with water. In the second part we made an experimental study of the rate of water exchange in uranyl complexes and investigated how this might depend on inter- and intramolecular hydrogen bond interactions. The experimental studies, made by using 17O NMR, with Tb3+ as a chemical shift reagent, gave the following kinetic parameters at 25 degrees C: kex = (1.30 +/- 0.05) x 10(6) s(-1); deltaH(not equal to) = 26.1 +/- 1.4 kJ/mol; deltaS(not equal to) = -40 +/- 5J J/(K mol). Additional mechanistic indicators were obtained from the known coordination geometry of U(VI) complexes with unidentate ligands and from the theoretical calculations. A survey of the literature shows that there are no known isolated complexes of UO2(2+) with unidentate ligands which have a coordination number larger than 5. This was corroborated by quantum chemical calculations which showed that the energy gains by binding an additional water to UO2(H2O)4(2+) and UO2(H2O)5(2+) are 29.8 and -2.4 kcal/mol, respectively. A comparison of the change in deltaU for the reactions UO2(H2O)5(2+)--> UO2(H2O)4(2+) + H2O and UO2(H2O)5(2+) + H2O --> UO2(H2O)6(2+) indicates that the thermodynamics favors the second (associative) reaction in gas phase at 0 K, while the thermodynamics of water transfer between the first and second coordination spheres, UO2(H2O)5(2+) --> UO2(H2O)4(H2O)2+ and UO2(H2O)5(H2O)2+ --> UO2(H2O)6(2+), favors the first (dissociative) reaction. The energy difference between the associative and dissociative reactions is small, and solvation has to be included in ab initio models in order to allow quantitative comparisons between experimental data and theory. Theoretical calculations of the activation energy were not possible because of the excessive computing time required. On the basis of theoretical and experimental studies, we suggest that the water exchange in UO2(H2O)5(2+) follows a dissociative interchange mechanism. The rates of exchange of water in UO2(oxalate)F(H2O)2- (and UO2(oxalate)F2(H2O)2- studied previously) are much slower than in the aqua ion, kex = 1.6 x 10(4) s(-1), an effect which we assign to hydrogen bonding involving coordinated water and fluoride. The kinetic parameters for the exchange of water in UO2(H2O)52+ and quenching of photo excited *UO2(H2O)5(2+) are very near the same, indicating similar mechanisms.     

A Raman and infrared spectroscopic study of the uranyl silicates--weeksite, soddyite and haiweeite: part 2

Raman spectroscopy has been used to study the molecular structure of a series of selected uranyl silicate minerals including weeksite K2[(UO2)2(Si5O13)].H2O, soddyite [(UO2)2SiO4.2H2O] and haiweeite Ca[(UO2)2(Si5O12(OH)2](H2O)3 with UO2(2+)/SiO2 molar ratio 2:1 or 2:5. Raman spectra clearly show well resolved bands in the 750-800 cm(-1) region and in the 950-1000 cm(-1) region assigned to the nu1 modes of the (UO2)2+ units and to the (SiO4)4- tetrahedra. Soddyite is characterized by Raman bands at 828.0, 808.6 and 801.8 cm(-1), 909.6 and 898.0 cm(-1), and 268.2, 257.8 and 246.9 cm(-1), attributed to the nu1, nu3, and nu2 (delta) (UO2)2+, respectively. Coincidences of the nu1 (UO2)2+ and the nu1 (SiO4)4- is expected. Bands at 1082.2, 1071.2, 1036.3, 995.1 and 966.3 cm(-1) are attributed to the nu3 (SiO4)4-. Sets of Raman bands in the 200-300 cm(-1) region are assigned to nu2 (delta) (UO2)2+ and UO ligand vibrations. Multiple bands indicate the non-equivalence of the UO bonds and the lifting of the degeneracy of nu2 (delta) (UO2)2+ vibrations. The (SiO4)4- tetrahedral are characterized by bands in the 470-550 cm(-1) and in the 390-420 cm(-1) region. These bands are attributed to the nu4 and nu2 (SiO4)4- bending modes. The minerals show characteristic OH stretching bands in the 2900-3500 and 3600-3700 cm(-1).     

