View source for Perxenate

{{Short description|Chemical compound}}
In [[chemistry]], '''perxenates''' are [[salt (chemistry)|salt]]s of the yellow<ref name="cotton1" /> [[xenon]]-containing anion {{chem|XeO|6|4-}}.<ref name="wiberg">{{Holleman&Wiberg|page=399}}</ref> This anion has [[octahedral molecular geometry]], as determined by [[Raman spectroscopy]], having O–Xe–O bond angles varying between 87° and 93°.<ref>{{Cite journal| last1 = Peterson | first1 = J. L.| last2 = Claassen | first2 = H. H.| last3 = Appelman | first3 = E. H.| title = Vibrational spectra and structures of xenate(VI) and perxenate(VIII) ions in aqueous solution| journal = Inorganic Chemistry | volume = 9| issue = 3| pages = 619–621 | date=March 1970 | doi = 10.1021/ic50085a037}}</ref> The Xe–O bond length was determined by X-ray crystallography to be 1.875&nbsp;Å.<ref name="hamilton1963">{{Cite journal | author1 = Hamilton| last3 = MacKenzie| last2 = Ibers | first2 = J. | first3 = D. | title = Geometry of the Perxenate Ion | journal = Science | volume = 141 | issue = 3580 | pages = 532–534 | date=Aug 1963 | issn = 0036-8075 | pmid = 17738629 | doi = 10.1126/science.141.3580.532|bibcode = 1963Sci...141..532H | s2cid = 27297165}}</ref>

== Synthesis ==

Perxenates are synthesized by the [[disproportionation]] of [[xenon trioxide]] when dissolved in strong [[Alkaline earth metal|alkali]]:<ref name="harding2002">{{cite book
| title = Elements of the ''p'' Block |volume=9 |series=Molecular World
| first1= Charlie |last1=Harding
| first2= David Arthur |last2=Johnson
| first3= Rob |last3=Janes
| publisher = Royal Society of Chemistry
| year = 2002
| isbn = 0-85404-690-9
| page = 93
}}</ref>
:2 XeO<sub>3</sub> ({{abbr|s|solid}}) + 4 OH<sup>−</sup> ({{abbr|aq|aqueous solution}}) → Xe ({{abbr|g|gas}}) + {{chem|XeO|6|4-}} ({{abbr|aq|aqueous solution}}) + O<sub>2</sub> ({{abbr|g|gas}}) + 2 H<sub>2</sub>O ({{abbr|l|liquid}})

When [[barium hydroxide|Ba(OH)<sub>2</sub>]] is used as the alkali, barium perxenate can be crystallized from the resulting solution.<ref name="harding2002"/>

==Perxenic acid==
[[File:Perxenonsäure Structural Formula V.1.svg|thumb|Perxenic acid]]
Perxenic acid is the unstable conjugate acid of the perxenate anion, formed by the solution of [[xenon tetroxide]] in [[water (molecule)|water]]. It has not been isolated as a free acid, because under acidic conditions it rapidly decomposes into [[xenon trioxide]] and [[oxygen]] gas:<ref name="klaening">{{Cite journal | last1 = Klaening | first1 = U. K.|last2 = Appelman | first2 =E. H.| title = Protolytic properties of perxenic acid| journal = Inorganic Chemistry | volume = 27 | issue = 21| pages = 3760–3762 | date=October 1988 | doi = 10.1021/ic00294a018}}</ref><ref>{{Holleman&Wiberg|page=400}}</ref>
:{{chem2|2 HXeO6(3-) + 6 H+ → 2 XeO3 + 4 H2O + O2}}

Its extrapolated formula, H<sub>4</sub>XeO<sub>6</sub>, is inferred from the [[octahedral molecular geometry|octahedral geometry]] of the perxenate ion ({{chem|XeO|6|4-}}) in its [[alkali metal]] salts.<ref name="klaening"/><ref name="hamilton1963"/>

