Perxenate

In chemistry, perxenates are salts of the yellow^[1] xenon-containing anion XeO⁴⁻
₆.^[2] This anion has octahedral molecular geometry, as determined by Raman spectroscopy, having O–Xe–O bond angles varying between 87° and 93°.^[3] The Xe–O bond length was determined by X-ray crystallography to be 1.875 Å.^[4]

Synthesis

Perxenates are synthesized by the disproportionation of xenon trioxide when dissolved in strong alkali:^[5]

2 XeO₃ (s) + 4 OH⁻ (aq) → Xe (g) + XeO⁴⁻
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.^[5]

Perxenic acid

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:^[6][7]

2 HXeO3−6 + 6 H⁺ → 2 XeO₃ + 4 H₂O + O₂

Its extrapolated formula, H₄XeO₆, is inferred from the octahedral geometry of the perxenate ion (XeO⁴⁻
₆) in its alkali metal salts.^[6][4]

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
₂XeO²⁻
₆ has a pK_a value of 10.81.^[8] Due to its rapid decomposition under acidic conditions as described above, however, it is most commonly known as perxenate salts, bearing the anion XeO⁴⁻
₆.^[6][2]

Properties

Perxenic acid and the anion XeO⁴⁻
₆ are both strong oxidizing agents,^[9] capable of oxidising silver(I), copper (II) and manganese(II) to (respectively) silver(III), copper(III),^[10] and permanganate.^[11] The perxenate anion is unstable in acidic solutions,^[10] being almost instantaneously reduced to HXeO⁻
₄.^[1]

The sodium, potassium, and barium salts are soluble.^[12] Barium perxenate solution is used as the starting material for the synthesis of xenon tetroxide (XeO₄) by mixing it with concentrated sulfuric acid:^[13]

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

Most metal perxenates are stable, except silver perxenate, which decomposes violently.^[10]

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.^[9]

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.