Xenon hexafluoride

Preparation

Xenon hexafluoride can be prepared by long-term heating of XeF₂ at about 300 °C and pressure 6 MPa (60 atmospheres).

With NiF
2 as catalyst, however, this reaction can proceed at 120 °C even in xenon-fluorine molar ratios as low as 1:5.

Structure

The structure of XeF₆ required several years to establish in contrast to the cases of XeF
2 and XeF
4. In the gas phase the compound is monomeric. VSEPR theory predicts that due to the presence of six fluoride ligands and one lone pair of electrons the structure lacks perfect octahedral symmetry, and indeed electron diffraction combined with high-level calculations indicate that the compound's point group is C_3v. O_h is only insignificantly higher, indicating that the minimum on the energy surface is very shallow. Konrad Seppelt, an authority on noble gas and fluorine chemistry, says, "the structure is best described in terms of a mobile electron pair that moves over the faces and edges of the octahedron and thus distorts it in a dynamic manner."

¹²⁹Xe and ¹⁹F NMR spectroscopy indicates that in solution the compound assumes a tetrameric structure: four equivalent xenon atoms are arranged in a tetrahedron surrounded by a fluctuating array of 24 fluorine atoms that interchange positions in a "cogwheel mechanism".

XeF
6 crystallizes in 6 possible modifications, including one that contains XeF+
5 ions with bridging F−
ions.

Hydrolysis

Xenon hexafluoride hydrolyzes stepwise, ultimately affording xenon trioxide:

XeF₆ + H₂O → XeOF₄ + 2 HFXeOF₄ + H₂O → XeO₂F₂ + 2 HFXeO₂F₂ + H₂O → XeO₃ + 2 HF3XeF₆ + 6H₂O → 2XeO₃ + Xe + 12HF

XeF₆ serves as a Lewis acid, binding one and two fluoride anions:

XeF₆ + F⁻ → XeF−
7XeF−
7 + F⁻ → XeF2−
8

Octafluoroxenates

Salts of the octafluoroxenate(VI) anion (XeF2−
8) are very stable, decomposing only above 400 °C. This anion has been shown to have square antiprismatic geometry, based on single-crystal X-ray counter analysis of its nitrosonium salt, (NO)
2XeF
8. The sodium and potassium salts are formed directly from sodium fluoride and potassium fluoride:

2 NaF + XeF
6 → Na
2XeF
82 KF + XeF
6 → K
2XeF
8

These are thermally less stable than the caesium and rubidium salts, which are synthesized by first forming the heptafluoroxenate salts:

CsF + XeF
6 → CsXeF
7RbF + XeF
6 → RbXeF
7

which are then pyrolysed at 50 °C and 20 °C, respectively, to form the yellow octafluoroxenate salts:

2 CsXeF
7 → Cs
2XeF
8 + XeF
62 RbXeF
7 → Rb
2XeF
8 + XeF
6

These salts are hydrolysed by water, yielding various products containing xenon and oxygen.

The two other binary fluorides of xenon do not form such stable adducts with fluoride.

With fluoride acceptors

XeF
6 reacts with strong fluoride acceptors such as RuF
5 and BrF
3·AuF
3 to form the XeF+
5 cation:

XeF
6 + RuF
5 → XeF+
5RuF−
6XeF
6 + BrF
3·AuF
3 → XeF+
5AuF−
4 + BrF
3