Xenon hexafluoride
Encyclopedia
Xenon hexafluoride is a noble gas compound
with the formula
XeF6 and the highest of the three binary fluorides of xenon
, the other two being XeF2
and XeF4
. All are exergonic
and stable at normal temperatures. XeF6 is the strongest fluorinating agent of the series. At room temperature, it is a colorless solid that readily sublimes into intensely yellow vapors.
With
as catalyst, however, this reaction can proceed at 120°C even in xenon-fluorine molar ratios as low as 1:5.
, and indeed electron diffraction
combined with high-level calculations indicate that the compound's point group is C3v. The calculated energy for the point group Oh 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".
crystallizes in 6 possible modifications, including one that contains XeF ions with bridging F ions.
:
XeF6 serves as a Lewis acid
, binding one and two fluoride anions:
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
and potassium fluoride
:
These are thermally less stable than the caesium
and rubidium
salts, which are synthesized by first forming the heptafluoroxenate salts:
which are then pyrolysed at 50°C and 20°C, respectively, to form the yellow octafluoroxenate salts:
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.
Noble gas compound
Noble gas compounds are chemical compounds that include an element from Group 18 of the periodic table, the noble gases.-History and background:...
with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
XeF6 and the highest of the three binary fluorides of xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
, the other two being XeF2
Xenon difluoride
Xenon difluoride is a powerful fluorinating agent with the chemical formula , and one of the most stable xenon compounds. Like most covalent inorganic fluorides it is moisture sensitive. It decomposes on contact with light or water vapour. Xenon difluoride is a dense, white crystalline solid. It...
and XeF4
Xenon tetrafluoride
Xenon tetrafluoride is a chemical compound with chemical formula . It was the first discovered binary compound of a noble gas. It is produced by the chemical reaction of xenon with fluorine, , according to the chemical equation:...
. All are exergonic
Exergonic
Exergonic means "releasing energy in the form of work". By thermodynamic standards, work, a form of energy, is defined as moving from the system to the surroundings...
and stable at normal temperatures. XeF6 is the strongest fluorinating agent of the series. At room temperature, it is a colorless solid that readily sublimes into intensely yellow vapors.
Preparation
Xenon hexafluoride can be prepared by long-term heating of XeF2 at about 300°C and pressure 6 MPa (60 atmospheres).With
Nickel(II) fluoride
Nickel fluoride is the chemical compound with the formula NiF2. Unlike many fluorides, NiF2 is stable in air. NiF2 comprises the passivating surface that forms on nickel alloys, e.g. monel, which is why such materials are good to store or transport hydrogen fluoride or elemental fluorine...
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. VSEPR theory predicts that due to the presence of six fluoride ligands and one lone pair of electrons the structure lacks perfect octahedral symmetryOctahedral symmetry
150px|thumb|right|The [[cube]] is the most common shape with octahedral symmetryA regular octahedron has 24 rotational symmetries, and a symmetry order of 48 including transformations that combine a reflection and a rotation...
, and indeed electron diffraction
Electron diffraction
Electron diffraction refers to the wave nature of electrons. However, from a technical or practical point of view, it may be regarded as a technique used to study matter by firing electrons at a sample and observing the resulting interference pattern...
combined with high-level calculations indicate that the compound's point group is C3v. The calculated energy for the point group Oh is only insignificantly higher, indicating that the minimum on the energy surface is very shallow. Konrad Seppelt
Konrad Seppelt
Professor Konrad Seppelt is a chemist who leads a research group at the Institute of Inorganic and Analytical Chemistry, Free University of Berlin .Positions held include:...
, 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
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
spectroscopy indicates that in solution the compound assumes a tetrameric structure: four equivalent xenon atoms are arranged in a tetrahedron
Tetrahedron
In geometry, a tetrahedron is a polyhedron composed of four triangular faces, three of which meet at each vertex. A regular tetrahedron is one in which the four triangles are regular, or "equilateral", and is one of the Platonic solids...
surrounded by a fluctuating array of 24 fluorine atoms that interchange positions in a "cogwheel mechanism".
crystallizes in 6 possible modifications, including one that contains XeF ions with bridging F ions.
Hydrolysis
Xenon hexafluoride hydrolyzes stepwise, ultimately affording xenon trioxideXenon 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 materials...
:
- XeF6 + H2O → XeOF4 + 2 HF
- XeOF4 + H2O → XeO2F2 + 2 HF
- XeO2F2 + H2O → XeO3 + 2 HF
XeF6 serves as a Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
, 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 antiprismSquare antiprism
In geometry, the square antiprism is the second in an infinite set of antiprisms formed by an even-numbered sequence of triangle sides closed by two polygon caps...
atic geometry, based on single-crystal X-ray counter analysis of its nitrosonium
Nitrosonium
The nitrosonium ion is NO+, 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. This ion is usually obtained as the following salts: NOClO4, NOSO4H , and NOBF4. The ClO and BF salts are slightly soluble in CH3CN...
salt,
Nitrosonium octafluoroxenate(VI)
Nitrosonium octafluoroxenate is a chemical compound of xenon with nitrogen, oxygen, and fluorine, having formula . It is an ionic compound containing well-separated nitrosonium cations and octafluoroxenate anions...
. The sodium and potassium salts are formed directly from sodium fluoride
Sodium fluoride
Sodium fluoride is an inorganic chemical compound with the formula NaF. A colorless solid, it is a source of the fluoride ion in diverse applications. Sodium fluoride is less expensive and less hygroscopic than the related salt potassium fluoride....
and potassium fluoride
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 metal halide and occurs naturally as the rare mineral carobbiite...
:
- 2 NaF + →
- 2 KF + →
These are thermally less stable than the caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
and rubidium
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...
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 +