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| Names | |
|---|---|
| Other names Hexafluoroarsenate(V) | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider | |
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| Properties | |
| AsF−6 | |
| Molar mass | 188.91 g/mol |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Thehexafluoroarsenate (sometimes shortened tofluoroarsenate) anion is achemical species with formulaAsF−6. Hexafluoroarsenate is relatively inert, being theconjugate base of the notionalsuperacidhexafluoroarsenic acid (HAsF6).
The first undisputed synthesis is due toOtto Ruff,Kurt Stäuber andHugo Graf, who began with the lower-valentarsenic trifluoride, usingsilver(I) fluoride as both a fluorine source andoxidant:[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 witharsenic pentafluoride (AsF5), whichabstractsfluoride from common donors, such ashydrogen fluoride (HF) orcis-difluorodiazine (N2F2).[2] Although the hexafluoroarsenate ion is stable against hydrolysis, the related hydroxyfluoroarsenate ion (AsF5OH−) is not; synthesis of hexafluoroarsenates frompentavalent arsenicoxides and aqueoushydrogen fluoride requires thermaldehydration or extensivestoichiometric excess of the latter.[3][4]
Like its pnictogen congeners, hexafluoroarsenate is anoncoordinating anion, acounterion used to stably store extremely reactivecations.[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 tometathesis withsilver(I),ammonium,potassium, orcaesium ions.[3][2] Unlike the former three, caesium hexafluoroarsenate isinsoluble in water.
Hexafluoroarsenic acid is an extremelystrong acid. The anhydrous compound has been analyzed byX-ray crystallography, which reveals hexafluoroarsenate with a proton attached to one fluoride.[7] The more commonly encountered hydrate isisostructural with the hydrates ofhexafluorophosphoric acid andhexafluoroantimonic acid.[8] These salts contain MF6− (M = P, As, Sb), HF, and water.
Intercalation compounds ofgraphite and hexafluoroarsenic acid exhibit unusually high conductivity, leading to early proposals that the acid might serve as anelectrode orelectrolyte in high-energy batteries.[9][4] Subsequent investigation revealed that the high conductivity occurs because bothelectron holes in the graphite and the hexafluoroarsenate ions themselves serve ascharge carriers.[9][10]
Dess 1955 citesMarignac, 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.