# Uranium hexachloride

Uranium hexachloride (UCl$_6$) is an inorganic chemical compound of uranium in the +6 oxidation state. UCl$_6$ 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 373.15 K. UCl$_6$ is stable in a vacuum, dry air, nitrogen and helium at room temperature. It is soluble in carbon tetrachloride (CCl$_4$). Compared to the other uranium halides, little is known about UCl$_6$.

## Structure and Bonding

Uranium hexachloride has an octahedral geometry, with point group O$_h$. 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

Uranium hexachloride is a highly hygroscopic compound and decomposes readily when exposed to ordinary atmospheric conditions. therefore it should be handled in either a vacuum apparatus or in a dry box.

### Thermal decomposition

UCl$_6$ is stable up to temperatures between 120 °C and 150 °C. The decomposition of UCl$_6$ results in a solid phase transition from one crystal form of UCl$_6$ to another more stable form. However, the decomposition of gaseous UCl$_6$ produces UCl$_5$. The activation energy for this reaction is about 40 kcal per mole.
    2 UCl$_6$ (g) → 2 UCl$_5$ (s) + Cl$_2$ (g)

### Solubility

UCl$_6$ is not a very soluble compound. It dissolves in CCl$_4$ to give a brown solution. It is slightly soluble in isobutyl bromide and in fluorocarbon (C$_7$F$_{16}$).
| Solvents | Temperature (°C) | Grams of UCl$_6$/100g of solution |
| CCl$_4$ | −18 | 2.64 |
| CCl$_4$ | 0 | 4.9 |
| CCl$_4$ | 20 | 7.8 |
| 6.6% Cl$_2$ : 93.4% CCl$_4$ | −20 | 2.4 |
| 12.5% Cl$_2$ : 87.5% CCl$_4$ | −20 | 2.23 |
| 12.5% Cl$_2$ : 87.5% CCl$_4$ | 0 | 3.98 |
| Liquid Cl$_2$ | −33 | 2.20 |
| CH$_3$Cl | −24 | 1.16 |
| Benzene | 80 | Insoluble |
| Freon 113 | 45 | 1.83 |

### Reaction with hydrogen fluoride

When UCl$_6$ is reacted with purified anhydrous liquid hydrogen fluoride (HF) at room temperature produces UF$_5$.
    2 UCl$_6$ + 10 HF → 2 UF$_5$ + 10 HCl + Cl$_2$

## Synthesis

Uranium hexachloride can be synthesized from the reaction of uranium trioxide (UO$_3$) with a mixture of liquid CCl$_4$ and hot chlorine (Cl$_2$). The yield can be increased if the reaction carried out in the presence of UCl$_5$. The UO$_3$ is converted to UCl$_5$, which in turn reacts with the excess Cl$_2$ to form UCl$_6$. 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 UO$_3$ + 5 Cl$_2$ → 2 UCl$_5$ + 3 O$_2$
Step 2: 2 UCl$_5$ + Cl$_2$ → 2 UCl$_6$
Overall reaction: 2 UO$_3$ + 6 Cl$_2$ → 2 UCl$_6$ + 3 O$_2$
This metal hexahalide can also be synthesized by blowing Cl$_2$ gas over sublimed UCl$_4$ at 350 °C.
Step 1: 2 UCl$_4$ + Cl$_2$ → 2 UCl$_5$
Step 2: 2 UCl$_5$ + Cl$_2$ → 2 UCl$_6$
Overall Reaction: UCl$_4$ + Cl$_2$ → UCl$_6$