Theuranyl ion is anoxycation ofuranium having the formulaUO2+
2; it is the most common form of uranium(VI). Uranyl is linear with two short U–O bonds of 180picometers.[1] Some important uranyl compounds areuranyl nitrate and severaluranyl chlorides.
The uranyl ion is linear and symmetrical, specifically belonging to the D∞h point group,[2] having both U–O bond lengths about 180 pm. The bond lengths are indicative of the presence of multiple bonding between the uranium and oxygen atoms. Since uranium(VI) has theelectronic configuration of the precedingnoble gas,radon, the electrons used in forming the U–O bonds are supplied by the oxygen atoms. The electrons are donated into emptyatomic orbitals on the uranium atom. The empty orbitals of lowest energy are 7s, 5f and 6d. In terms ofvalence bond theory, thesigma bonds may be formed using dz2 and fz3 to construct sd, sf and dfhybrid orbitals (thez-axis passes through the oxygen atoms). (dxz, dyz) and (fxz2 and fyz2) may be used to formpi bonds. Since the pair of d or f orbitals used in bonding aredoubly degenerate, this equates to an overallbond order of three.[3]
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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 [UO2Cl4]2−, the uranium has a distortedoctahedral environment. In many cases more than four ligands occupy the equator.[5]
Inuranyl fluoride, UO2F2, the uranium atom achieves acoordination 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 someuranates, such as calcium uranate, CaUO4, which may be written as Ca(UO2)O2 even though the structure does not contain isolated uranyl groups.[6]
The colour of uranyl compounds is due to ligand-to-metal charge transfer transitions at ca. 420 nm, on the blue edge of thevisible spectrum.[7][8] The exact location of the absorption band andNEXAFS bands depends on the nature of the equatorial ligands.[9] Compounds containing the uranyl ion are usually yellow, though some compounds are red, orange or green.[10]
Uranyl compounds also exhibitluminescence. The first study of the green luminescence ofuranium glass, byBrewster[11] in 1849, began extensive studies of the spectroscopy of the uranyl ion. Detailed understanding of this spectrum was obtained 130 years later.[12] It is now well-established that the uranyl luminescence is more specifically aphosphorescence, as it is due to a transition from the lowest triplet excited state to the singlet ground state.[13] The luminescence from K2UO2(SO4)2 was involved in the discovery ofradioactivity.[citation needed]
The uranyl ion has characteristicνU–O stretchingvibrations at ca. 880 cm−1 (Raman spectrum) and 950 cm−1 (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 thespectrochemical series.[14]
The aqueous uranyl ion is aweak acid.
As pH increases polymeric species with stoichiometry [(UO2)2(OH)2]2+ and [(UO2)3(OH)5]+ are formed before the hydroxide UO2(OH)2 precipitates. The hydroxide dissolves in strongly alkaline solution to give hydroxo complexes of the uranyl ion.[citation needed]
The uranyl ion can bereduced by mild reducing agents, such as zinc metal, to the oxidation state +4. Reduction to uranium(III) can be done using aJones reductor.[citation needed]
Though the oxygen ligands of the uranyl group are often treated as inert, this is not entirely the case.[16]
The uranyl ion behaves as ahard acceptor and forms weaker complexes with nitrogen-donor ligands than with fluoride and oxygen donor ligands, such as hydroxide,carbonate,nitrate,sulfate andcarboxylate. There may be 4, 5 or 6 donor atoms in the equatorial plane. In uranyl nitrate, [UO2(NO3)2]·2H2O, for example, there are six donor atoms in the equatorial plane, four frombidentate nitrato ligands and two from water molecules. The structure is described ashexagonal bipyramidal. Other oxygen-donor ligands includephosphine oxides andphosphate esters.[17]As discovered byChristian Friedrich Bucholz already in 1805,[18] uranyl nitrate, UO2(NO3)2, can beextracted from relatively concentrated aqueous solutions intodiethyl 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 notablehydrophobic 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 withtransition metal andlanthanide ions. For this reason only uranyl and other actinyl ions, including theplutonyl ion,PuO2+
2, 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.[19]
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 withtributyl phosphate (TBP, (CH3CH2CH2CH2O)3PO) 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 [UO2(NO3)4]2− which are more soluble in the aqueous phase. Uranyl nitrate is recovered by evaporating the solution.[17]
The uranyl ion occurs in minerals derived fromuranium ore deposits by water-rock interactions that occur in uranium-rich mineral seams. Examples of uranyl containing minerals include:[citation needed]
These minerals are of little commercial value as most uranium is extracted frompitchblende.
Uranyl salts are used to stain samples for electron and electromagnetic microscopy studies of DNA.[20] Some uranyl complexes have also emerged as visible-light catalysts for the selective fluorination of unactivated C-H bonds, which is of utility in organic synthesis, pharmaceutical, agricultural, and materials chemistry.[21]
Uranyl salts are toxic and can cause severechronic kidney disease andacute tubular necrosis. Target organs include thekidneys,liver,lungs andbrain. Uranyl ion accumulation in tissues including gonocytes[22] producescongenital disorders, and in white blood cells causes immune system damage.[23] Uranyl compounds are alsoneurotoxins. Uranyl ion contamination has been found on and arounddepleted uranium targets.[24]
All uranium compounds areradioactive. However, uranium is usually in depleted form, except in the context of the nuclear industry. Depleted uranium consists mainly of238U which decays byalpha decay with a half-life of4.468(3)×109 years. Even if the uranium contained235U which decays with a similar half-life of about7.038×108 years, both of them would still be regarded as weak alpha emitters and their radioactivity is only hazardous with direct contact or ingestion.[citation needed]
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