Uranium hexachloride

Uranium hexachloride
Names
IUPAC name
Uranium(VI) chloride
Other names
Uranium hexachloride
Peruranic chloride
Identifiers
13763-23-0
Properties
UCl6
Molar mass 450.745 g/mol
Appearance dark green crystalline solid
Density 3600 kg/m3
Melting point 177 °C (351 °F; 450 K)
Boiling point 75 °C (167 °F; 348 K)
Except where noted otherwise, data is 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 Oh. 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 120oC and 150oC. 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 65oC to 170oC depending on the amount of reactants (ideal temp: 100oC - 125oC) 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 350oC. [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 UFn and UCln (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 Xα Calculations". J. Chem.Phys. 70 (11): 6. doi:10.1063/1.437313.