Xenon difluoride 4733324 225585411 2008-07-14T12:47:03Z Samuel13 7172777 {{Chembox new | Name = Xenon difluoride | ImageFile = Xenon-difluoride-2D.png <!-- | ImageSize = 140px --> | ImageName = Xenon difluoride | ImageFile1 = Xenon-difluoride-3D-vdW.png <!-- | ImageSize1 = 140px --> | ImageName1 = Xenon difluoride | IUPACName = xenon(II) fluoride | OtherNames = xenon fluoride, xenon difluoride | Section1 = {{Chembox Identifiers | SMILES = FXeF | CASNo = 13709-36-9 }} | Section2 = {{Chembox Properties | Formula = XeF<sub>2<sub> | MolarMass = 169.2968 g mol<sup>−1</sup> | Appearance = Moisture sensitive white solid | Density = 4.32 g cm<sup>−3</sup>, solid | Solubility = Decomposes<br />0.042 g/100mL @ 999°C | BoilingPt = 114 °C | VaporPressure = 5.2 kPa | pKa = | Viscosity = }} | Section3 = {{Chembox Structure | MolShape = Linear | CrystalStruct = parallel linear XeF<sub>2</sub> units | Dipole = 0 [[Debye|D]] }} | Section7 = {{Chembox Hazards | ExternalMSDS = | MainHazards = Explosive, corrosive, highly toxic }} | Section8 = {{Chembox Related | Function = [[noble gas compounds|xenon compounds]] | OtherFunctn = [[xenon tetrafluoride|XeF<sub>4</sub>]], [[xenon trioxide|XeO<sub>3<sub>]] | Function = [[fluoride]]s | OtherFunctn = [[krypton difluoride|KrF<sub>2<sub>]], [[beryllium fluoride|BeF<sub>2<sub>]], [[oxygen difluoride|OF<sub>2<sub>]] }} }} '''Xenon difluoride''' is a powerful [[fluorinating agent]], with the chemical formula Xe<sub>F2</sub>, is one of the most stable [[xenon compounds]]. Like most covalent inorganic [[fluoride]]s it is moisture sensitive. It decomposes on contact with light or water vapour. Xenon difluoride is a dense, white [[crystal]]line solid. It has a nauseating odour but low vapor pressure (Weeks, 1966). Xenon difluoride is a [[Linear (chemistry)|linear]] molecule. It has a strong characteristic [[infra-red]] doublet at 550 [[wavenumber|cm<sup>−1</sup>]] and 556 cm<sup>−1</sup>. ==Synthesis== Synthesis proceeds by the simple formula Xe + F<sub>2</sub> → XeF<sub>2</sub>. The reaction requires heat, irradiation, or an electrical discharge. The product is gaseous, but can be condensed at -30 °C. It is purified by [[fractional distillation]] or selective condensation using a vacuum line. The synthesis of XeF<sub>2</sub> was first reported by Weeks, Cherwick, and Matheson of [[Argonne National Laboratory]] in 1962. They used an all-nickel system with sapphire windows. Equal parts Xe and F<sub>2</sub> gases react at low pressure upon irradiation by an ultraviolet source to give XeF<sub>2</sub>. Williamson reported that the reaction works equally well at atmospheric pressure in a dry Pyrex glass bulb using sunlight as a source. It was noted that the synthesis worked even on cloudy days. In the previous syntheses the F<sub>2</sub> reactant had been purified to remove H<sub>2</sub>. Šmalc and Lutar found that if this step is skipped the reaction rate actually proceeds at 4 times the original rate. ===Safety considerations=== Xenon difluoride (XeF<sub>2</sub>) was first made by combining xenon and oxygen difluoride (OF<sub>2</sub>) in a nickel tube at 300 degree Celsius under pressure. The same compound can be made now from xenon and fluorine. An evacuated glass container of fluorine and xenon is exposed to daylight. The usual precautions associated with use of F<sub>2</sub> are required: grease-free, preferably fluorine passivated metal system or very dry glasswear. Air must be excluded to preclude formation of [[xenon trioxide]], an explosive (this is only true if the XeF<sub>2</sub> sample contains XeF<sub>4</sub> which hydrolyzes to xenon trioxide). ==Coordination chemistry== XeF<sub>2</sub> can act as a ligand in coordination complexes when accompanied by AsF<sub>6</sub>. One such example is the reaction in HF solution: :Mg(AsF<sub>6</sub>)<sub>2</sub> + 4 XeF<sub>2</sub> → [Mg(XeF<sub>2</sub>)<sub>4</sub>](AsF<sub>6</sub>)<sub>2</sub>. Crystallographic analysis shows that the magnesium is coordinated to 6 fluorine atoms. Four of the fluorines are attributed to the four xenon difluoride ligands while the other two are a pair of cis AsF<sub>6</sub> ligands. A similar reaction is Mg(AsF<sub>6</sub>)<sub>2</sub> + 2 XeF<sub>2</sub> → [Mg(XeF<sub>2</sub>)<sub>2</sub>](AsF<sub>6</sub>)<sub>2</sub>. In