Xenon hexafluoride

Xenon hexafluoride is the chemical compound with the formula XeF₆. This colorless crystalline compound is one of the three binary fluorides 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.

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Xenon hexafluoride can be prepared by long-term heating of XeF₂ at about 300 degrees and pressure 6 MPa.

Structure

The structure of XeF₆ required several years to establish in contrast to the cases of xenon difluoride and xenon tetrafluoride. In the gas phase it adopts a slightly distorted octahedral geometry. It lacks perfect octahedral symmetry, as predicted by VSEPR theory due to the presence of six fluoride ligands and one lone pair of electrons. According to Konrad Seppelt, then Associate Professor of Chemistry at the University of Heidelberg, "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.".^[1] NMR spectra of¹²⁹Xe¹⁹F₆ indicate that it is tetrameric: 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".

Reactions

Xenon hexafluoride hydrolyzes stepwise, ultimately affording xenon trioxide:^[2]

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^- → XeF₇^-

XeF₇- + F^- → XeF₈^2-

The two other binary fluorides do not form such stable adducts with fluoride. The salt Rb₂XeF₈ is among the most robust xenon compounds, decomposing only above 400 °C.^[3]

References

1. ^ 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.
2. ^ 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.
3. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.