Xenon difluoride

Xenon difluoride is a linear 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 theory, which predicts that there are 3 pairs of non-bonding electrons around the equatorial region of the xenon atom. A 2011 theoretical study cast doubt on these experimental results, suggesting that xenon difluoride remains stable up to 200 GPa, at which point it dissociates into an ionic solid. 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 or selective condensation using a vacuum line. In 1965, it was also synthesized by reacting xenon gas with dioxygen difluoride. Solubility is soluble in interhalogen solvents such as Bromine pentafluoride|, Bromine trifluoride|, Iodine pentafluoride|, and others like anhydrous hydrogen fluoride, and acetonitrile, without reduction or oxidation. Solubility in hydrogen fluoride is high, at 167 g per 100 g HF at 29.95 °C. The XeF+ cation is formed by combining xenon difluoride with a strong fluoride acceptor, such as an excess of liquid antimony pentafluoride (): : + → + Adding xenon gas to this pale yellow solution at a pressure of 2–3 atmospheres produces a green solution containing the paramagnetic ion, which contains a Xe−Xe bond: ("apf" denotes solution in liquid ) : 3 Xe(g) + (apf) + (l) 2 (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 indicates that the Xe–Xe bond length in this compound is 309 pm, indicating a very weak bond. Coordination chemistry Bonding in the XeF2 molecule is adequately described by the three-center four-electron bond model. XeF2 can act as a ligand in coordination complexes of metals. A similar reaction is: :Mg(AsF6)2 + 2 XeF2 → [Mg(XeF2)2](AsF6)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 [Mx(XeF2)n](AF6)x have been observed, where M can be calcium, strontium, barium, lead, silver, lanthanum, or neodymium and A can be arsenic, antimony or phosphorus. Some of these compounds feature extraordinarily high coordination numbers at the metal center. 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. 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, but leaves the methyl groups untouched: : + → + Xe XeF2 can similarly be used to prepare N-fluoroammonium salts, useful as fluorine transfer reagents in organic synthesis (e.g., Selectfluor), from the corresponding tertiary amine: :[R–(CH2CH2)3N:][] + XeF2 + NaBF4 → [R–(CH2CH2)3–F][]2 + NaF + Xe will also oxidatively decarboxylate carboxylic acids to the corresponding fluoroalkanes: :RCOOH + XeF2 → RF + CO2 + Xe + HF Silicon tetrafluoride 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, particularly in the production of microelectromechanical systems (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==