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| [[File:Uranyl-3D-balls.png|right|thumb|[[Ball-and-stick model]] of [UO<sub>2</sub>]<sup>2+</sup>]]
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| [[File:Uranyl-ion-structure.png|right|thumb|The uranyl ion, showing the U-O bond order of 3]]
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| The '''uranyl''' ion is an [[oxycation]] of [[uranium]] in the [[oxidation state]] +6, with the [[chemical formula]] [UO<sub>2</sub>]<sup>2+</sup>. It has a linear structure with short U-O bonds, indicative of the presence of [[multiple bond]]s between uranium and oxygen. Four or more [[ligand]]s are bound to the uranyl ion in an equatorial plane. The uranyl ion forms many [[complex (chemistry)|complexes]], particularly with ligands that have oxygen donor atoms. Complexes of the uranyl ion are important in the extraction of uranium from its ores and in [[nuclear fuel reprocessing]].
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| ==Structure and bonding==
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| [[File:F4M0.png|thumb|100px| <math>f_{z^3}</math> orbital]]
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| The uranyl ion is linear and symmetrical, with both U-O bond lengths of about 180 pm. The bond lengths are indicative of the presence of multiple bonding between the uranium and oxygen atoms. Since uranium(+6) has the [[electronic configuration]] [ [[radon]] ], the electrons used in forming the U-O bonds are supplied by the oxygen atoms. The electrons are donated into empty [[atomic orbitals]] on the uranium atom. The empty orbitals of lowest energy are 7s, 5f and 6d. In terms of [[valence bond theory]] the [[sigma bond]]s may be formed using <math>d_{z^2}</math> and <math>f_{z^3}</math> to construct sd, sf and df [[hybrid orbitals]] (the ''z'' axis passes through the oxygen atoms). (<math>d_{xz}</math>, <math>d_{yz}</math>) and (<math>f_{xz^2}</math> and <math>f_{yz^2}</math>) may be used to form [[pi bond]]s. Since the pair of d or f orbitals used in bonding are [[degenerate orbital|doubly degenerate]] this equates to an overall [[bond order]] of three.<ref name="cotton">{{cite book | author= Cotton, S | year= 1991 | title= Lanthanides and Actinides | location= New York | publisher= Oxford University Press | page= 128 }}</ref>
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| The uranyl ion is always associated with other ligands. The most common arrangement is for the so-called equatorial ligands to lie in a plane perpendicular to the O-U-O line and passing through the uranium atom. With four ligands, as in [UO<sub>2</sub>Cl<sub>4</sub>]<sup>2-</sup> the uranium has a distorted [[octahedral]] environment. In many cases there are more than four equatorial ligands. The presence of the equatorial ligands lowers the [[molecular symmetry|symmetry]] of the uranyl ion from point group D<sub>∞h</sub> for the isolated ion to, for example, D<sub>4h</sub> in a distorted octahedral complex; this permits the involvement of d and f orbitals other than those used in U-O bonds.
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| In [[uranyl fluoride]], UO<sub>2</sub>F<sub>2</sub>, the uranium atom achieves a [[coordination number]] of 8 by forming a layer structure with two oxygen atoms in a uranyl configuration and six fluoride ions bridging between uranyl groups. A similar structure is found in α-[[uranium trioxide]], with oxygen in place of fluoride, except that in that case the layers are connected by sharing oxygen atom from "uranyl groups", which are identified by having relatively short U-O distances. A similar structure occurs in some uranates, such as calcium uranate, CaUO<sub>4</sub>, which may be written as Ca(UO<sub>2</sub>)O<sub>2</sub> even though the structure does not contain isolated uranyl groups.<ref>{{cite book|last=Wells|first=A.F|title=Structural Inorganic Chemistry|edition=3rd.|year=1962|publisher=Clarendon Press|location=Oxford|page=966|isbn=0-19-855125-8}}</ref>
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| == Spectroscopy ==
