Uranium hexachloride (UCl₆) is an

inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemis ...

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one ele ...

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weakly ...

in the +6

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. ...

. UCl₆ is a metal halide composed of

and

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...

. It is a multi-luminescent dark green crystalline solid with a vapor pressure between 1-3 mmHg at 373.15 K. UCl₆ is stable in a

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...

, dry air,

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seve ...

and

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

room temperature Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...

. It is soluble in

carbon tetrachloride Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVACR) is an organic compound with the chemi ...

(CCl₄). Compared to the other uranium halides, little is known about UCl₆.

Structure and Bonding

Uranium hexachloride has an

octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...

geometry, with

point group In geometry, a point group is a mathematical group of symmetry operations ( isometries in a Euclidean space) that have a fixed point in common. The coordinate origin of the Euclidean space is conventionally taken to be a fixed point, and every ...

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 Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...

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

UCl₆ is stable up to temperatures between 120 °C and 150 °C. The

decomposition Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is ...

of UCl₆ results in a solid phase transition from one crystal form of UCl₆ to another more stable form. However, the

of gaseous UCl₆ produces UCl₅. The

activation energy In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...

for this reaction is about 40 kcal per mole. :2 UCl₆ (g) → 2 UCl₅ (s) + Cl₂ (g)

Solubility

UCl₆ is not a very

soluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solub ...

compound. It dissolves in CCl₄ to give a brown solution. It is slightly soluble in isobutyl bromide and in fluorocarbon (C₇F₁₆).

Reaction with hydrogen fluoride

When UCl₆ is reacted with purified anhydrous liquid hydrogen fluoride (HF) at room temperature produces UF₅. :2 UCl₆+ 10 HF → 2 UF₅ + 10 HCl + Cl₂

Synthesis

Uranium hexachloride can be synthesized from the reaction of

uranium trioxide Uranium trioxide (UO3), also called uranyl oxide, uranium(VI) oxide, and uranic oxide, is the hexavalent oxide of uranium. The solid may be obtained by heating uranyl nitrate to 400 °C. Its most commonly encountered polymorph, γ-UO3, is a ...

(UO₃) with a mixture of liquid CCl₄ and hot

(Cl₂). The yield can be increased if the reaction carried out in the presence of UCl₅. 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 Reaction may refer to a process or to a response to an action, event, or exposure: Physics and chemistry *Chemical reaction *Nuclear reaction *Reaction (physics), as defined by Newton's third law * Chain reaction (disambiguation). Biology and me ...

to take place; the temperature range is from 65 °C to 170 °C depending on the amount of

reactant In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

(ideal temperature 100 °C - 125 °C). The reaction is carried out in a closed gas-tight vessel (for example a

glovebox A glovebox (or glove box) is a sealed container that is designed to allow one to manipulate objects where a separate atmosphere is desired. Built into the sides of the glovebox are gloves arranged in such a way that the user can place their han ...

) 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 can also be synthesized by blowing Cl₂ gas over sublimed UCl₄ at 350 °C. Step 1: 2 UCl₄ + Cl₂ → 2 UCl₅ Step 2: 2 UCl₅ + Cl₂ → 2 UCl₆ Overall Reaction: UCl₄ + Cl₂ → UCl₆

References

{{Chlorides Uranium(VI) compounds Chlorides Actinide halides