Uranium hexachloride

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 373.15 K.^[2]

Uranium hexachloride

Names
IUPAC name Uranium(VI) chloride
Other names Uranium hexachloride Peruranic chloride
Identifiers
CAS Number	161280-02-0
3D model (JSmol)	Interactive image
ChemSpider	57564875
PubChem CID	57346050
InChI InChI=1S/6ClH.U/h6*1H;/p-6 Key: XFCORTPUZRSUIZ-UHFFFAOYSA-H
SMILES [Cl-].[Cl-].[Cl-].[Cl-].[Cl-].[Cl-].[U]
Properties
Chemical formula	UCl6
Molar mass	450.745 g/mol
Appearance	dark green crystalline solid
Density	3.6 g/cm3
Melting point	177 °C (351 °F; 450 K)
Related compounds
Other anions	Uranium hexafluoride
Other cations	Tungsten hexachloride
Related uranium chlorides	Uranium(III) chloride Uranium(IV) chloride Uranium(V) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Structure and bonding

Uranium hexachloride has an octahedral geometry, with point group O_h.^[3][4] 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),^[5] and the distance between two adjacent chlorine atoms is 3.65 Å.

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:

2 UCl₆ (g) → 2 UCl₅ (s) + Cl₂ (g)

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

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).^[6]

Solvents	Temperature (°C)	Grams of UCl6/100g of solution
CCl4	−18	2.64
CCl4	0	4.9
CCl4	20	7.8
6.6% Cl2 : 93.4% CCl4	−20	2.4
12.5% Cl2 : 87.5% CCl4	−20	2.23
12.5% Cl2 : 87.5% CCl4	0	3.98
Liquid Cl2	−33	2.20
CH3Cl	−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, UCl₆ produces UF₅.^[6]

2 UCl₆ + 10 HF → 2 UF₅ + 10 HCl + Cl₂

Synthesis

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