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| The colour of uranyl compounds is due to LMCT [[charge transfer complex|charge transfer]] transitions at ca. 420 nm, on the blue edge of the [[visible spectrum]].<ref>{{cite journal |last=Umreiko |first=D.S. |year=1965 |title=Symmetry in the electronic absorption spectra of uranyl compounds|journal=J. Appl. Spectrosc. |volume=2|issue=5|pages=302–304|doi=10.1007/BF00656800 |bibcode=1965JApSp...2..302U}}</ref><ref>{{cite journal |last=Berto |first=Silvia |year=2006 |title=Dioxouranium(VI)-Carboxylate Complexes. Interaction with dicarboxylic acids in Aqueous Solution: Speciation and Structure |journal=Annali di Chimica |volume=96 |issue=7–8 |pages=399–420 |doi=10.1002/adic.200690042 |last2=Crea |first2=Francesco |last3=Daniele |first3=Pier G. |last4=De Stefano |first4=Concetta |last5=Prenesti |first5=Enrico |last6=Sammartano |first6=Silvio |pmid=16948430}}</ref> The exact location of the absorption band and [[NEXAFS]] bands depends on the nature of the equatorial ligands.<ref>{{cite journal|last=Fillaux|first=C.|coauthors=Guillaumont, D.; Berthet, J-C; Copping, R.; Shuh, D.K.; Tyliszczak, T.; Den Auwer, C.|year=2010|title=Investigating the electronic structure and bonding in uranyl compounds by combining NEXAFS spectroscopy and quantum chemistry|journal=Phys. Chem. Chem. Phys.|pmid=20886130|volume=12|issue=42|pages=14253–14262|doi=10.1039/C0CP00386G|bibcode = 2010PCCP...1214253F }}</ref> Compounds containing the uranyl ion are usually yellow, though some compounds are red, orange or green.
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| Uranyl compounds also exhibit [[luminescence]]. The first study of the green luminescence of [[uranium glass]], by [[David Brewster|Brewster]]<ref>{{cite journal |last=Brewster |first=D. |year=1849 |journal=[[Transactions of the Royal Society of Edinburgh]] |volume=16 |pages=111–121}}</ref> in 1849, began extensive studies of the spectroscopy of the uranyl ion. Detailed understanding of this spectrum was obtained 130 years later.<ref>{{cite journal |last=Denning |first=R. G. |year=2007 |title=Electronic Structure and Bonding in Actinyl Ions and their Analogs |journal=[[J. Phys. Chem. A]] |volume=111 |issue=20 |pages=4125–4143 |doi=10.1021/jp071061n |pmid=17461564}}</ref> It is now well-established that the uranyl luminescence is more specifically a [[phosphorescence]], as it is due to a transition from the lowest triplet excited state to the singlet ground state.<ref>{{cite book|last=V. Balzani and V. Carassiti |first=|title=Photochemistry of Coordination Compounds|edition=|series=|year=1970|publisher=Academic Press|isbn=0-12-077250-7}}</ref> The luminescence from K<sub>2</sub>UO<sub>2</sub>(SO<sub>4</sub>)<sub>2</sub> was involved in the discovery of [[radioactivity]].
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| The uranyl ion has characteristic ν U-O stretching [[molecular vibration|vibrations]] at ca. 880 cm<sup>−1</sup> ([[Raman spectrum]]) and 950 cm<sup>−1</sup> ([[infrared spectrum]]). These frequencies depend somewhat on which ligands are present in the equatorial plane. Correlations are available between the stretching frequency and U-O bond length. It has also been observed that the stretching frequency correlates with the position of the equatorial ligands in the [[spectrochemical series]].<ref>{{cite book|last=Nakamoto|first=K.|title=Infrared and Raman spectra of Inorganic and Coordination compounds|edition=5th|series=Part A|year=1997|publisher=Wiley|isbn=0-471-16394-5}}{{cite book|series=Part B|isbn=0-471-16392-9}} Part A, p 167. Part B, p 168</ref>
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| == Aqueous chemistry ==
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| [[File:Uranium fraction diagram with no carbonate.png|thumb|240px|Hydrolysis of uranium(VI) as a function of pH.|alt=A graph of potential vs. pH showing stability regions of various uranium compounds]]
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| The uranyl ion can be viewed as the end result of extensive hydrolysis of the highly charged, hypothetical, U<sup>6+</sup> cation.
