Uranium hexachloride is the inorganic compound with the formula . It features uranium in the +6 oxidation state. hydrolyzes readily but is stable under inert atmosphere. It is soluble in carbon tetrachloride (). It is a multi-luminescent dark green or black solid with a vapor pressure between 1-3 mmHg at 373.15 K.

Structure and bonding

Uranium hexachloride has an octahedral geometry, with point group Oh. 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), and the distance between two adjacent chlorine atoms is 3.65 Å.

Chemical properties is stable up to temperatures between 120 °C and 150 °C. The decomposition of results in a solid phase transition from one crystal form of to another more stable form, before fully decomposing into UCl5. It decomposes as follows:

The activation energy for this reaction is about 40 kcal per mole.

Solubility is not a very soluble compound. It dissolves in to give a brown solution. It is slightly soluble in isobutyl bromide and in fluorocarbon ().

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

Reaction with hydrogen fluoride When treated with liquid hydrogen fluoride (HF) at room temperature, produces .

Synthesis Uranium hexachloride is efficiently prepared from uranium hexafluoride by halide exchange using boron trichloride according to the following idealized equation:

Uranium hexachloride can also be synthesized from the reaction of uranium trioxide () with a mixture of liquid and hot chlorine (). The yield can be increased if the reaction carried out in the presence of . The is converted to , which in turn reacts with the excess to form . 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:

Step 2:

Overall reaction:

This metal hexahalide also form upon combining and at 350 °C.

Step 1:

Step 2:

Overall Reaction:

References