exception of XeCl2 and XeCl4. Xenon dichloride, formed by the high-frequency irradiation of a mixture of xenon, fluorine, and silicon or carbon tetrachloride, is reported to be an endothermic, colorless, crystalline compound that decomposes into the elements at 80 °C. However, may be merely a van der Waals molecule of weakly bound Xe atoms and molecules and not a real compound. Theoretical calculations indicate that the linear molecule is less stable than the van der Waals complex. Xenon tetrachloride is so unstable that it cannot be synthesized by chemical reactions. It was created by radioactive decay. Oxides and oxohalides Three oxides of xenon are known: xenon trioxide () and xenon tetroxide (), both of which are dangerously explosive and powerful oxidizing agents, and xenon dioxide (XeO2), which was reported in 2011 with a coordination number of four. XeO2 forms when xenon tetrafluoride is poured over ice. Its crystal structure may allow it to replace silicon in silicate minerals. The XeOO+ cation has been identified by infrared spectroscopy in solid argon. Xenon does not react with oxygen directly; the trioxide is formed by the hydrolysis of : + 3 → + 6 HF is weakly acidic, dissolving in alkali to form unstable xenate salts containing the anion. These unstable salts easily disproportionate into xenon gas and perxenate salts, containing the anion. Barium perxenate, when treated with concentrated sulfuric acid, yields gaseous xenon tetroxide: + 2 → 2 + 2 + To prevent decomposition, the xenon tetroxide thus formed is quickly cooled into a pale-yellow solid. It explodes above −35.9 °C into xenon and oxygen gas, but is otherwise stable. A number of xenon oxyfluorides are known, including , , , and . is formed by reacting with xenon gas at low temperatures. It may also be obtained by partial hydrolysis of . It disproportionates at −20 °C into and . is formed by the partial hydrolysis of , or the reaction of with sodium perxenate, . The latter reaction also produces a small amount of . reacts with CsF to form the anion, while XeOF3 reacts with the alkali metal fluorides KF, RbF and CsF to form the anion. Other compounds Xenon can be directly bonded to a less electronegative element than fluorine or oxygen, particularly carbon. Electron-withdrawing groups, such as groups with fluorine substitution, are necessary to stabilize these compounds. Numerous such compounds have been characterized, including: , where C6F5