Uranium hexachloride

Uranium hexachloride (UCl₆) is an inorganic chemical compound of uranium in the +6 oxidation state.[1][2] UCl₆ is a metal halide composed of uranium and chlorine. It is a multi-luminescent dark green crystalline solid with a vapor pressure between 1-3 mmHg at 373.15 K.[3] UCl₆ is stable in a vacuum, dry air, nitrogen and helium at room temperature. It is soluble in carbon tetrachloride (CCl₄). Compared to the other uranium halides, little is known about UCl₆.

Uranium hexachloride

Names
IUPAC name Uranium(VI) chloride
Other names Uranium hexachloride Peruranic chloride
Identifiers
CAS Number	161280-02-0
3D model (JSmol)	Interactive image
ChemSpider	57564875
PubChem CID	57346050
InChI InChI=1S/6ClH.U/h6*1H;/p-6 Key: XFCORTPUZRSUIZ-UHFFFAOYSA-H
SMILES [Cl-].[Cl-].[Cl-].[Cl-].[Cl-].[Cl-].[U]
Properties
Chemical formula	UCl6
Molar mass	450.745 g/mol
Appearance	dark green crystalline solid
Density	3600 kg/m^3
Melting point	177 °C (351 °F; 450 K)
Boiling point	75 °C (167 °F; 348 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Structure and Bonding

Uranium hexachloride has an octahedral geometry, with point group O_h. Its lattice (dimensions: 10.95 ± 0.02 Å x 6.03 ± 0.01 Å) is hexagonal in shape with three molecules per cell; the average theoretical U-Cl bond is 2.472 Å long (the experimental U-Cl length found by X-ray diffraction is 2.42 Å),[4] and the distance between two adjacent chlorine atoms is 3.65 Å.

Chemical Properties

Uranium hexachloride is a highly hygroscopic compound and decomposes readily when exposed to ordinary atmospheric conditions.[5] therefore it should be handled in either a vacuum apparatus or in a dry box.

Thermal decomposition

UCl₆ is stable up to temperatures between 120 °C and 150 °C. The decomposition of UCl₆ results in a solid phase transition from one crystal form of UCl₆ to another more stable form.[6] However, the decomposition of gaseous UCl₆ produces UCl₅. The activation energy for this reaction is about 40 kcal per mole.

2 UCl₆ (g) → 2 UCl₅ (s) + Cl₂ (g)

Solubility

UCl₆ is not a very soluble compound. It dissolves in CCl₄ to give a brown solution. It is slightly soluble in isobutyl bromide and in fluorocarbon (C₇F₁₆).[7]

Solvents	Temperature (°C)	Grams of UCl6/100g of solution
CCl4	−18	2.64
CCl4	0	4.9
CCl4	20	7.8
6.6% Cl2 : 93.4% CCl4	−20	2.4
12.5% Cl2 : 87.5% CCl4	−20	2.23
12.5% Cl2 : 87.5% CCl4	0	3.98
Liquid Cl2	−33	2.20
CH3Cl	−24	1.16
Benzene	80	Insoluble
Freon 113	45	1.83

Reaction with hydrogen fluoride

When UCl₆ is reacted with purified anhydrous liquid hydrogen fluoride (HF) at room temperature produces UF₅.[8]

2 UCl₆+ 10 HF → 2 UF₅ + 10 HCl + Cl₂

Synthesis

Uranium hexachloride can be synthesized from the reaction of uranium trioxide (UO₃) with a mixture of liquid CCl₄ and hot chlorine (Cl₂). The yield can be increased if the reaction carried out in the presence of UCl₅.[9] The UO₃ is converted to UCl₅, which in turn reacts with the excess Cl₂ to form UCl₆. It requires a substantial amount of heat for the reaction to take place; the temperature range is from 65 °C to 170 °C depending on the amount of reactant (ideal temperature 100 °C - 125 °C). The reaction is carried out in a closed gas-tight vessel (for example a glovebox) that can withstand the pressure that builds up.

Step 1: 2 UO₃ + 5 Cl₂ → 2 UCl₅ + 3 O₂

Step 2: 2 UCl₅ + Cl₂ → 2 UCl₆

Overall reaction: 2 UO₃ + 6 Cl₂ → 2 UCl₆ + 3 O₂

This metal hexahalide can also be synthesized by blowing Cl₂ gas over sublimed UCl₄ at 350 °C.[10]

Step 1: 2 UCl₄ + Cl₂ → 2 UCl₅

Step 2: 2 UCl₅ + Cl₂ → 2 UCl₆

Overall Reaction: UCl₄ + Cl₂ → UCl₆

References

1. Zachariasen, W. H. (1948). "Crystal chemical studies of the 5f-series of elements. V. The crystal structure of uranium hexachloride". Acta Crystallographica. 1 (6): 285–287. doi:10.1107/S0365110X48000788.
2. Taylor, J. C.; Wilson, P. W. (1974). "Neutron and X-ray powder diffraction studies of the structure of uranium hexachloride". Acta Crystallographica Section B. 30 (6): 1481. doi:10.1107/S0567740874005115.
3. Van Dyke, R. E.; Evers, E. C. (1955). "Preparation of Uranium Hexachloride". Google Patents: 2.
4. Batista, E. R.; Martin, R. L.; Hay, P. J. (2004). "Density Functional Investigations of the Properties and Thermodynamics of UF_n and UCl_n (n=1,...,6)". J. Chem. Phys. 121 (22): 8. doi:10.1063/1.1811607. PMID 15634063.
5. Lipkin, D.; Wessman, S. (1955). "Process and Apparatus for protecting Uranium hexachloride from Deterioration and Contamination". Google Patents: 2.
6. Katz, J.J.; Rabinowitch,E. (1951). The Chemistry of Uranium. Ann Arbor: The McGraw-Hill Book Company.
7. Katz,J.J; Rabinowitch,E. (1951). The Chemistry of Uranium. Ann Arbor: The McGraw-Hill Book Company.
8. Katz,J.J; Rabinowitch,E. (1951). The Chemistry of Uranium. Ann Arbor: The McGraw-Hill Book Company.
9. Van Dyke, R. E.; Evers, E. C. (1955). "Preparation of Uranium Hexachloride". Google Patents: 2.
10. Thornton, G.; Edelstein, N.; Rösch, N.; Woodwark, D.R.; Edgell, R.G. (1979). "The Electronic Structure of UCl₆: Photoelectron Spectra and Scattered Wave X_α Calculations". J. Chem. Phys. 70 (11): 6. Bibcode:1979JChPh..70.5218T. doi:10.1063/1.437313.