uranyl

Ball-and-stick model of [UO₂]²⁺

The uranyl ion, showing the U-O bond order of 3

Diagram of a uranyl ion. The formation of two short U-O bonds in the uranyl ion prevents the closer approach of a third oxygen atom.^[1]

The uranyl ion is the dipositive cation [UO₂]²⁺, which forms salts with acids. In this ion, uranium is in its +6 oxidation state. The other common oxidation state of uranium is uranium(IV), called uranous. The uranyl ion is the most common species encountered in the aqueous chemistry of uranium. Solid uranyl compounds are often colored red, yellow, orange or green. Like all uranium compounds, uranyl compounds are toxic. The toxicity of soluble uranyl salts is higher due to their faster incorporation into tissues.

Usage

Uranyl generally refers to uranyl(VI), the oxycation of uranium in the +6 oxidation state. There is also uranyl(V), oxycation of uranium in +5 oxidation state. However these compounds are quite uncommon, and in general, uranyl can be assumed to be the +6 oxidation state.

Examples

Examples of uranyl compounds include:

• Uranium trioxide, UO₃
• Uranyl acetate, UO₂(C₂H₃O₂)₂
• Uranyl ammonium carbonate, UO₂CO₃·2(NH₄)₂CO₃
• Uranyl carbonate, UO₂CO₃
• Uranyl chloride, UO₂Cl₂
• Uranyl fluoride, UO₂F₂
• Uranyl hydroxide, UO₂(OH)₂, also (UO₂)₂(OH)₄ in aqueous solution
• Uranyl nitrate, UO₂(NO₃)₂
• Uranyl peroxide, UO₂O₂ or UO₄
• Uranyl sulfate, UO₂SO₄
• Uranyl zinc acetate, ZnUO₂(CH₃COO)₄

See also Diuranate.

Minerals

Such minerals occur in oxidised portions of uranium ore deposits. Common uranyl minerals include tyuyamunite (Ca(UO₂)₂V₂O₈·8H₂O), autunite (Ca(UO₂)₂(PO₄)₂·8-12H₂O), torbernite (Cu(UO₂)₂ (PO₄)·8-12H₂O) and uranophane (H₃O)₂Ca (UO₂)₂(SiO₄)·3H₂O) (Hutchinson and Blackwell, 1984). Uranyl minerals, which contain uranium(VI) can help show the genesis of uranium deposits and the water-rock interactions that occur in uranium-rich mineral seams.

Chemistry

Uranium chemistry has traditionally revolved around the aqueous chemistry of the uranyl ion, and related molecular species. One important use of this chemistry is for preparation of uranium dioxide ceramic pellets that are used as the fuel in light water nuclear reactors. Often the fuel materials start to break down chemically before the uranium is completely spent, and this too is an active area of investigation, as many of the corrosion products are of the uranyl group.

Uses

Partially soluble uranyl salts such as uranyl acetate are used for microscopy stains. These stains are used for electron and electromagnetic microscopy studies of DNA.^[2]

Structure and Bonding

The close approach in the uranyl ion of the two oxygen atoms to the uranium atom, with each linear O-U-O bond from 1.7 to 1.9 Å, prevents the close approach to the uranium atom of additional oxygen atoms or other atoms. d-p and f-p bonding have been suggested to explain the short U-O bonds. In particular, short bond lengths are required for f orbital participation in bonding because the f orbitals have a smaller radial extent than the d orbitals.^[1]

Spectroscopy

The first study of the green fluorescense of uranium glass, by Brewster^[3] in 1849, began extensive studies of the spectroscopy of the uranyl ion. Detailed understanding of this spectrum was obtained by Denning and coworkers^[4] 130 years later. This fluorescence, from K₂UO₂(SO₄)₂, was involved in the discovery of radioactivity.

Health and environmental issues

Uranyl nitrate is an oxidizing and highly toxic compound and should not be ingested; it causes severe renal insufficiency and acute tubular necrosis and is a lymphocyte mitogen.

Target organs include the kidneys, liver, lungs and brain. Uranyl ion accumulation in tissues including gonocytes^[5] produces congenital disorders, and in white blood cells causes immune system damage.^[6] Uranyl compounds are also neurotoxins.

Combustion of uranium

Aerial oxidation of any uranium compound eventually results in the formation of a uranyl compound.^[1] Uranyl ion contamination has been found on and around depleted uranium targets.^[7]

History

Uranyl group, UO₂, was at one time believed to be an element. This is due to the fact that UO₂ is quite stable and participates in chemical reactions quite similar to other group 2 elements such as magnesium or calcium. Also uranium dioxide (UO₂) looks quite similar to a metal. In fact, it is now known, this is because uranium requires strongly reducing conditions to bring its oxidation state lower than +4, that in the uranyl group.

References