Isolation of a tetrameric cation-cation complex of pentavalent uranyl

A polymetallic assembly containing mutually coordinated highly reactive UO2+ groups was isolated in the presence of dibenzoylmethanate. NMR studies showed unambiguously the presence of the cation-cation complex in pyridine solution while more polar solvents lead to the disruption of the UO2+/UO2+ interaction and increased stability.     

Enhancement of hydrolysis through the formation of mixed hetero-metal species: dioxouranium(VI)-cadmium(II) mixtures

In order to continue the investigation on the formation of hetero-metal polynuclear hydrolytic species, in this paper we report some results (at I = 0.16 mol L(-1) in NaNO3, at t = 25 degrees C by potentiometry, ISE-H+, glass electrode) on the hydrolysis of several mixtures (in different ratios) of the dioxouranium(VI) and cadmium(II) cations. The same experimental and calculation procedure of previously investigated systems was followed, and all measurements were performed by two different operators, using completely independent instruments and reagents. Many different speciation models were considered in the calculations, and a simple statistical analysis of obtained results was proposed too. UO2(2+) and Cd2+ form two hetero-metal polynuclear hydrolytic species, namely UO2Cd(OH)3+ and (UO2)2Cd(OH)4(2+), with logbeta(pqr) = -3.25 +/- 0.25 and -13.75 +/- 0.10, respectively. The formation of hetero-metal hydrolytic species is thermodynamically favored with respect to the homo-metal ones, and causes an enhancement of the percentage of hydrolyzed metal cations; comparisons with previously studied systems reveal that the hydrolytic behavior of UO2(2+)/Cd2+ mixtures is more similar to that observed for UO2(2+)/Cu2+ than for UO2(2+)/(C2H5)2Sn2+, and the tendency to form hetero polynuclear hydrolytic species with dioxouranium(VI) by other cations follows the trend (C2H5)2Sn2+ > Cu2+ > or = Cd2+.     

基于非血红素铁模型配合物[FeⅣ(O)(N4Py)]2+的新型N杂环卡宾配合物的理论设计

基于四价非血红素铁模型配合物[FeⅣ(O)(N4Py)]2+, 通过理论计算设计出一种新型N杂环卡宾配合物[FeⅣ(O)(N4Py)]2+. 采用密度泛函理论B3LYP方法, 计算了[FeⅣ(O)(N4Py)]2+的几何结构和电子结构, 并研究了[FeⅣ(O)(N4Py)]2+使环己烷C-H键羟基化的反应机理. 结果表明, [FeⅣ(O)(N4Py)]2+的五重态能量比基态三重态能量高约5.7 kJ/mol, 故五重态几乎不能参与反应. 赤道方向的配位基N杂环卡宾(NHC)对FeO单元的σ-贡献要大于N4Py的贡献, 而它的空间位阻效应也大于N4Py, 因此2+的稳定性强于[FeⅣ(O)(N4Py)]2+. [FeⅣ(O)(N4Py)]2+的三重态的反应能垒比[FeⅣ(O)(N4Py)]2+的三重态反应能垒高2.0 kJ/mol, 且为单态反应, 所以[FeⅣ(O)(N4Py)]2+的反应活性要高于[FeⅣ(O)(N4Py)]2+.     

UO22+对某些细胞毒性的初探

建立了由多种金属离子和小分子配体组成的多相细胞液热力学平衡模型,模拟研究了UO22+在组织液和细胞液的形态。体外培养了SD大鼠成骨细胞和人肾小管上皮细胞,通过体外细胞生长抑制实验探索了UO22+对成骨细胞及肾小管上皮细胞的毒性。研究表明,在细胞液中,当各形态UO22+物质总浓度[U]=8.4×10-6mol/L时,当pH为6.0～6.5时,UO22+主要以固相(UO2)3(PO4)2存在,当pH为6.8～7.5时,UO22+主要以水溶性[UO2(CO3)3]4-存在;当[U]=1.3×10-3mol/L时,在整个细胞液pH范围内,固相(UO2)3(PO4)2占主导地位。体外细胞生长抑制实验表明,UO22+对成骨细胞的生长具有抑制作用,能显著降低肾小管上皮细胞的存活率,具有明显的细胞毒性。     

