Hexafluoroarsenate
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Hexafluoroarsenate(V)
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3D model (JSmol)
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| 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).
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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).
Synthesis[edit]
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]
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[edit]
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 caesium ions.[3][2] 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 off hexafluorophosphoric acid and hexafluoroantimonic acid.[8] These salts contain MF6− (M = P, As, Sb), HF, and water.
Applications[edit]
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[edit]
- Superacid – Extremely strong acid
- Hexafluorophosphate — phosphorus analogue
- Arsenic pentafluoride – chemical compound
- Hexafluoroantimonate – chemical compound
References[edit]
- ^ 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.
- ^ a b c 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.
- ^ a b 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.
- ^ a b Lawless, Edward W.; Wiegand, C. J. Wesley; Mizumoto, Yukio; Weis, Constance (July 28, 1970). "Lithium Hexafluoroarsenate and Hexafluoroarsenic Acid". Inorganic Chemistry. 10 (5) (published 1971): 1084–1086. doi:10.1021/ic50099a048.
- ^ 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. 94 (2). The Netherlands: Elsevier Sequoia: 305–308. doi:10.1016/0022-5088(83)90029-2.
- ^ 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. 25 (3). The Netherlands: Elsevier Sequoia S.A. (published 1984): 387–394. doi:10.1016/S0022-1139(00)81212-9.
- ^ Axhausen, Joachim; Lux, Karin; Kornath, Andreas (2014). "The Existence of Hexafluoroarsenic(V) Acid". Angewandte Chemie International Edition. 53 (14). Wiley: 3720–3721. doi:10.1002/anie.201308023. PMID 24446235.
- ^ 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. 20 (published 1981): 2013–2016. doi:10.1021/ic50221a016. Also published as NRCC 18823.
- ^ a b 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. 31. The Netherlands/Lausanne, Switzerland: Elsevier Sequoia S.A.: 261–265. doi:10.1016/0025-5416(77)90043-X.
- ^ Milliken, J. W.; Fischer, J. E. (1 May 1983). "Ionic Salt Limit in Graphite–Fluoroarsenate Intercalation Compounds". J. Chem. Phys. 78 (9) (published 31 August 1998): 5800–5808. Bibcode:1983JChPh..78.5800M. doi:10.1063/1.445423.