the crystal structure of this product the magnesium is octahedrally coordinated and the XeF<sub>2</sub> ligands are axial while the AsF<sub>6</sub> ligands are equatorial. Many such reactions of the form [M<sup>x</sup>(XeF<sub>2</sub>)<sub>n</sub>](AF<sub>6</sub>)<sub>x</sub> have been observed where M can be Ca, Sr, Ba, Pb, Ag, La, or Nd and A can be As, Sb or P. Recently a compound has been synthesised where a metal is coordinated solely by XeF<sub>2</sub> fluorines. The reaction is :2 Ca (AsF<sub>6</sub> )<sub>2</sub> + 9 XeF<sub>2</sub> → Ca<sub>2</sub>(XeF<sub>2</sub>)<sub>9</sub>(AsF<sub>6</sub>)<sub>4</sub>. This reaction requires a large excess of xenon difluoride. The structure of the salt is such that one half of Ca ions is coordinated by fluorines from xenon difluoride while the [[coordination sphere]] of the other Ca ion bears both XeF<sub>2</sub> and AsF<sub>6</sub> ligands. ==Fluorination reactions== ===Oxidative fluorination=== An example of inorganic oxidative fluorination is Ph<sub>3</sub>TeF + XeF<sub>2</sub> → Ph<sub>3</sub>TeF<sub>3</sub> + Xe ===Reductive fluorination=== Examples of reductive fluorination are: :2CrO<sub>2</sub>F<sub>2</sub> + XeF<sub>2</sub> → 2CrOF<sub>3</sub> + Xe +O<sub>2</sub> ===Aromatic fluorination=== [[Image:fluor1.png|250px]][[Image:fluor2.png|250px]] ===Alkene fluorination=== [[Image:fluor3.png|500px]]. ===Decarboxylation=== Xenon difluoride will oxidatively decarboxylate carboxylic acids to the corresponding fluoroalkanes : RCO<sub>2</sub>H + XeF<sub>2</sub> → RF + CO<sub>2</sub> + Xe + HF ==Use as an etchant== Xenon difluoride is used as an etchant for silicon, particularly in the production of [[Microelectromechanical systems]], or MEMS. From<ref>Brazzle, J.D.; Dokmeci, M.R.; Mastrangelo, C.H.; {{doi-inline|10.1109/MEMS.2004.1290690|Modeling and characterization of sacrificial polysilicon etching using vapor-phase xenon difluoride}}, 17th IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 2004, pages 737-740.</ref>: <blockquote>The mechanism of the etch is as follows. First, the XeF<sub>2</sub> absorbs and dissociates to xenon (Xe) and fluorine (F) on the surface of silicon. Fluorine is the main etchant in the silicon etching process. The reaction describing the silicon with XeF is :2 XeF<sub>2</sub> + Si → 2Xe + SiF<sub>4</sub> XeF<sub>2</sub> has a relatively high etch rate and does not require ion bombardment or external energy sources in order to etch silicon. </blockquote> ==References== *Tius, M. A., ''Tetrahedron'', Volume 51, Issue 24, 12 June 1995, Pages 6605-6634 , {{doi-inline|10.1016/0040-4020(95)00362-C|Xenon Difluoride in Synthesis}}. A good review. *{{cite journal | title = Recent Advances in Electrophilic Fluorination | author = Taylor, S.; Kotoris, C.; Hum, G., | journal = Tetrahedron | volume = 55 | issue = 43 | pages = 12431–12477 | year = 1999 | url = | doi = 10.1016/S0040-4020(99)00748-6 }} A review of fluorination in general. *{{cite journal | title = Photochemical Preparation of Xenon Difluoride" Photochemical Preparation of Xenon Difluoride | author = Weeks, J.; Matheson, M.; Chernick, C., | journal = J. Am. Chem. Soc. | volume = 84 | issue = 23 | pages = 4612–4613 | year = 1962 | url = | doi = 10.1021/ja00882a063 }} *Weeks, J.; Matheson, M., "Xenon Difluoride",<u> Inorganic Syntheses</u> (8) '''1966''' *Williamson, S., "Xenon Difluoride",<u> Inorganic Syntheses</u> (11) '''1968''' *Šmalc, A.; Lutar, K., "Xenon Difluoride (Modification)",<u> Inorganic Syntheses</u> (29) '''1992''' *Tramšek, M.; Benkič, P.; Žemva, B., ''Inorg. Chem.'', '''43''' (2), 699 -703, (2004) "First Compounds of Magnesium with XeF<sub>2</sub>" *Tramšek, M.; Benkič, P.; Žemva, B., ''Angewandte Chemie International Edition'', '''43''', (2), 3456 (2004) "The First Compound Containing a Metal Center in a Homoleptic Environment of XeF<sub>2</sub> Molecules" *Greenwood, N.; Earnshaw, A.,<u>Chemistry of the Elements</u> Second Edition, p. 894 '''1997''' *D. F. Halpem, "Xenon(II) Fluoride" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. <references/> [[Category:xenon compounds]] [[Category:fluorides]] [[Category:Inorganic compounds]] [[cs:Fluorid xenonatý]] [[de:Xenondifluorid]] [[zh:二氟化氙]]