Xenate and Perxenate

Shown below is a mechanism for the first step. Hydroxide (or water) attacks as a nucleophile fluoride leaves in the next step, as expected for an 8 2-81 process, followed by deprotonation of the Xe-bound hydroxide. 

The XeOp4 so produced may be hydrolyzed further to give Xe03 explosive as a solid ) 

Xe03 is typically obtained by hydrolysis of XeFg (as described above) it s soluble in water to quite high concentrations but does at one point crystallize out as colorless, dangerously explosive crystals. In alkaline solution, Xe03 forms the anion of xenic acid  

The mechanism is a simple A process (i.e., nucleophile-electrophile association)  

The xenate anion is unstable with respect to disproportionation, yielding perxenate (XeOg ) and elemental Xe  

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Xenates and Perxenates. Alkali metal xenates of composition MHXe04-1.5H20, where M is sodium, potassium, mbidium, or cesium, have been prepared by free2e-dryiQg mixtures of xenon trioxide and the corresponding metal hydroxides ia 1 1 molar ratios. The xenates are unstable, explosive solids. 

As mentioned above, xenon trioxide is an endothermic compound which explodes violently at the slightest provocation. Aqueous solutions are stable but powerfully oxidizing. These solutions are weakly acidic ( xenic acid ) and contain molecular XeOj When these solutions are made basic. HXeO ions are formed and alkali hydrogen xenates, MHXe04, may be isolated from them. Hydrogen xenale ions disproportionate in alkaline solution to yield perxenates ... 

Xenate solutions may also be oxidized directly to perxenate with ozone. Solid perxenates are rather insoluble and are unusually stable for xenon-oxygen compounds Most do not decompose until healed above 200 eC. X-ray crystallographic structures have been determined for several perxenates. and they have been found to contain the octahedral XeC ion. which persists in aqueous solution (possibly with proionation to HXeOj"). 

CsHXe04 has been prepared by the reaction of xenon trioxide with aqueous cesium hydroxide in the presence of fluoride ion. The role of the fluoride ion in the preparation of CsHXe04 is not clear, but it seems that it prevents the formation of perxenate. In the absence of the fluoride ion, xenon trioxide and cesium hydroxide form a mixed perxenate and xenate salt. CsHXe04 is stable in air. 

S1S.1 As the self-test states, xenate (HXeOO decomposes to perxenate (XeOs", xenon, and oxygen, so the equation that must be balanced is ... 

Xenates contain Xe (+ VI) and they slowly disproportionate in solution to perxenates (which contain Xe (+ VIII) and Xe... 

The addition of alkali to Xe03 solutions yields xenate ions [HXeOJ. Although salts of this species can be isolated, solutions containing [HXe04] disproportionate to give the perxenate ion, [Xe06]4", and xenon ... 

Hydrolysis of xenon hexafluoride or tetrafluoride with a sodium hydroxide solution results in the precipitation of a stable xenon (VIII) salt, sodium perxenate, Na4Xe06, which on heating does not decompose until around 300°. Similar hydrolysis with potassium hydroxide can yield a yellow precipitate which is a mixed potassium perxenate-xenon trioxide salt, and is explosive even when damp. With more concentrated base, the much more stable hydrated per> xenate may be isolated. 

Pulse radiolysis ° of aqueous solutions of xenon trioxide at pH 8-9, and of xenate, HXe04, at pH 11-13 gave unstable species with xenon in the formal oxidation states 5 and 7. Pulse radiolysis and flash photolysis of aqueous solutions of perxenate HXeOe gives xenon(IX) compounds. These show similarities to the corresponding iodine species in oxidation states 4, 6, and 8. 

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