Xenon difluoride
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Xenon difluoride is a powerful
fluorinating agent In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polyme ...
with the chemical formula , and one of the most stable
xenon compounds Xenon is a chemical element with the Symbol (chemistry), symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Atmosphere of Earth, Earth's atmosphere in trace amounts. Although generally unreactive, it can under ...
. Like most
covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...
inorganic fluorides it is moisture-sensitive. It decomposes on contact with
water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous pha ...
, but is otherwise stable in storage. Xenon difluoride is a dense, colourless
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
line solid. It has a nauseating odour and low
vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phas ...
.


Structure

Xenon difluoride is a
linear Linearity is the property of a mathematical relationship (''function'') that can be graphically represented as a straight line. Linearity is closely related to '' proportionality''. Examples in physics include rectilinear motion, the linear r ...
molecule with an Xe–F bond length of in the vapor stage, and 200 pm in the solid phase. The packing arrangement in solid shows that the fluorine atoms of neighbouring molecules avoid the equatorial region of each molecule. This agrees with the prediction of
VSEPR 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 ...
theory, which predicts that there are 3 pairs of non-bonding electrons around the equatorial region of the xenon atom. At high pressures, novel, non-molecular forms of xenon difluoride can be obtained. Under a pressure of ~50 
GPa Grading in education is the process of applying standardized measurements for varying levels of achievements in a course. Grades can be assigned as letters (usually A through F), as a range (for example, 1 to 6), as a percentage, or as a numbe ...
, transforms into a semiconductor consisting of units linked in a two-dimensional structure, like
graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
. At even higher pressures, above 70 GPa, it becomes metallic, forming a three-dimensional structure containing units. However, a recent theoretical study has cast doubt on these experimental results. The Xe–F bonds are weak. XeF2 has a total bond energy of , with first and second bond energies of and , respectively. However, XeF2 is much more robust than KrF2, which has a total bond energy of only .


Chemistry


Synthesis

Synthesis proceeds by the simple reaction: :Xe + F2 → XeF2 The reaction needs heat, irradiation, or an electrical discharge. The product is a solid. It is purified by
fractional distillation Fractional distillation is the separation of a mixture into its component parts, or fractions. Chemical compounds are separated by heating them to a temperature at which one or more fractions of the mixture will vaporize. It uses distillation to ...
or selective condensation using a vacuum line. The first published report of XeF2 was in October 1962 by Chernick, et al. However, though published later, XeF2 was probably first created by
Rudolf Hoppe Rudolf Hoppe (29 October 1922 – 24 November 2014), a German chemist, discovered the first covalent noble gas compounds. Academic career Hoppe studied chemistry at the Christian-Albrechts-University of Kiel and was awarded his doctorate at the ...
at the
University of Münster The University of Münster (german: Westfälische Wilhelms-Universität Münster, WWU) is a public university, public research university located in the city of Münster, North Rhine-Westphalia in Germany. With more than 43,000 students and over ...
, Germany, in early 1962, by reacting fluorine and xenon gas mixtures in an electrical discharge. Shortly after these reports, Weeks, Cherwick, and Matheson of
Argonne National Laboratory Argonne National Laboratory is a science and engineering research United States Department of Energy National Labs, national laboratory operated by University of Chicago, UChicago Argonne LLC for the United States Department of Energy. The facil ...
reported the synthesis of XeF2 using an all-nickel system with transparent alumina windows, in which equal parts xenon and fluorine gases react at low pressure upon irradiation by an
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
source to give XeF2. Williamson reported that the reaction works equally well at atmospheric pressure in a dry
Pyrex Pyrex (trademarked as ''PYREX'' and ''pyrex'') is a brand introduced by Corning Inc. in 1915 for a line of clear, low-thermal-expansion borosilicate glass used for laboratory glassware and kitchenware. It was later expanded to include kitchenw ...
glass bulb using sunlight as a source. It was noted that the synthesis worked even on cloudy days. In the previous syntheses the fluorine gas reactant had been purified to remove
hydrogen fluoride Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...
. Šmalc and Lutar found that if this step is skipped the reaction rate proceeds at four times the original rate. In 1965, it was also synthesized by reacting xenon gas with
dioxygen difluoride Dioxygen difluoride is a compound of fluorine and oxygen with the molecular formula O2F2. It can exist as an orange-colored solid which melts into a red liquid at . It is an extremely strong oxidant and decomposes into oxygen and fluorine even ...
.


Solubility

is soluble in solvents such as , , , anhydrous
hydrogen fluoride Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...
, and
acetonitrile Acetonitrile, often abbreviated MeCN (methyl cyanide), is the chemical compound with the formula and structure . This colourless liquid is the simplest organic nitrile (hydrogen cyanide is a simpler nitrile, but the cyanide anion is not clas ...
, without reduction or oxidation. Solubility in hydrogen fluoride is high, at 167 g per 100 g HF at 29.95 °C.


Derived xenon compounds

Other xenon compounds may be derived from xenon difluoride. The unstable organoxenon compound can be made by irradiating hexafluoroethane to generate radicals and passing the gas over . The resulting waxy white solid decomposes completely within 4 hours at room temperature. The XeF+ cation is formed by combining xenon difluoride with a strong fluoride acceptor, such as an excess of liquid
antimony pentafluoride Antimony pentafluoride is the inorganic compound with the formula Sb F5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, formed when mixing liquid HF with liquid SbF5 in a 2:1 ratio. ...
(): : + → + Adding xenon gas to this pale yellow solution at a pressure of 2–3
atmospheres The standard atmosphere (symbol: atm) is a unit of pressure defined as Pa. It is sometimes used as a ''reference pressure'' or ''standard pressure''. It is approximately equal to Earth's average atmospheric pressure at sea level. History The s ...
produces a green solution containing the paramagnetic ion, which contains a Xe−Xe bond: ("apf" denotes solution in liquid ) : 3 Xe(g) + (apf) + (l)(apf) + (apf) This reaction is reversible; removing xenon gas from the solution causes the ion to revert to xenon gas and , and the color of the solution returns to a pale yellow. In the presence of liquid HF, dark green crystals can be precipitated from the green solution at −30 °C: : (apf) + 4 (apf)(s) + 3 (apf)
X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
indicates that the Xe–Xe bond length in this compound is 309  pm, indicating a very weak bond. The ion is
isoelectronic Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in the ...
with the ion, which is also dark green.


