Xenon hexafluoride

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Xenon hexafluoride
Identifiers
CAS number 13693-09-9 N
PubChem 139546
ChemSpider 123066 YesY
Jmol-3D images {{#if:F[Xe](F)(F)(F)(F)F|Image 1
Properties
Molecular formula XeF6
Molar mass 245.28 g mol-1
Density 3.56 g cm-3
Melting point 49.25 °C
Boiling point 75.6 °C
Solubility in water hydrolyzes
Thermochemistry
Std enthalpy of
formation ΔfHo298		 −294 kJ·mol−1[1]
 N (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Xenon hexafluoride is a noble gas compound with the formula XeF6 and the highest of the three known binary fluorides of xenon, the other two being XeF2 and XeF4. All known 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.

Preparation

Xenon hexafluoride can be prepared by long-term heating of XeF2 at about 300°C and pressure 6 MPa (60 atmospheres).

With NiF
2 as catalyst, however, this reaction can proceed at 120°C even in xenon-fluorine molar ratios as low as 1:5.[2]

Structure

The structure of XeF6 required several years to establish in contrast to the cases of XeF
2 and XeF
4. 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 symmetry, 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."[3]

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".

XeF
6 crystallizes in 6 possible modifications,[4] including one that contains XeF+
5 ions with bridging F−
 ions.[5]

Reactions

Hydrolysis

Xenon hexafluoride hydrolyzes stepwise, ultimately affording xenon trioxide:[6]

XeF6 + H2O → XeOF4 + 2 HF
XeOF4 + H2O → XeO2F2 + 2 HF
XeO2F2 + H2O → XeO3 + 2 HF

XeF6 serves as a Lewis acid, binding one and two fluoride anions:

XeF6 + F− → XeF−
7
XeF−
7 + F− → XeF2−
8

Octafluoroxenates

Salts of the octafluoroxenate(VI) anion (XeF2−
8) are very stable, decomposing only above 400 °C.[7][8][9] This anion has been shown to have square antiprismatic geometry, based on single-crystal X-ray counter analysis of its nitrosonium salt, (NO)
2XeF
8.[10] The sodium and potassium salts are formed directly from sodium fluoride and potassium fluoride:[9]

2 NaF + XeF
6 → Na
2XeF
8
2 KF + XeF
6 → K
2XeF
8

These are thermally less stable than the caesium and rubidium salts, which are synthesized by first forming the heptafluoroxenate salts:

CsF + XeF
6 → CsXeF
7
RbF + XeF
6 → RbXeF
7

which are then pyrolysed at 50°C and 20°C, respectively, to form the yellow[11] octafluoroxenate salts:[7][8][9]

2 CsXeF
7 → Cs
2XeF
8 + XeF
6
2 RbXeF
7 → Rb
2XeF
8 + XeF
6

These salts are hydrolysed by water, yielding various products containing xenon and oxygen.[9]

The two other binary fluorides of xenon do not form such stable adducts with fluoride.

With fluoride acceptors

XeF
6 reacts with strong fluoride acceptors such as RuF
5[5] and BrF
3·AuF
3[12] to form the XeF+
5 cation:

XeF
6 + RuF
5 → XeF+
5RuF−
6
XeF
6 + BrF
3·AuF
3 → XeF+
5AuF−
4 + BrF
3

References

  1. ↑ Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A23. ISBN 0-618-94690-X. 
  2. ↑ Melita Tramšek; Boris Žemva (December 5, 2006). "Synthesis, Properties and Chemistry of Xenon(II) Fluoride" (PDF). Acta Chim. Slov. 53 (2): 105–116. doi:10.1002/chin.200721209. 
  3. ↑ Seppelt, Konrad (June 1979). "Recent Developments in the Chemistry of Some Electronegative Elements". Accounts of Chemical Research 12 (6): 211–216. doi:10.1021/ar50138a004. 
  4. ↑ Hoyer, S.; Emmler, K.; Seppelt, T. (October 2006). "The structure of xenon hexafluoride in the solid state". Journal of Fluorine Chemistry 127 (10): 1415–1422. doi:10.1016/j.jfluchem.2006.04.014. ISSN 0022-1139. 
  5. ↑ 5.0 5.1 James E. House (2008). Inorganic Chemistry. Academic Press. p. 569. ISBN 0-12-356786-6. 
  6. ↑ Appelman, E. H.; and J. G. Malm (June 1964). "Hydrolysis of Xenon Hexafluoride and the Aqueous Solution Chemistry of Xenon". Journal of the American Chemical Society 86 (11): 2141–2148. doi:10.1021/ja01065a009. 
  7. ↑ 7.0 7.1 Holleman, A. F.; Wiberg,, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5. 
  8. ↑ 8.0 8.1 Riedel, Erwin; Janiak, Christoph (2007). Anorganische Chemie (7th ed.). Walter de Gruyter. p. 393. ISBN 3-11-018903-8. 
  9. ↑ 9.0 9.1 9.2 9.3 Chandra, Sulekh (2004). Comprehensive Inorganic Chemistry. New Age International. p. 308. ISBN 81-224-1512-1. 
  10. ↑ Peterson, W.; Holloway, H.; Coyle, A.; Williams, M. (Sep 1971). "Antiprismatic Coordination about Xenon: the Structure of Nitrosonium Octafluoroxenate(VI)". Science 173 (4003): 1238–1239. Bibcode:1971Sci...173.1238P. doi:10.1126/science.173.4003.1238. ISSN 0036-8075. PMID 17775218. 
  11. ↑ "Xenon". Encyclopaedia Britannica. Encyclopaedia Britannica Inc. 1995. 
  12. ↑ Cotton (2007). Advanced Inorganic Chemistry (6th ed.). Wiley-India. p. 591. ISBN 81-265-1338-1. 

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