The [[pKa|p''K''<sub>a</sub>]] of aqueous perxenic acid has been indirectly calculated to be below 0, making it an extremely strong acid. Its first ionization yields the anion {{chem|H|3|XeO|6|-}}, which has a p''K''<sub>a</sub> value of 4.29, still relatively acidic. The twice deprotonated species {{chem|H|2|XeO|6|2-}} has a p''K''<sub>a</sub> value of 10.81.<ref>{{cite book
| title = Advances in Inorganic Chemistry
| volume = 46
| author1 = John H. Holloway
| author2 = Eric G. Hope
| editor = A. G. Sykes
| publisher = Academic Press
| year = 1998
| isbn = 0-12-023646-X
| page = 67
}}</ref> Due to its rapid decomposition under acidic conditions as described above, however, it is most commonly known as perxenate salts, bearing the anion {{chem|XeO|6|4-}}.<ref name="klaening" /><ref name="wiberg" />

==Properties==

Perxenic acid and the anion {{chem|XeO|6|4-}} are both strong [[oxidizing agent]]s,<ref name="holcomb">{{Cite journal| first1 = H. P. | title = Analytical Oxidation of Americium with Sodium Perxenate | journal = Analytical Chemistry | volume = 37| issue = 3 | page = 415| last1 = Holcomb | date=March 1965 | doi = 10.1021/ac60222a002}}</ref> capable of oxidising silver(I), copper (II) and manganese(II) to (respectively) silver(III), copper(III),<ref name="bard1" /> and [[permanganate]].<ref>{{cite book
| title = General Chemistry
| author = Linus Pauling
| edition = 3rd
| publisher = Courier Dover Publications
| year = 1988
| isbn = 0-486-65622-5
| page = [https://archive.org/details/generalchemistry00paul_0/page/251 251]
| url-access = registration
| url = https://archive.org/details/generalchemistry00paul_0/page/251
}}</ref> The perxenate anion is unstable in acidic solutions,<ref name="bard1" /> being almost instantaneously reduced to {{chem|HXeO|4|-}}.<ref name="cotton1">{{Cotton&Wilkinson6th|page=593}}</ref>

The [[sodium]], [[potassium]], and [[barium]] salts are soluble.<ref>{{cite book
| title = Concise encyclopedia chemistry
| author = Thomas Scott
| author2 = Mary Eagleson
| publisher = Walter de Gruyter
| year = 1994
| isbn = 3-11-011451-8
| page = 1183
}}</ref> Barium perxenate solution is used as the starting material for the synthesis of [[xenon tetroxide]] (XeO<sub>4</sub>) by mixing it with concentrated [[sulfuric acid]]:<ref>{{cite book
| title = Elements of the ''p'' block
| author = Charlie Harding
| author2 = David Arthur Johnson
| author3 = Rob Janes
| location = Great Britain
| publisher = Royal Society of Chemistry
| year = 2002
| isbn = 0-85404-690-9
| pages = 92–93
}}</ref>
: Ba<sub>2</sub>XeO<sub>6</sub> (s) + 2 H<sub>2</sub>SO<sub>4</sub> (l) →  XeO<sub>4</sub> (g) + 2 BaSO<sub>4</sub> (s) + 2 H<sub>2</sub>O (l)

Most metal perxenates are stable, except silver perxenate, which decomposes violently.<ref name="bard1">{{cite book
| title = Standard Potentials in Aqueous Solution
| author = Allen J. Bard
| author2 = Roger Parsons
| author3 = Joseph Jordan
| author4 = International Union of Pure and Applied Chemistry
| publisher = CRC Press
| year = 1985
| isbn = 0-8247-7291-1
| page = [https://archive.org/details/standardpotentia0000unse/page/778 778]
| url = https://archive.org/details/standardpotentia0000unse/page/778
}}</ref>

==Applications==
Sodium perxenate, Na<sub>4</sub>XeO<sub>6</sub>, can be used for the analytic separation of trace amounts of [[americium]] from [[curium]]. The separation involves the oxidation of Am<sup>3+</sup> to Am<sup>4+</sup> by sodium perxenate in acidic solution in the presence of [[lanthanum|La<sup>3+</sup>]], followed by treatment with [[calcium fluoride]], which forms insoluble fluorides with Cm<sup>3+</sup> and La<sup>3+</sup>, but retains Am<sup>4+</sup> and [[plutonium|Pu<sup>4+</sup>]] in solution as soluble fluorides.<ref name="holcomb"/>

==References==
{{reflist}}

{{Xenon compounds}}
{{Noble gas compounds}}

[[Category:Oxyanions]]
[[Category:Salts]]
[[Category:Xenon(VIII) compounds]]
[[Category:Octahedral compounds]]