Perxenate

 ( Oxyanions ) 

In chemistry, perxenates are salts of the yellow xenon-containing anion . This anion has octahedral molecular geometry, as determined by Raman spectroscopy, having O–Xe–O bond angles varying between 87° and 93°. The Xe–O bond length was determined by X-ray crystallography to be 1.875 Å.

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Synthesis Perxenates are synthesized by the disproportionation of xenon trioxide when dissolved in strong alkali:

(2025). 9780854046904, Royal Society of Chemistry. ISBN 9780854046904

2 XeO₃ () + 4 OH⁻ () → Xe () + () + O₂ () + 2 H₂O ()

When barium hydroxide is used as the alkali, barium perxenate can be crystallized from the resulting solution.

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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:

Its extrapolated formula, H₄XeO₆, is inferred from the octahedral geometry of the perxenate ion () in its alkali metal salts.

The pKa of aqueous perxenic acid has been indirectly calculated to be below 0, making it an extremely strong acid. Its first ionization yields the anion , which has a p K_a value of 4.29, still relatively acidic. The twice deprotonated species has a p K_a value of 10.81.

Due to its rapid decomposition under acidic conditions as described above, however, it is most commonly known as perxenate salts, bearing the anion .

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Properties

Perxenic acid and the anion are both strong , capable of oxidising silver(I), copper (II) and manganese(II) to (respectively) silver(III), copper(III), and permanganate.

Linus Pauling (1988). 9780486656229, Courier Dover Publications. . ISBN 9780486656229

The perxenate anion is unstable in acidic solutions, being almost instantaneously reduced to .

The sodium, potassium, and barium salts are soluble.

Thomas Scott (1994). 9783110114515, Walter de Gruyter. ISBN 9783110114515

Barium perxenate solution is used as the starting material for the synthesis of xenon tetroxide (XeO₄) by mixing it with concentrated sulfuric acid:

Charlie Harding (2025). 9780854046904, Royal Society of Chemistry. ISBN 9780854046904

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

Most metal perxenates are stable, except silver perxenate, which decomposes violently.

Allen J. Bard (1985). 9780824772918, CRC Press. . ISBN 9780824772918

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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 lanthanum, followed by treatment with calcium fluoride, which forms insoluble fluorides with Cm³⁺ and La³⁺, but retains Am⁴⁺ and plutonium in solution as soluble fluorides.

Categories: Oxyanions, Salts, Xenon Compounds, Octahedral Compounds

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