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Concept
Hexafluorure de xénon
Résumé
Xenon hexafluoride is a noble gas compound with the formula XeF6. It is one of the three binary fluorides of xenon that have been studied experimentally, 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. It is a colorless solid that readily sublimes into intensely yellow vapors.
Xenon hexafluoride can be prepared by heating of XeF2 at about 300 °C under 6 MPa (60 atmospheres) of fluorine. With NiF2 as catalyst, however, this reaction can proceed at 120 °C even in xenon-fluorine molar ratios as low as 1:5.
The structure of XeF6 required several years to establish in contrast to the cases of XeF2 and XeF4. 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. It is a fluxional molecule. Oh is only insignificantly higher, indicating that the minimum on the energy surface is very shallow.
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".
Six polymorphs of XeF6 are known. including one that contains XeF ions with bridging F ions.
Xenon hexafluoride hydrolyzes, ultimately affording xenon trioxide:
XeF6 + H2O → XeOF4 + 2 HF
XeOF4 + H2O → XeO2F2 + 2 HF
XeO2F2 + H2O → XeO3 + 2 HF
XeF6 + 3 H2O → XeO3 + 6 HF
XeF6 is a Lewis acid, binding one and two fluoride anions:
XeF6 + F− → XeF
XeF + F− → XeF
Salts of the octafluoroxenate(VI) anion (XeF) are very stable, decomposing only above 400 °C. This anion has been shown to have square antiprismatic geometry, based on single-crystal X-ray counter analysis of its nitrosonium salt, (NO)2XeF8.
Source officielle
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