Coordination chemistry

Bonding in the XeF2 molecule is adequately described by the
three-center four-electron bond The 3-center 4-electron (3c–4e) bond is a model used to explain bonding in certain hypervalent molecules such as tetratomic and hexatomic interhalogen compounds, sulfur tetrafluoride, the xenon fluorides, and the bifluoride ion. It is also know ...
model. XeF2 can act as a
ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
in
coordination complexes A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many m ...
of metals. For example, in HF solution: :Mg(AsF6)2 + 4 XeF2g(XeF2)4AsF6)2 Crystallographic analysis shows that the magnesium atom is coordinated to 6 fluorine atoms. Four of the fluorine atoms are attributed to the four xenon difluoride ligands while the other two are a pair of ''cis''- ligands. A similar reaction is: :Mg(AsF6)2 + 2 XeF2g(XeF2)2AsF6)2 In the crystal structure of this product the magnesium atom is octahedrally-coordinated and the XeF2 ligands are axial while the ligands are equatorial. Many such reactions with products of the form ''x''(XeF2)''n''AF6)''x'' have been observed, where M can be
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
,
strontium Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...
,
barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...
,
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
,
silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
,
lanthanum Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between lantha ...
, or
neodymium Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarnishes i ...
and A can be
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but ...
,
antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient time ...
or
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
. In 2004, results of synthesis of a solvate where part of cationic centers were coordinated solely by XeF2 fluorine atoms were published. Reaction can be written as: :2 Ca(AsF6)2 + 9 XeF2 → Ca2(XeF2)9(AsF6)4. This reaction requires a large excess of xenon difluoride. The structure of the salt is such that half of the Ca2+ ions are coordinated by fluorine atoms from xenon difluoride, while the other Ca2+ ions are coordinated by both XeF2 and .


Applications


As a fluorinating agent

Xenon difluoride is a strong fluorinating and oxidizing agent. With fluoride ion acceptors, it forms and species which are even more powerful fluorinators. Among the fluorination reactions that xenon difluoride undergoes are: * ''Oxidative fluorination'': ::Ph3TeF + XeF2 → Ph3TeF3 + Xe * ''Reductive fluorination'': ::2 CrO2F2 + XeF2 → 2 CrOF3 + Xe +O2 * ''Aromatic fluorination'': :: :: * ''Alkene fluorination'': :: * '' Radical fluorination'' in radical decarboxylative fluorination reactions, in Hunsdiecker-type reactions where xenon difluoride is used to generate the radical intermediate as well as the fluorine transfer source, and in generating aryl radicals from aryl silanes: :: :: is selective about which atom it fluorinates, making it a useful reagent for fluorinating heteroatoms without touching other substituents in organic compounds. For example, it fluorinates the arsenic atom in
trimethylarsine Trimethylarsine (abbreviated TMA or TMAs) is the chemical compound with the formula (CH3)3As, commonly abbreviated As Me3 or TMAs. This organic derivative of arsine has been used as a source of arsenic in microelectronics industry, a building bloc ...
, but leaves the
methyl group In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in many ...
s untouched: : + → + Xe XeF2 can similarly be used to prepare ''N''-fluoroammonium salts, useful as fluorine transfer reagents in organic synthesis (e.g.,
Selectfluor Selectfluor, a trademark of Air Products and Chemicals, is a reagent in chemistry that is used as a fluorine donor. This compound is a derivative of the nucleophillic base DABCO. It is a colourless salt that tolerates air and even water. It has ...
), from the corresponding tertiary amine: : –(CH2CH2)3N:] + XeF2 + NaBF4–(CH2CH2)3–F]2 + NaF + Xe will also oxidatively decarboxylate
carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...
s to the corresponding fluoroalkanes: :RCOOH + XeF2 → RF + CO2 + Xe + HF
Silicon tetrafluoride Silicon tetrafluoride or tetrafluorosilane is a chemical compound with the formula Si F4. This colorless gas is notable for having a narrow liquid range: its boiling point is only 4 °C above its melting point. It was first prepared in 1771 ...
has been found to act as a catalyst in fluorination by .


As an etchant

Xenon difluoride is also used as an isotropic gaseous etchant for
silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
, particularly in the production of
microelectromechanical systems Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...
(MEMS), as first demonstrated in 1995. Commercial systems use pulse etching with an expansion chamber Brazzle, Dokmeci, et al. describe this process: The mechanism of the etch is as follows. First, the XeF2 adsorbs and dissociates to xenon and fluorine atoms on the surface of silicon. Fluorine is the main etchant in the silicon etching process. The reaction describing the silicon with XeF2 is :2 XeF2 + Si → 2 Xe + SiF4 XeF2 has a relatively high etch rate and does not require ion bombardment or external energy sources in order to etch silicon.


References


Further reading

*


External links


WebBook page for XeF2
{{fluorine compounds Xenon(II) compounds Fluorides Nonmetal halides Fluorinating agents