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In chemistry, perxenates are salts of the yellow xenon-containing anion XeO
₆. 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.

Synthesis

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

2 XeO₃ (s) + 4 OH (aq) → Xe (g) + XeO
₆ (aq) + O₂ (g) + 2 H₂O (l)

When Ba(OH)₂ is used as the alkali, barium perxenate can be crystallized from the resulting solution.

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:

2 HXeO3−6 + 6 H → 2 XeO₃ + 4 H₂O + O₂

Its extrapolated formula, H₄XeO₆, is inferred from the octahedral geometry of the perxenate ion (XeO
₆) in its alkali metal salts.

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 H
₃XeO
₆, which has a pK_a value of 4.29, still relatively acidic. The twice deprotonated species H
₂XeO
₆ has a pK_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 XeO
₆.

Properties

Perxenic acid and the anion XeO
₆ are both strong oxidizing agents, capable of oxidising silver(I), copper (II) and manganese(II) to (respectively) silver(III), copper(III), and permanganate. The perxenate anion is unstable in acidic solutions, being almost instantaneously reduced to HXeO
₄.

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

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

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

Applications

Sodium perxenate, Na₄XeO₆, can be used for the analytic separation of trace amounts of americium from curium. The separation involves the oxidation of Am to Am by sodium perxenate in acidic solution in the presence of La, followed by treatment with calcium fluoride, which forms insoluble fluorides with Cm and La, but retains Am and Pu in solution as soluble fluorides.

References

1. ^ Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, p. 593, ISBN 0-471-19957-5
2. ^ Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, p. 399, ISBN 0-12-352651-5
3. Peterson, J. L.; Claassen, H. H.; Appelman, E. H. (March 1970). "Vibrational spectra and structures of xenate(VI) and perxenate(VIII) ions in aqueous solution". Inorganic Chemistry. 9 (3): 619–621. doi:10.1021/ic50085a037.
4. ^ Hamilton; Ibers, J.; MacKenzie, D. (Aug 1963). "Geometry of the Perxenate Ion". Science. 141 (3580): 532–534. Bibcode:1963Sci...141..532H. doi:10.1126/science.141.3580.532. ISSN 0036-8075. PMID 17738629. S2CID 27297165.
5. ^ Harding, Charlie; Johnson, David Arthur; Janes, Rob (2002). Elements of the p Block. Molecular World. Vol. 9. Royal Society of Chemistry. p. 93. ISBN 0-85404-690-9.
6. ^ Klaening, U. K.; Appelman, E. H. (October 1988). "Protolytic properties of perxenic acid". Inorganic Chemistry. 27 (21): 3760–3762. doi:10.1021/ic00294a018.
7. Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, p. 400, ISBN 0-12-352651-5
8. John H. Holloway; Eric G. Hope (1998). A. G. Sykes (ed.). Advances in Inorganic Chemistry. Vol. 46. Academic Press. p. 67. ISBN 0-12-023646-X.
9. ^ Holcomb, H. P. (March 1965). "Analytical Oxidation of Americium with Sodium Perxenate". Analytical Chemistry. 37 (3): 415. doi:10.1021/ac60222a002.
10. ^ Allen J. Bard; Roger Parsons; Joseph Jordan; International Union of Pure and Applied Chemistry (1985). Standard Potentials in Aqueous Solution. CRC Press. p. 778. ISBN 0-8247-7291-1.
11. Linus Pauling (1988). General Chemistry (3rd ed.). Courier Dover Publications. p. 251. ISBN 0-486-65622-5.
12. Thomas Scott; Mary Eagleson (1994). Concise encyclopedia chemistry. Walter de Gruyter. p. 1183. ISBN 3-11-011451-8.
13. Charlie Harding; David Arthur Johnson; Rob Janes (2002). Elements of the p block. Great Britain: Royal Society of Chemistry. pp. 92–93. ISBN 0-85404-690-9.

• Oxyanions
• Salts
• Xenon(VIII) compounds
• Octahedral compounds