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| :[U(H<sub>2</sub>O)<sub>n</sub>]<sup>6+</sup> → [UO<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]<sup>2+</sup> + 4H<sup>+</sup> + n-4 H<sub>2</sub>O
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| The driving force for this hypothetical reaction is the reduction in charge density on the uranium atom. The number of water molecules attached to the uranyl ion in aqueous solution is mostly five.<ref>{{cite journal|last=Hagberg D, Karlström G, Roos BO, Gagliardi L|first=|year=2005|title=The Coordination of Uranyl in Water: A Combined Quantum Chemical and Molecular Simulation Study|journal=J. Am. Chem. Soc.|volume=127|issue=41|pages=14250–14256|doi=10.1021/ja0526719|first1=Daniel|last2=Karlström|first2=Gunnar|last3=Roos|first3=Björn O.|last4=Gagliardi|first4=Laura|pmid=16218619}}</ref> Further hydrolysis occurs, with a further reduction in charge density when one or more equatorial water molecules is replaced by an [[hydroxide]] ion. In fact the aqueous uranyl ion is a [[weak acid]].
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| :[UO<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]<sup>2+</sup> {{eqm}} [UO<sub>2</sub>(H<sub>2</sub>O)<sub>3</sub>(OH) ]<sup>+</sup> + H<sup>+</sup>; [[acid dissociation constant|pK<sub>a</sub>]] = ca. 4.2<ref name=scdb>{{Cite web|url=http://www.acadsoft.co.uk/scdbase/scdbase.htm IUPAC SC-Database|title= A comprehensive database of published data on equilibrium constants of metal complexes and ligands|work=Academic Software}}</ref> | |
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| As pH increases polymeric species with stoichiometry [(UO<sub>2</sub>)<sub>2</sub>(OH)<sub>2</sub>]<sup>2+</sup> and [(UO<sub>2</sub>)<sub>3</sub>(OH)<sub>5</sub>]<sup>+</sup> are formed before the hydroxide UO<sub>2</sub>(OH)<sub>2</sub> precipitates. The hydroxide dissolves in strongly alkaline solution to give hydroxo complexes of the uranyl ion.
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| The uranyl ion can be [[reduction (chemistry)|reduced]] by mild reducing agents, such as zinc metal, to the oxidation state +4. Reduction to uranium(3+) can be done using a [[Jones reductor]].
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| === Complexes ===
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| [[File:Uranium fraction diagram with carbonate present.png|thumb|240px| Carbonate and hydoxo complexes of uranium(VI) as a function of pH]]
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| The uranyl ion behaves as a [[hsab theory|hard]] acceptor and forms weaker complexes with nitrogen-donor ligands than with fluoride and oxygen donor ligands, such as hydroxide, [[carbonate]], [[nitrate]], [[sulfate]] and [[carboxylate]]. There may be 4, 5 or 6 donor atoms in the equatorial plane. In uranyl nitrate, [UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>].2H<sub>2</sub>O, for example, there are six donor atoms in the equatorial plane, four from [[bidentate]] nitrato ligands and two from water molecules. The structure is described as [[hexagonal bipyramid]]al. Other oxygen-donor ligands include [[phosphine oxide]]s and [[phosphate ester]]s.<ref name=nng>{{Greenwood&Earnshaw2nd|pages=1273–1274}}</ref>
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| [[File:extracteduraniumcomplex.jpg|170px|thumb|left| UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>.2(triethylphosphate)]]
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| Uranyl nitrate, UO<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>, can be [[solvent extraction|extracted]] from aqueous solution into [[diethyl ether]]. The complex that is extracted has two nitrato ligands bound to the uranyl ion, making a complex with no electrical charge and also the water molecules are replaced by ether molecules, giving the whole complex notable [[hydrophobic]] character. Electroneutrality is the most important factor in making the complex soluble in organic solvents. The nitrate ion forms much stronger complexes with the uranyl ion than it does with [[transition metal]] and [[lanthanide]] ions. For this reason only uranyl and other actinyl ions, including the plutonyl ion, PuO<sub>2</sub><sup>2+</sup>, can be extracted from mixtures containing other ions. Replacing the water molecules that are bound to the uranyl ion in aqueous solution by a second, hydrophobic, ligand increases the solubility of the neutral complex in the organic solvent. This has been called a synergic effect.<ref>{{cite journal|last=Irving|first=H.M.N.H.|year=1965|title=Synergic Effects in Solvent Extraction|journal=Angewandte Chemie International Edition|volume=4|issue=1|pages=95–96|doi=10.1002/anie.196500951}}</ref>
