Chemistry:Uranium hexachloride

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	UCl 6
Molar mass	450.745 g/mol
Appearance	dark green crystalline solid
Density	3600 kg/m3
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

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
7F
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 3Cl	−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