Uranium hexachloride

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	3.6 g/cm³
Melting point	177 °C (351 °F; 450 K)
Related compounds
Other anions	Uranium hexafluoride
Other cations	Tungsten hexachloride
Related uranium chlorides	Uranium(III) chloride Uranium(IV) chloride Uranium(V) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Chemical compound

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.

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).^[6]

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.^[6]

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.^[7] 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.^[8]

Step 1: 2 UCl₄ + Cl₂ → 2 UCl₅

Step 2: 2 UCl₅ + Cl₂ → 2 UCl₆

Overall Reaction: UCl₄ + Cl₂ → UCl₆

References

^ 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. Bibcode:1948AcCry...1..285Z. doi:10.1107/S0365110X48000788.
^ 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. Bibcode:1974AcCrB..30.1481T. doi:10.1107/S0567740874005115.
^ Van Dyke, R. E.; Evers, E. C. (1955). "Preparation of Uranium Hexachloride". Google Patents: 2.
^ 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): 11104–11. doi:10.1063/1.1811607. PMID 15634063.
^ Lipkin, D.; Wessman, S. (1955). "Process and Apparatus for protecting Uranium hexachloride from Deterioration and Contamination". Google Patents: 2.
^ ^a ^b ^c Katz, J.J.; Rabinowitch,E. (1951). The Chemistry of Uranium. Ann Arbor: The McGraw-Hill Book Company.
^ Van Dyke, R. E.; Evers, E. C. (1955). "Preparation of Uranium Hexachloride". Google Patents: 2.
^ 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.