Potentiometric and multinuclear NMR study of the binary and ternary uranium(VI)-L-fluoride systems, where L is alpha-hydroxycarboxylate or glycine

Equilibria, structures, and ligand-exchange dynamics in binary and ternary U(VI)-L-F- systems, where L is glycolate, alpha-hydroxyisobutyrate, or glycine, have been investigated in 1.0 M NaClO4 by potentiometry and 1H, 17O, and 19F NMR spectroscopy. L may be bonded in two ways: either through the carboxylate end or by the formation of a chelate. In the glycolate system, the chelate is formed by proton dissociation from the alpha-hydroxy group at around pH 3, indicating a dramatic increase, a factor of at least 10(13), of its dissociation constant on coordination to uranium(VI). The L exchange in carboxylate-coordinated UO2LF3(2-) follows an Eigen-Wilkins mechanism, as previously found for acetate. The water exchange rate, k(aq) = 4.2 x 10(5) s(-1), is in excellent agreement with the value determined earlier for UO2(2+)(aq). The ligand-exchange dynamics of UO2(O-CH2-COO)2F3- and the activation parameters for the fluoride exchange in D2O (k(obs) = 12 s(-1), deltaH(double dagger) = 45.8 +/- 2.2 kJ mo(-1), and deltaS(double dagger) = -55.8 +/- 3.6 J K(-1) mol(-1)) are very similar to those in the corresponding oxalate complex, with two parallel pathways, one for fluoride and one for the alpha-oxocarboxylate. The same is true for the L exchange in UO2(O-CH2-COO)2(2-) and UO2(oxalate)2(2-). The exchange of alpha-oxocarboxylate takes place by a proton-assisted chelate ring opening followed by dissociation. Because we cannot decide if there is also a parallel H+-independent pathway, only an upper limit for the rate constant, k1 < 1.2 s(-1), can be given. This value is smaller than those in previously studied ternary systems. Equilibria and dynamics in the ternary uranium(VI)-glycine-fluoride system, investigated by 19F NMR spectroscopy, indicate the formation of one major ternary complex, UO2LF3(2-), and one binary complex, UO2L2 (L = H2N-CH2COO-), with chelate-bonded glycine; log beta(9) = 13.80 +/- 0.05 for the equilibrium UO2(2+) + H2N-CH2COO- + 3F- = UO2(H2N-CH2COO)F3(2-) and log beta(11) = 13.0 +/- 0.05 for the reaction UO2(2+) + 2H2N-CH2COO- = UO2(H2N-CH2COO)2. The glycinate exchange consists of a ring opening followed by proton-assisted steps. The rate of ring opening, 139 +/- 9 s(-1), is independent of both the concentration of H+ and the solvent, H2O or D2O.     

Determination of the equilibrium formation constants of two U(VI)-peroxide complexes at alkaline pH

The formation of uranyl-peroxide complexes was studied at alkaline media by using UV-Visible spectrophotometry and the STAR code. Two different complexes were found at a H(2)O(2)/U(VI) ratio lower than 2. A graphical method was used in order to obtain the formation constants of such complexes and the STAR program was used to refine the formation constants values because of its capacity to treat multiwavelength absorbance data and refining equilibrium constants. The values obtained for the two complexes identified were: UO(2)(2+) + H(2)O(2) + 4OH(-) UO(2)(O(2))(OH)(2)(2-) + 2H(2)O: log β°(1,1,4) = 28.1 ± 0.1 (1). UO(2)(2+) + 2H(2)O(2) + 6OH(-) UO(2)(O(2))(2)(OH)(2)(4-) + 4H(2)O: log β°(1,2,6) = 36.8 ± 0.2 (2). At hydrogen peroxide concentrations higher than 10(-5) mol dm(-3), and in the absence of carbonate, the UO(2)(O(2))(2)(OH)(2)(4-) complex is predominant in solution, indicating the significant peroxide affinity of peroxide ions for uranium and the strong complexes of uranium(VI) with peroxide.     

堆积模型在萃取中的应用I: 水相介质协同效应

根据推积模型提出一种新的协萃体系,即:(简单阴离子)1+(简单阴离子)2+萃取剂,并以实验证实了水相混合介质的协同效应.研究了UO2/OAc,C1/TBP-二甲苯体系的协萃效应,测定了萃合物的组成以及各种影响分配比的因素.     

