Other Oxidation State Uranyl

As already shown, DFT methods can be used to describe and explain the reactivity of actinide complexes, even when SET is involved. Interestingly, it is worth noting that DFT allowed theoretical treatment of the experimental systems. In particular, the geometry and 

In these cases, interaction between actinides and the ligands were analyzed [74] and confirmed the participation of actinides 5f orbitals for chemical bonding, leading to new modes of actinide-ligand interactions. DFT was also used in order to study the reactivity 

As already mentioned earlier, the chemistry of high-valent uranium is dominated by the dioxo or uranyl dication, [U02] +, which is found both in aqueous solutions and in the solid state. It is chemically robust and shows little propensity to participate in the myriad reactions that are characteristic of its Group 6 transition metal analogues, [M02] . Furthermore, this stability coupled with its mobility in the aqueous phase means that it is a problematic environmental contaminant. There is in the literature a long range of experimental and theoretical study on actinyl, and particularly uranyl. [46,77] All of these theoretical and computational studies are focused on structure and electronic properties of actinyl using DPT and post Hartree-Fock calculations methods. But often, the theoretical studied systems are not corresponding to the real experimental one. With the increase of computational power, it is now possible to treat full experimental systems, and even more to study the reactivity of these species at the DFT level adequately. 

A theoretical investigation of the reductive oxo-group silylation reaction of the uranyl dication held in a Pacman macrocylic environment has been carried out by us. [78] The effect of the modeling of the Pacman ligand on the reaction profiles is found to be important, with the dipotassiation of a single oxo group identified as a key component in promoting the reaction between the Si X and uranium-oxo bonds. This reductive silylation reaction 

Uranium (III) redox reactivity with small molecules always consists in the preliminary double reduction of the latter by two U(III) complexes. Thus, the reactivity concerns only U(IV) bimetallic complexes. DFT calculations can be used for this problem thanks to the recent development of 5f-in-core ECPs by Moritz et al. and a methodology to compute the preliminary redox step via the combination of small core and 5f-in-core calculations. Recent theoretical studies have mainly concerned the reduction of CO2 and CO, but attention has also to be paid to the reduction of CS2, COS, PhNCO, and PhNs, the C-C coupling of terminal bis-alkynes to form U(IV) vinyl complexes and the reduction of arenes. However, in order to have more elements to discuss about actinide properties, one must perform calculations at a multireference post Hartree-Fock level to take into account the important effect of electronic correlation in these systems however, there is the obstacle of the computational power to perform calculations of real systems at this level that stands still. 

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Among the elements known before transuranium elements started to be synthesized in 1940, uranium has a unique characteristic, the extreme stability of the triatomic uranyl ion OUO+z. Not only are the numbers of uranium(VI) compounds larger than of U(IV), and far larger than of the two other oxidation states U(V) and U(III) known from non-metallic compounds, but until the preparation of UOFj discussed below, the only two U(VI) compounds known to contain less than two oxygen atoms per uranium atom were the octahedral molecules UFe and UQ6. 

Uranium can be dissolved in dilute acids, when uranium(IV) ions, U4+, and hydrogen gas are formed. Uranium(IV) ions can easily be oxidized to the hexavalent state, which is the most stable oxidation state of uranium. At this oxidation state, depending on the pH of the solution, two ions can be formed the uranyl cation, UO +, is stable in acid solutions, while the diuranate anion, u2or, in alkaline media. The two ions are in equilibrium with each other ... 

Uranium occurs in sea water in its highest oxidation state +6 owing to the carbonate content of sea water, uranium predominantly should exist in sea water as the tricarbonato uranylate anion [U02(C03)3]4 , an extremely stable complex with a formation constant of log ji = 22.6. However, there is no experimental evidence for the occurrence of this complex ion in natural sea water due to its extremely low concentration. According to equilibrium constants also other uranium species are expected to occur in sea water (Table 2). 

The situation is similar in actinide compounds but here the 5f-orbitals play a more active role and contribute to the chemical bond. Most applications have so far been to systems with a high oxidation state of the actinide ion. A typical case is the uranyl ion, UO + where the uranium ion has a formal charge of - -6. As a result, three strongly covalent bonds are formed to each of the oxygen atoms. The resulting active space consists of 12 electrons in 12 orbitals [43]. This active space can also be used when the uranyl ion forms complexes with other ligands, such as carbonate [44]. Additional active orbitals are needed for the neutral UO2 molecule [45]. Wahlgren and co-workers have... 

The most common oxides are uranium dioxide (UO2), uranium trioxide (UO3), triuranium octoxide (UjOg), and uranyl peroxide (UO4 or UO2 Oj). The main properties of these compounds are summarized in Table 1.5. It should be noted that there are several other oxides in which the oxidation states are not well defined as either tetra-valent or hexavalent, like UO2.1, U2O5, U3O7, U12O35, etc. Detailed discussions of the uranium-oxygen system and its complex phase diagram were presented elsewhere, for example, by Allen and Tempest (1982) and Grenthe (2006). 

Measurements of the electrode potentials in NaCl-2CsCl and NaCl-KCl melts showed that, by the end of the experiments, the system had reached equilibrium. On the other hand, the data in Table 6.13.2 indicate that the average oxidation state of uranium in the melt was below five and in some experiments the final melts contained essentially only uranyl(V) species. This means that in the melt at equilibrium with uranium dioxide some uranyl(V) ions are present. As the total uranium concentration in the melt decreases the ratio of uranyl(V)-to-uranyl(VI) concentrations in the melt increases, and at the low uranium content the melt equilibrated with uranium dioxide would contain essentially only uranyl(V) ions. 

General. For the most part, studies of aqueous solutions of fissile materials for use in homogeneous reactors have dealt with hexavalent uranium salts of the strong mineral acids. Hexavalent uranium in aqueous solutions appears as the divalent uranyl ion, IJOj Tetravalcnt uranium salts in aqueous solutions arc relatively unstalile, being oxidized to the hexavalent condition in the presence of air. Other valence states of uranium either disproportionate or form very insoluble compounds and have not been seriously proposed as fuel solutes. 

TJew references to uranium borates appear in the literature. Larson (12) reported that yellow crystals, whose composition was assumed to be 3UO3 B2O3 (uranyl orthoborate), were obtained among other products from a melt of uranium niobate in boric oxide. Bruhat and Dubois (2) stated that perborate solutions react with uranium oxide to give an anhydrous stable yellow salt of the composition UBO4. No further information has appeared on either of these compounds. 

As mentioned earlier, the two most common and stable types of uranium compounds are those in which uranium is in the tetravalent and hexavalent states. As far as the NFC is concerned, the binary oxides, binary fluorides, and oxyfluorides are of major importance, although several other compounds (e.g., uranyl nitrate, uranyl sulfate, and ammonium uranates) also play a role in the processing and handling of uranium (Table 1.5). Uranium metal and especially uranium alloys also play a significant role in commercial and military applications of uranium. 

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