Uranyl

of The uranyl ion is an oxycation of uranium having the formula ; it is the most common form of uranium(VI). Uranyl is linear with two short U–O bonds of 180 picometers. Some important uranyl compounds are uranyl nitrate and several uranyl chlorides.

of ]] The uranyl ion is an oxycation of uranium having the formula ; it is the most common form of uranium(VI). Uranyl is linear with two short U–O bonds of 180 picometers. Some important uranyl compounds are uranyl nitrate and several uranyl chlorides. ==Structure and bonding== The uranyl ion is linear and symmetrical, specifically belonging to the D ∞h point group, 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 the electronic configuration of the preceding noble gas, 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 bonds may be formed using d z 2 and f z 3 to construct sd, sf and df hybrid orbitals (the z-axis passes through the oxygen atoms). (d xz , d yz ) and (f xz 2 and f yz 2 ) may be used to form pi bonds. Since the pair of d or f orbitals used in bonding are doubly degenerate, this equates to an overall bond order of three. dihydrate (UO 2 (H 2 O) 2 (NO 3 ) 2 ). In the uranyl group, the O=U=O angle is linear. In the equatorial plane of the complex are six U-O bonds to bidentate nitrate and two water ligands. At 245-151 pm, these U-O bonds are much longer than the U=O bonds of the uranyl center. ]] 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 2 Cl 4 ] 2− , the uranium has a distorted octahedral environment. In many cases more than four ligands occupy the equator. In uranyl fluoride, UO 2 F 2 , 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 4 , which may be written as Ca(UO 2 )O 2 even though the structure does not contain isolated uranyl groups. [UO 2 (H 2 O) 4 ] 2+ [UO 2 (H 2 O) 3 (OH)] + + H + ; pK a = ca. 4.2 As pH increases, polymeric species with stoichiometry [(UO 2 ) 2 (OH) 2 ] 2+ and [(UO 2 ) 3 (OH) 5 ] + are formed before the hydroxide species UO 2 (OH) 2 precipitates. The hydroxide dissolves in strongly alkaline solution to give hydroxo complexes of the uranyl ion. The uranyl ion can be reduced by mild reducing agents, such as zinc metal, to the +4 oxidation state. Reduction to uranium(III) can be achieved using a Jones reductor. ==Health and environmental issues== Uranyl salts are toxic and can cause severe chronic kidney disease and acute tubular necrosis. Target organs include the kidneys, liver, lungs and brain. Uranyl ion accumulation in tissues including gonocytes produces congenital disorders, and in white blood cells causes immune system damage. Uranyl compounds are also neurotoxins. Uranyl ion contamination has been found on and around depleted uranium targets. 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 238 U which decays by alpha decay with a half-life of . Even if the uranium contained 235 U which decays with a similar half-life of about , 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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