Xenon hexafluoride is a noble gas compound with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

XeF₆. It is one of the three binary fluorides of

xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...

, the other two being XeF₂ and XeF₄. All known are

exergonic An exergonic process is one which there is a positive flow of energy from the system to the surroundings. This is in contrast with an endergonic process. Constant pressure, constant temperature reactions are exergonic if and only if the Gibbs ...

and stable at normal temperatures. XeF₆ is the strongest fluorinating agent of the series. It is a colorless solid that readily sublimes into intensely yellow vapors.

Preparation

Xenon hexafluoride can be prepared by heating of XeF₂ at about 300 °C under 6 MPa (60 atmospheres) of fluorine. With 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 and . In the gas phase the compound is

monomeric In chemistry, a monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification Mo ...

VSEPR theory Valence shell electron pair repulsion (VSEPR) theory ( , ), is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is also named the Gillespie-Nyholm the ...

predicts that due to the presence of six fluoride ligands and one lone pair of electrons the structure lacks perfect

octahedral symmetry A regular octahedron has 24 rotational (or orientation-preserving) symmetries, and 48 symmetries altogether. These include transformations that combine a reflection and a rotation. A cube has the same set of symmetries, since it is the polyhedr ...

, and indeed

electron diffraction Electron diffraction refers to the bending of electron beams around atomic structures. This behaviour, typical for waves, is applicable to electrons due to the wave–particle duality stating that electrons behave as both particles and waves. S ...

combined with high-level calculations indicate that the compound's point group is '' C_3v''. It is a

fluxional molecule In chemistry and molecular physics, fluxional (or non-rigid) molecules are molecules that undergo dynamics such that some or all of their atoms interchange between symmetry-equivalent positions. Because virtually all molecules are fluxional in ...

. '' O_h'' is only insignificantly higher, indicating that the minimum on the energy surface is very shallow. ¹²⁹Xe and ¹⁹F NMR spectroscopy indicates that in solution the compound assumes a tetrameric structure: four equivalent xenon atoms are arranged in a

tetrahedron In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all ...

surrounded by a fluctuating array of 24 fluorine atoms that interchange positions in a "cogwheel mechanism". Six polymorphs of are known. including one that contains XeF ions with bridging F ions.

Reactions

Hydrolysis

Xenon hexafluoride hydrolyzes, ultimately affording

xenon trioxide Xenon trioxide is an unstable compound of xenon in its +6 oxidation state. It is a very powerful oxidizing agent, and liberates oxygen from water slowly, accelerated by exposure to sunlight. It is dangerously explosive upon contact with organic ma ...

: :XeF₆ + H₂O → XeOF₄ + 2 HF :XeOF₄ + H₂O → XeO₂F₂ + 2 HF :XeO₂F₂ + H₂O → XeO₃ + 2 HF :XeF₆ + 3 H₂O → XeO₃ + 6 HF XeF₆ is a

Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

, binding one and two fluoride anions: :XeF₆ + F⁻ → XeF :XeF + F⁻ → XeF

Octafluoroxenates

Salts of the octafluoroxenate(VI) anion (XeF) 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 The nitrosonium ion is , in which the nitrogen atom is bonded to an oxygen atom with a bond order of 3, and the overall diatomic species bears a positive charge. It can be viewed as nitric oxide with one electron removed. This ion is usually ob ...

salt, . The sodium and potassium salts are formed directly from

sodium fluoride Sodium fluoride (NaF) is an inorganic compound with the formula . It is used in trace amounts in the fluoridation of drinking water, in toothpaste, in metallurgy, and as a flux. It is a colorless or white solid that is readily soluble in water. I ...

and potassium fluoride: :2 NaF + → :2 KF + → These are thermally less stable than the

caesium Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...

and

rubidium Rubidium is the chemical element with the symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have a density higher ...

salts, which are synthesized by first forming the heptafluoroxenate salts: :CsF + → :RbF + → which are then pyrolysed at 50 °C and 20 °C, respectively, to form the yellow octafluoroxenate salts: :2 → + :2 → + 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

reacts with strong fluoride acceptors such as and to form the XeF cation: : + → XeFRuF : + → XeFAuF +

References

External links

WebBook page for XeF₆
{{fluorine compounds Xenon(VI) compounds Hexafluorides Nonmetal halides Fluorinating agents Gases with color