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| The complexes formed by the uranyl ion in aqueous solution are of major importance both in the extraction of uranium from its ores and in nuclear fuel reprocessing. In industrial processes uranyl nitrate is extracted with [[tributyl phosphate]], (CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>O)<sub>3</sub>PO, TBP, as the preferred second ligand, and kerosene the preferred organic solvent. Later in the process, uranium is stripped from the organic solvent by treating it with strong nitric acid, which forms complexes such as [UO<sub>2</sub>(NO<sub>3</sub>)<sub>4</sub>]<sup>2-</sup> which are more soluble in the aqueous phase. Uranyl nitrate is recovered by evaporating the solution.<ref name=nng/>
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| == Minerals ==
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| The uranyl ion occurs in minerals derived from [[Uraninite|uranium ore]] deposits by water-rock interactions that occur in uranium-rich mineral seams. [[Tyuyamunite]] (Ca(UO<sub>2</sub>)<sub>2</sub>V<sub>2</sub>O<sub>8</sub>•8H<sub>2</sub>O), [[autunite]] (Ca(UO<sub>2</sub>)<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>•8-12H<sub>2</sub>O), [[torbernite]] (Cu(UO<sub>2</sub>)<sub>2</sub> (PO<sub>4</sub>)•8-12H<sub>2</sub>O) and [[uranophane]] (H<sub>3</sub>O)<sub>2</sub>Ca (UO<sub>2</sub>)<sub>2</sub>(SiO<sub>4</sub>)•3H<sub>2</sub>O) are examples of uranyl containing minerals. These minerals are of little commercial value as most uranium is extracted from [[pitchblende]].
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| == Uses ==
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| Uranyl salts are used to stain samples for electron and electromagnetic microscopy studies of DNA.<ref>{{cite journal | author = Zobel R., Beer M. | year = 1961 | title = ELECTRON STAINS : I. Chemical Studies on the Interaction of DNA with Uranyl Salts | url = http://www.jcb.org/cgi/content/abstract/10/3/335 | journal = J. Cell Biology | volume = 10 | issue = 3| pages = 335–346 | doi = 10.1083/jcb.10.3.335 }}</ref>
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| ==Health and environmental issues==
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| Uranyl salts are toxic and can cause severe [[renal insufficiency]] and [[acute tubular necrosis]]. Target organs include the [[kidney]]s, [[liver]], [[lungs]] and [[brain]].
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| Uranyl ion accumulation in tissues including gonocytes<ref>{{cite journal | author= Arfsten DP, Still KR, Ritchie GD | title= A review of the effects of uranium and depleted uranium exposure on reproduction and fetal development | journal= Toxicology and Industrial Health | year= 2001 | volume= 17 | pages=180–191 | doi= 10.1191/0748233701th111oa | pmid= 12539863 | issue= 5–10 }}</ref> produces [[congenital disorder]]s, and in white blood cells causes immune system damage.<ref>{{cite journal | author= Schröder H, Heimers A, Frentzel-Beyme R, Schott A, Hoffman W | title= Chromosome Aberration Analysis in Peripheral Lymphocytes of Gulf War and Balkans War Veterans | journal= Radiation Protection Dosimetry | year= 2003 | volume= 103 | pages= 211–219 | pmid= 12678382 | issue= 3 | url=http://www.cerrie.org/committee_papers/INFO_9-H.pdf | doi= 10.1093/oxfordjournals.rpd.a006135}}</ref> Uranyl compounds are also [[neurotoxin]]s. Uranyl ion contamination has been found on and around [[depleted uranium]] targets.<ref>{{cite journal | author= Salbu B, Janssens K, Linda OC, Proost K, Gijsels L, Danesic PR | title= Oxidation states of uranium in depleted uranium particles from Kuwait | journal= Journal of Environmental Radioactivity | year= 2004 | volume= 78 | pages= 125–135 | doi= 10.1016/j.jenvrad.2004.04.001 | pmid= 15511555 | issue= 2 }}</ref>
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| All uranium compounds are [[radioactive]]. However, uranium is usually in depleted form, except in the context of the nuclear industry. Depleted uranium consists mainly of [[Isotopes of uranium|<sup>238</sup>U]] which decays by [[alpha decay]] with a half-life of 4.468(3)×10<sup>9</sup> y. Even if the uranium contained [[Isotopes of uranium|<sup>235</sup>U]] which decays with a similar half-life of about 7,038×10<sup>8</sup> y, both of them would still be regarded as weak alpha emitters and their radioactivity is only hazardous with direct contact or ingestion.
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| == References ==
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| {{reflist}}
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| [[Category:Uranyl compounds| ]]
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| [[Category:Oxycations]]
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