Perxenate

Template:Short description In chemistry, perxenates are salts of the yellow<ref name="cotton1" /> xenon-containing anion Template:Chem.<ref name="wiberg">Template:Holleman&Wiberg</ref> This anion has octahedral molecular geometry, as determined by Raman spectroscopy, having O–Xe–O bond angles varying between 87° and 93°.<ref>Template:Cite journal</ref> The Xe–O bond length was determined by X-ray crystallography to be 1.875 Å.<ref name="hamilton1963">Template:Cite journal</ref>

Synthesis

Perxenates are synthesized by the disproportionation of xenon trioxide when dissolved in strong alkali:<ref name="harding2002">Template:Cite book</ref>

2 XeO₃ (Template:Abbr) + 4 OH⁻ (Template:Abbr) → Xe (Template:Abbr) + Template:Chem (Template:Abbr) + O₂ (Template:Abbr) + 2 H₂O (Template:Abbr)

When Ba(OH)₂ 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

Perxenic acid

Perxenic acid is the unstable conjugate acid of the perxenate anion, formed by the solution of xenon tetroxide in 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">Template:Cite journal</ref><ref>Template:Holleman&Wiberg</ref>

Template:Chem2

Its extrapolated formula, H₄XeO₆, is inferred from the octahedral geometry of the perxenate ion (Template:Chem) in its alkali metal salts.<ref name="klaening"/><ref name="hamilton1963"/>

The pK_a of aqueous perxenic acid has been indirectly calculated to be below 0, making it an extremely strong acid. Its first ionization yields the anion Template:Chem, which has a pK_a value of 4.29, still relatively acidic. The twice deprotonated species Template:Chem has a pK_a value of 10.81.<ref>Template:Cite book</ref> Due to its rapid decomposition under acidic conditions as described above, however, it is most commonly known as perxenate salts, bearing the anion Template:Chem.<ref name="klaening" /><ref name="wiberg" />

Properties

Perxenic acid and the anion Template:Chem are both strong oxidizing agents,<ref name="holcomb">Template:Cite journal</ref> capable of oxidising silver(I), copper (II) and manganese(II) to (respectively) silver(III), copper(III),<ref name="bard1" /> and permanganate.<ref>Template:Cite book</ref> The perxenate anion is unstable in acidic solutions,<ref name="bard1" /> being almost instantaneously reduced to Template:Chem.<ref name="cotton1">Template:Cotton&Wilkinson6th</ref>

The sodium, potassium, and barium salts are soluble.<ref>Template:Cite book</ref> Barium perxenate solution is used as the starting material for the synthesis of xenon tetroxide (XeO₄) by mixing it with concentrated sulfuric acid:<ref>Template:Cite book</ref>

Ba₂XeO₆ (s) + 2 H₂SO₄ (l) → XeO₄ (g) + 2 BaSO₄ (s) + 2 H₂O (l)

Most metal perxenates are stable, except silver perxenate, which decomposes violently.<ref name="bard1">Template:Cite book</ref>

Applications

Sodium perxenate, Na₄XeO₆, can be used for the analytic separation of trace amounts of americium from curium. The separation involves the oxidation of Am³⁺ to Am⁴⁺ by sodium perxenate in acidic solution in the presence of La³⁺, followed by treatment with calcium fluoride, which forms insoluble fluorides with Cm³⁺ and La³⁺, but retains Am⁴⁺ and Pu⁴⁺ in solution as soluble fluorides.<ref name="holcomb"/>