Uranium hexachloride

Uranium hexachloride

Names
IUPAC name Uranium(VI) chloride
Other names Uranium hexachloride Peruranic chloride
Identifiers
CAS Number	13763-23-0
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

Uranium hexachloride is an inorganic chemical compound of uranium in the +6 oxidation state.^[1][2]The chemical compound Uranium hexachloride (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 373.15K^[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 a point group of O_h. Its lattice (dimensions: 10.95 ± 0.02Å x 6.03 ± 0.01Å) is hexagon in shape with three molecules per cell; the average theoretical U-Cl bond is 2.472Å long (X-ray diffraction experimental U-Cl length is 2.42Å)^[4] and the distance between two adjacent chlorine atoms is 3.65Å .

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^oC and 150^oC. 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 40kcal per mole.

2UCl₆ (g) → 2UCl₅ (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 (^oC)	Grams of UCl6/100g of solution
CCl4
2.64
CCl4	0	4.9
CCl4	20	7.8
6.6% Cl2 : 93.4% CCl4
2.4
12.5% Cl2 : 87.5% CCl4
2.23
12.5% Cl2 : 87.5% CCl4	0	3.98
Liquid Cl2
2.20
CH3Cl
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]

2UCl₆+ 10HF → 2UF₅ + 10HCl + Cl₂

Synthesis

Uranium hexachloride can be synthesized from the reaction of uranium trioxide (UO3) with a mixture of liquid CCl₄ and hot chlorine (Cl₂). However 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^oC to 170^oC depending on the amount of reactants (ideal temp: 100^oC - 125^oC) The reaction is carried out in a closed gas-tight vessel (for example a glovebox) that can withstand the pressure the builds up.

Step 1: 2UO₃ + 5Cl₂ → 2UCl₅ + 3O₂

Step 2: 2UCl₅ + Cl₂ → 2UCl₆

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

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

Step 1: 2UCl₄ + Cl₂ → 2UCl₅

Step 2: 2UCl₅ + Cl₂ → 2UCl₆

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. 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 Structural Crystallography and Crystal Chemistry 30 (6): 1481. doi:10.1107/S0567740874005115.
3. Van Dyke, R. E.; Evers, E. C. (1955). "Preparation of Uranium Hexachloride". google patent: 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.
5. Lipkin, D.; Wessman, S. (1955). "Process and Apparatus for protecting Uranium hexachloride from Deterioration and Contamination". google patent: 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 patent: 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. doi:10.1063/1.437313.