Uranium hexachloride is the inorganic compound with the formula UCl6. It features uranium in the +6 oxidation state.[1]UCl6 hydrolyzes readily but is stable under inert atmosphere. It is soluble in carbon tetrachloride (CCl4). It is a multi-luminescent dark green or black solid with a vapor pressure between 1-3 mmHg at 373.15K.[2]
Structure and bonding
Uranium hexachloride has an octahedral geometry, with point group Oh.[3][4] Its lattice (dimensions: 10.95 ± 0.02Å x 6.03 ± 0.01Å) is hexagonal in shape with three molecules per cell; the average theoretical U-Cl bond is 2.472Å long (the experimental U-Cl length found by X-ray diffraction is 2.42Å),[5] and the distance between two adjacent chlorine atoms is 3.65Å.
Chemical properties
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] It decomposes as follows:
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]
Uranium hexachloride is efficiently prepared from uranium hexafluoride by halide exchange using boron trichloride according to the following idealized equation:[1]
UF6 + 2 BCl3 → UCl6 + 2 BF3
Uranium hexachloride can also 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 also form upon combining Cl2 and 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
12O'Donnell, T. A.; Wilson, P. W. (1976). "Uranium Hexachloride: ( Hexachlorouranium )". Uranium Hexachloride: (Hexachlorouranium ). Inorganic Syntheses. Vol.16. p.143. doi:10.1002/9780470132470.ch39. ISBN978-0-470-13178-7.
↑Van Dyke, R. E.; Evers, E. C. (1955). "Preparation of Uranium Hexachloride". Google Patents: 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. Bibcode:1974AcCrB..30.1481T. doi:10.1107/S0567740874005115.
