Xenon hexafluoride is a
noble gas compound
In chemistry, noble gas compounds are chemical compounds that include an element from the noble gases, group 18 of the periodic table. Although the noble gases are generally unreactive elements, many such compounds have been observed, particular ...
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 betwee ...
XeF
6. 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 fr ...
and stable at normal temperatures. XeF
6 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 XeF
2 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 XeF
6 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 theo ...
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. Si ...
combined with high-level calculations indicate that the compound's point group is ''
C3v''. 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 so ...
. ''
Oh'' is only insignificantly higher, indicating that the minimum on the energy surface is very shallow.
129Xe and
19F
NMR
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with ...
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 the o ...
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 Noble gas compounds, 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 co ...
:
:XeF
6 + H
2O → XeOF
4 + 2 HF
:XeOF
4 + H
2O →
XeO2F2 + 2 HF
:XeO
2F
2 + H
2O → XeO
3 + 2 HF
:XeF
6 + 3 H
2O → XeO
3 + 6 HF
XeF
6 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:
:XeF
6 + 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 antiprism
In geometry, the square antiprism is the second in an infinite family of antiprisms formed by an even-numbered sequence of triangle sides closed by two polygon caps. It is also known as an ''anticube''.
If all its faces are regular, it is a sem ...
atic geometry, based on single-crystal X-ray counter analysis of its
nitrosonium 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
Potassium fluoride is the chemical compound with the formula KF. After hydrogen fluoride, KF is the primary source of the fluoride ion for applications in manufacturing and in chemistry. It is an alkali halide and occurs naturally as the rare m ...
:
: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