Uranyl

Source: Wikipedia, the free encyclopedia.
Not to be confused with Urinal.
Ball-and-stick model of UO2+
2
The uranyl ion, showing the U–O bond order of 3

The uranyl ion is an

nuclear fuel reprocessing
.

Structure and bonding

fz3 orbital

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(VI) has the

doubly degenerate, this equates to an overall bond order of three.[1]

pm, these U-O bonds are much longer than the U=O bonds of the uranyl center.[2]

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 distorted

octahedral environment. In many cases more than four ligands occupy the equator.[citation needed
]

In uranyl fluoride, UO2F2, 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, CaUO4, which may be written as Ca(UO2)O2 even though the structure does not contain isolated uranyl groups.[3]

Spectroscopy

The colour of uranyl compounds is due to ligand-to-metal charge transfer transitions at ca. 420 nm, on the blue edge of the

NEXAFS bands depends on the nature of the equatorial ligands.[6] Compounds containing the uranyl ion are usually yellow, though some compounds are red, orange or green.[citation needed
]

Uranyl compounds also exhibit

radioactivity.[citation needed
]

The uranyl ion has characteristic νU–O stretching

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.[10]

Aqueous chemistry

A graph of potential versus pH showing stability regions of various uranium compounds
Hydrolysis of uranium(VI) as a function of pH.

The aqueous uranyl ion is a

weak acid
.

[UO2(H2O)4]2+ ⇌ [UO2(H2O)3(OH)]+ + H+;  pKa = ca. 4.2[11]

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 be

reduced by mild reducing agents, such as zinc metal, to the oxidation state +4. Reduction to uranium(III) can be done using a Jones reductor.[citation needed
]

Complexes

Carbonate and hydoxo complexes of uranium(VI) as a function of pH

The uranyl ion behaves as a

phosphate esters.[12]
As discovered by
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, 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.[14]

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 (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.[12]

Minerals

The uranyl ion occurs in minerals derived from uranium 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 from

pitchblende
.

Uses

Uranyl salts are used to stain samples for electron and electromagnetic microscopy studies of DNA.[15]

Health and environmental issues

Uranyl salts are toxic and can cause severe

congenital disorders, and in white blood cells causes immune system damage.[17] Uranyl compounds are also neurotoxins. Uranyl ion contamination has been found on and around depleted uranium targets.[18]

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 238U which decays by alpha decay with a half-life of 4.468(3)×109 years. Even if the uranium contained 235U which decays with a similar half-life of about 7.038×108 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
]

References

  1. ^ Cotton, S (1991). Lanthanides and Actinides. New York: Oxford University Press. p. 128.
  2. doi:10.1021/ic50096a021
    .
  3. ISBN 0-19-855125-8
    .
  4. S2CID 96229881
    .
  5. PMID 16948430
    .
  6. PMID 20886130
    .
  7. S2CID 94834106
    .
  8. PMID 17461564
    .
  9. ISBN 0-12-077250-7
    .
  10. ISBN 0-471-16392-9
    .
  11. ^ "IUPAC SC-Database: A comprehensive database of published data on equilibrium constants of metal complexes and ligands". Academic Software. Archived from the original on 2020-05-09. Retrieved 2011-01-27.
  12. ^
    ISBN 978-0-08-037941-8
    .
  13. ^ Neues allgemeines Journal der Chemie (in German). Frölich. 1805.
  14. doi:10.1002/anie.196500951
    .
  15. PMID 13788706
    .
  16. S2CID 25310165
    .
  17. PMID 12678382. Archived from the original
    (PDF) on 2014-01-08. Retrieved 2014-01-08.
  18. PMID 15511555
    .
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