Xenon tetrafluoride

Xe + 2 F
₂ → XeF
₄

This reaction is exothermic, releasing an energy of 251 kJ/mol.

Its discovery in 1962 was inspired by the discovery earlier in the same year by Neil Bartlett of the first xenon compound, XePtF₆, which showed that it was possible for a xenon compound to exist.

Xenon tetrafluoride is a colorless crystalline solid that sublimes at 117 °C. Its structure was determined by both NMR spectroscopy and X-ray crystallography in 1963. The structure is square planar, as has been confirmed by neutron diffraction studies. According to VSEPR theory, in addition to four fluoride ligands, the xenon center has two lone pairs of electrons. These lone pairs are mutually trans.

Synthesis

The original synthesis of xenon tetrafluoride occurred through direct 1:5-molar-ratio combination of the elements in a nickel (Monel) vessel at 400 °C. The nickel does not catalyze the reaction, but rather protects the container surfaces against fluoride corrosion. Controlling the process against impurities is difficult, as xenon difluoride (XeF
₂), tetrafluoride, and hexafluoride (XeF
₆) are all in chemical equilibrium, the difluoride favored at low temperatures and little fluorine and the hexafluoride favored at high temperatures and excess fluorine. Fractional sublimation (xenon tetrafluoride is particularly involatile) or other equilibria generally allow purification of the product mixture.

The elements combine more selectively when γ- or UV-irradiated in a nickel container or dissolved in anhydrous hydrogen fluoride with catalytic oxygen. That reaction is believed selective because dioxygen difluoride at standard conditions is too weak an oxidant to generate xenon(VI) species.

Alternatively, fluoroxenonium perfluorometallate salts pyrolyze to XeF₄.