Uranium hexachloride

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 O<sub>h</sub>. Its lattice (dimensions: 10.95 ÃÂ± 0.02 ÃÂ x 6.03 ÃÂ± 0.01 ÃÂ) is hexagonal in shape with three molecules per cell; the average theoretical U-Cl bond is 2.472 ÃÂ long (the experimental U-Cl length found by X-ray diffraction is 2.42 ÃÂ), and the distance between two adjacent chlorine atoms is 3.65 ÃÂ.

Chemical properties

is stable up to temperatures between 120 ÃÂ°C and 150 ÃÂ°C. The decomposition of results in a solid phase transition from one crystal form of to another more stable form. 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 ().

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 ÃÂ°C to 170 ÃÂ°C depending on the amount of reactant (ideal temperature 100 ÃÂ°C - 125 ÃÂ°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 ÃÂ°C.