Xenon tetroxide

Xenon tetroxide (molecular formula XeO4) is a yellow crystalline solid that is stable below −35.9 °C. The xenon atom has an oxidation state of +8 and oxygen of −2. All eight valence shell electrons of xenon are involved, making it an unstable compound. Above -35.9°C it explodes to give xenon and oxygen gas. Oxygen is the only element that can bring xenon up to its highest oxidation state; even fluorine can only give XeF6, probably for steric reasons. Xenon tetroxide dissolves in water to give perxenic acid, and in alkalis to give the perxenate ion.

Another known oxide is xenon trioxide. The dioxide remains elusive and only the XeOO+ cation has been identified by infrared spectroscopy in solid argon.

Reactions
A spontaneous explosion occurs at temperature above −35.9 °C, with ΔH = -643 kJ/mol.
 * XeO4 → Xe + 2 O2

The two other short lived xenon compounds with an oxidation state of +8 are accessible by the reaction of xenon tetroxide with xenon hexafluoride. XeO3F2 and XeO2F4 can be detected with mass spectroscopy.

Synthesis
All syntheses start from the perxenates, which are accessible from the xenates through two methods. One is disproportionation of xenates to perxenates and xenon:
 * 2 XeO42− → XeO64− + Xe + O2.

The other is oxidation of the xenates with ozone:
 * 2 XeO42− + 4 e- + 2 O3 → 2 XeO6 4- + 2 O2.

Barium perxenate is reacted with sulfuric acid and the unstable perxenic acid is dehydrated to give xenon tetroxide:
 * Ba2XeO6 + 2 H2SO4 → 2 BaSO4 + (H4XeO6 → 2 H2O + XeO4).

The unstable perxenic acid slowly undergoes a disproportionation reaction to the xenic acid and oxygen:
 * H4XeO6 → 1/2 O2 + H2XeO4 + H2O.