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Names | |
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IUPAC name Uranium(VI) chloride | |
Other names Uranium hexachloride Peruranic chloride | |
Identifiers | |
3D model (JSmol) | |
ChemSpider | |
PubChem CID | |
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Properties | |
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 | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). |
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 has an octahedral geometry, with point group Oh. [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.
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:
The activation energy for this reaction is about 40 kcal per mole.
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 |
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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 |
When treated with liquid hydrogen fluoride (HF) at room temperature, UCl6 produces UF5. [6]
Uranium hexachloride is efficiently prepared from uranium hexafluoride by halide exchange using boron trichloride according to the following idealized equation: [1]
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