Uranium pentafluoride

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Uranium pentafluoride

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Molar mass	333.02 g/mol
Appearance	Pale yellow crystalline solid
Density	5.823 g/cm3 (alpha polymorph)

Uranium pentafluoride is the inorganic compound with the chemical formula UF₅. It is a pale yellow paramagnetic solid. The compound has attracted interest because it is related to uranium hexafluoride, which is widely used to produce uranium fuel. It crystallizes in two polymorphs, called α- and β-UF₅.

Synthesis and structure

Uranium pentafluoride is an intermediate in the conversion of uranium tetrafluoride to volatile UF₆:

2UF₄ + F₂ → 2UF₅

2UF₅ + F₂ → 2UF₆

It can be produced by reduction of the hexafluoride with carbon monoxide at elevated temperatures.

2UF₆ + CO → 2UF₅ + COF₂

Other reducing agents have been examined.

The α form is a linear coordination polymer consisting of chains of octahedral uranium centers in which one of the five fluoride anion forms a bridge to the next uranium atom. The structure is reminiscent of that for vanadium pentafluoride.

In the β form, the uranium centers adopt a square antiprismatic structure. The β polymorph gradually converts to α at 130 °C.

Monomeric UF₅

Of theoretical interest, molecular UF₅ can be generated as a transient monomer by UV-photolysis of uranium hexafluoride. It is thought to adopt a square pyramidal geometry.

File:UF5 monomer.png

Structure of molecular UF₅.