Hexafluoroarsenate Explained

The hexafluoroarsenate (sometimes shortened to fluoroarsenate) anion is a chemical species with formula . Hexafluoroarsenate is relatively inert, being the conjugate base of the notional superacid hexafluoroarsenic acid .

Synthesis

The first undisputed synthesis is due to Otto Ruff, Kurt Stäuber and Hugo Graf, who began with the lower-valent arsenic trifluoride, using silver(I) fluoride as both a fluorine source and oxidant:^[1] AsF3 + 3AgF + NOCl -> NOAsF6 + AgCl + 2AgIn the following reaction, one mole of arsenic trifluoride, three moles of silver fluoride, and one mole of nitrosyl chloride are reacted to produce one mole of nitrosyl hexafluoroarsenate, one mole of silver chloride, and two moles of elemental silver.

Modern syntheses usually begin with arsenic pentafluoride, which abstracts fluoride from common donors, such as hydrogen fluoride or cis-difluorodiazine .^[2] Although the hexafluoroarsenate ion is stable against hydrolysis, the related hydroxyfluoroarsenate ion is not; synthesis of hexafluoroarsenates from pentavalent arsenic oxides and aqueous hydrogen fluoride requires thermal dehydration or extensive stoichiometric excess of the latter.^{[3] [4]}

Conjugate acid and other salts

Like its pnictogen congeners, hexafluoroarsenate is a noncoordinating anion, a counterion used to stably store extremely reactive cations.^[5] Through the appropriate choice of fluorine donor, the synthesis of hexafluoroarsenate can also double as preparation of an exotic cation.^[6] The resulting salts are typically stable to metathesis with silver(I), ammonium, potassium, or caesium ions. Unlike the former three, caesium hexafluoroarsenate is insoluble in water.

Hexafluoroarsenic acid is an extremely strong acid. The anhydrous compound has been analyzed by X-ray crystallography, which reveals hexafluoroarsenate with a proton attached to one fluoride.^[7] The more commonly encountered hydrate is isostructural with the hydrates of hexafluorophosphoric acid and hexafluoroantimonic acid.^[8] These salts contain MF₆⁻ (M = P, As, Sb), HF, and water.

Applications

Intercalation compounds of graphite and hexafluoroarsenic acid exhibit unusually high conductivity, leading to early proposals that the acid might serve as an electrode or electrolyte in high-energy batteries. Subsequent investigation revealed that the high conductivity occurs because both electron holes in the graphite and the hexafluoroarsenate ions themselves serve as charge carriers.^{[9] [10]}

See also

• • Hexafluorophosphate - phosphorus analogue

Notes and References

1. Über Verbindungen des Arsen­pentafluorids und Antimon­pentafluorids mit Nitrosyl­fluorid. de. On the Fusion of Arsenic and Antimony Pentafluorides with Nitrosyl Fluoride. Ruff. Otto. Stäuber. Kurt. Graf. Hugo. 325–337. 1 May 1908. 10.1002/zaac.19080580130. Zeitschrift für anorganische und allgemeine Chemie. Wiley.

   cites Sur Quelques Fluosels de l'Antimoine et de l'Arsenic. fr. On Some Fluorine Salts of Antimony and Arsenic. Marignac. M. C.. 371–385. Annales de chimie et de physique. Gallica. 1867.,. but discounts it as describing an implausibly easy synthesis with a hydrolyzable product.

2. The Preparation of Fluorodiazonium Hexafluoroarsenate (N₂F⁺AsF⁻) from cis-Difluorodiazine. Moy. David. Young. Archie R. II. . 87. 9. May 5, 1965. 1889–1892. 10.1021/ja01087a010.
3. The Preparation and Properties of Complex Fluoroarsenate Compounds. Dess. Harry Martin. Feb 9, 1955. University of Michigan. Excerpted in the Journal of the American Chemical Society, DOI 10.1021/ja01564a018.
4. Inorganic Chemistry. 10. 5. 1971. Lithium Hexafluoroarsenate and Hexafluoroarsenic Acid. Edward W.. Lawless. C. J. Wesley. Wiegand. Yukio. Mizumoto. Constance. Weis. July 28, 1970. 1084–1086. 10.1021/ic50099a048 .
5. Journal of the Less Common Metals. 94. 3 February 1983. 305–308. Hexafluoroarsenate as a Non-Coordinating Anion in Lanthanide Complexes with the Diphenyl Sulphoxide Ligand. Maia Melo. Sérgio. Sousa Silveira. Alexandre. 2 . Elsevier Sequoia. The Netherlands. 10.1016/0022-5088(83)90029-2.
6. Journal of Fluorine Chemistry. 25. 1984. 387–394. December 5, 1983. Novel Ammonium Hexafluoroarsenate Salts from Reaction of (CF₃)₂NH, CF₃N(OCF₃)H, CF₃N[OCF(CF₃)₂]H, CF₃NHF and SF₅NHF with the Strong Acid HF/AsF₅. Darryl D.. Desmarteau. William Y.. Lam. Brian A.. O'Brien. Shi-Ching Chang. 3 . Elsevier Sequoia S.A.. The Netherlands. 10.1016/S0022-1139(00)81212-9.
7. The Existence of Hexafluoroarsenic(V) Acid. Angewandte Chemie International Edition. 10.1002/anie.201308023. Joachim. Axhausen. Karin. Lux. Andreas. Kornath. 2014. Wiley. 53. 14 . 3720–3721. 24446235 .
8. Hydrogen Fluoride Containing Isostructural Hydrates of Hexafluoro­phosphoric, Hexafluoro­arsenic, and Hexafluoro­antimonic Acids. Davidson. D. W.. Calvert. L. D.. Lee. F.. Ripmeester. J. A.. 31 July 1980. Inorg. Chem.. 1981. 20. 2013–2016. 10.1021/ic50221a016. Also published as NRCC 18823.
9. Materials Science and Engineering. 31. 1977. 261–265. Elsevier Sequoia S.A.. The Netherlands/Lausanne, Switzerland. High Electrical Conductivity in Graphite Intercalated with Acid Fluorides. F. L.. Vogel. G. M. T.. Foley. C.. Zeller. E. R.. Falardeau. J.. Gan. 10.1016/0025-5416(77)90043-X.
10. Ionic Salt Limit in Graphite–Fluoroarsenate Intercalation Compounds. J. Chem. Phys.. 78. 5800–5808. 10.1063/1.445423. 1 May 1983. 31 August 1998. J. W.. Milliken. J. E.. Fischer. 9. 1983JChPh..78.5800M .