Hexafluoroarsenate

The hexafluoroarsenate (sometimes shortened to fluoroarsenate) anion is a chemical species with formula AsF−6. Hexafluoroarsenate is relatively inert, being the conjugate base of the notional superacid hexafluoroarsenic acid (HAsF6).

Hexafluoroarsenate
Names
Other names
Hexafluoroarsenate(V)
Identifiers
3D model (JSmol)
ChemSpider
EC Number
  • acid: 241-128-9
  • InChI=1S/AsF6/c2-1(3,4,5,6)7/q-1
  • Key: ZENAVJHBLNXNHN-UHFFFAOYSA-N
  • F[As-](F)(F)(F)(F)F
Properties
AsF−6
Molar mass 188.91 g/mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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]

In 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 (AsF5), which abstracts fluoride from common donors, such as hydrogen fluoride (HF) or cis-difluorodiazine (N2F2).[2] Although the hexafluoroarsenate ion is stable against hydrolysis, the related hydroxyfluoroarsenate ion (AsF5OH−) 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.[2][6] The resulting salts are typically stable to metathesis with silver(I), ammonium, potassium, or cesium ions.[3][2] Unlike the former three, cesium 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 off 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.[9][4] 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

References
  1. Ruff, Otto; Stäuber, Kurt; Graf, Hugo (1 May 1908). "Über Verbindungen des Arsen­pentafluorids und Antimon­pentafluorids mit Nitrosyl­fluorid" [On the Fusion of Arsenic and Antimony Pentafluorides with Nitrosyl Fluoride]. Zeitschrift für anorganische und allgemeine Chemie (in German). Wiley: 325–337. doi:10.1002/zaac.19080580130.

    Dess 1955 cites Marignac, M. C. (1867). "Sur Quelques Fluosels de l'Antimoine et de l'Arsenic" [On Some Fluorine Salts of Antimony and Arsenic]. Annales de chimie et de physique (in French): 371–385 – via Gallica, but discounts it as describing an implausibly easy synthesis with a hydrolyzable product.

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