Dioxouranium(VI)-carboxylate complexes. Speciation of UO2(2+)-1,2,3-propanetricarboxylate system in NaCl(aq) at different ionic strengths and at t=25 degrees C

The formation constants of dioxouranium(VI)-1,2,3-propanetricarboxylate [tricarballylate (3-), TCA] complexes were determined in NaCl aqueous solutions at 0 < or = I/mol L(-1) < or = 1.0 and t=25 degrees C, by potentiometry, ISE-[H+] glass electrode. The speciation model obtained at each ionic strength includes the following species: ML-, MLH0, ML2(4-) and ML2H3- (M = UO2(2+) and L = TCA). The dependence on ionic strength of protonation constants of 1,2,3-propanetricarboxylate and of the metal-ligand complexes was modeled by the SIT (Specific ion Interaction Theory) approach and by the Pitzer equations. The formation constants at infinite dilution are [for the generic equilibrium p UO22+ + q (L3-) + r H+ = (UO2(2+))p(L)qHr(2p-3q+r); betapqr]: log beta110 = 6.222 +/- 0.030, log beta111 = 11.251 +/- 0.009, log beta121 = 7.75 +/- 0.02, log beta121 = 14.33 +/- 0.06. The sequestering ability of 1,2,3-propanetricarboxylate towards UO2(2+) was quantified by using a sigmoid Boltzman type equation.     

A density functional study of the various forms of UN4O12 containing uranyl nitrate

In this paper we report the computational results of a density functional study of 73 UN4O12 isomers containing uranyl nitrate, UO2(NO3)2, as a component. The isomers are grouped into three categories and 19 types. Forty-four isomers of 14 types are dinitrogen tetroxide adducts of uranyl nitrate, UO2(NO3)2.N2O4, 22 are nitrosonium salt adducts of uranyl nitrate, NO+UO2(NO3)3-, NO+UO2(NO3)2O(NO2)-, NO+UO2(NO3)2(ONOO)-, or (NO+)2UO2(NO3)2O22-, and 7 are bis(nitrogen dioxide) adducts of uranyl nitrate, UO2(NO3)2.2NO2. The 22 most stable isomers in solution, representing the 20 most stable gas-phase isomers, were selected for analysis. Of these selected structures only two categories and six types were represented. Structures, frequencies, gas-phase and solution energetics, atomic charges, dipole moments, and the bonding within the N2O4 unit and between NO+ and UO2(NO3)3- components have been analyzed in detail. On the basis of relative Gibbs free energy calculations five isomers (the N2O4 adducts a1, a2, and a3 and the nitrosonium salts b1 and b2) were identified as strong candidates to exist and possibly predominate in the gas phase, with a1 and a2 being the strongest candidates. Similarly, four isomers (a6, a5, a8, and a1, all of them N2O4 adducts) were identified as strong candidates to exist and possibly predominate in a nonaqueous solution of nitromethane/dinitrogen tetroxide. Of these, a6 was determined to be the most likely candidate to predominate in solution. The possibility of dissociation in solution has been addressed briefly. In addition, computational evidence for the existence of four new N2O4 isomers 20, 22, 27, and 28 in both the gas and the solution phases is presented for the first time.     

Local structure and charge distribution in the UO(2)-U(4)O(9) system

Analysis of X-ray absorption fine structure spectra of UO(2+x) for x = 0-0.20 (UO(2)--U(4)O(9)) reveals that the adventitious O atoms are incorporated as oxo groups with U--O distances of 1.74 A, most likely associated with U(VI), that occur in clusters so that the UO(2) fraction of the material largely remains intact. In addition to the formation of some additional longer U--O bonds, the U sublattice consists of an ordered portion that displays the original U--U distance and a spectroscopically silent, glassy part. This is very different from previous models derived from neutron diffraction that maintained long U--O distances and high U--O coordination numbers. UO(2+x) also differs from PuO(2+x) in its substantially shorter An-oxo distances and no sign of stable coordination with H(2)O and its hydrolysis products.