Hexafluoroarsenate

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:{{cite journal|title=Über Verbindungen des Arsen­pentafluorids und Antimon­pentafluorids mit Nitrosyl­fluorid|language=de|trans-title=On the Fusion of Arsenic and Antimony Pentafluorides with Nitrosyl Fluoride|last1=Ruff|first1=Otto|last2=Stäuber|first2=Kurt|last3=Graf|first3=Hugo|pages=325–337|date=1 May 1908|doi=10.1002/zaac.19080580130|journal=Zeitschrift für anorganische und allgemeine Chemie|publisher=Wiley|url=https://zenodo.org/record/1898421 }}

{{harvnb|Dess|1955}} cites {{cite journal|title=Sur Quelques Fluosels de l'Antimoine et de l'Arsenic|language=fr|trans-title=On Some Fluorine Salts of Antimony and Arsenic|last1=Marignac|first1=M. C.|pages=371–385|journal=Annales de chimie et de physique|via=Gallica|year=1867|postscript=,}} but discounts it as describing an implausibly easy synthesis with a hydrolyzable product.

:{{chem2 | AsF3 + 3 AgF + NOCl -> NOAsF6 + AgCl + 2 Ag }}

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 ({{chem2|AsF5}}), which abstracts fluoride from common donors, such as hydrogen fluoride ({{chem2|HF}}) or cis-difluorodiazine ({{chem2|N2F2}}).{{cite journal|title=The Preparation of Fluorodiazonium Hexafluoroarsenate (N₂F⁺AsF⁻ {{sic}}) from cis-Difluorodiazine|last1=Moy|first1=David|last2=Young|first2=Archie R. II|journal=Journal of the American Chemical Society |volume=87|number=9|date=May 5, 1965|pages=1889–1892|doi=10.1021/ja01087a010|bibcode=1965JAChS..87.1889M }} Although the hexafluoroarsenate ion is stable against hydrolysis, the related hydroxyfluoroarsenate ion ({{chem2|AsF5OH-}}) is not; synthesis of hexafluoroarsenates from pentavalent arsenic oxides and aqueous hydrogen fluoride requires thermal dehydration or extensive stoichiometric excess of the latter.{{cite thesis|title=The Preparation and Properties of Complex Fluoroarsenate Compounds|last=Dess|first=Harry Martin|date=Feb 9, 1955|publisher=University of Michigan}} Excerpted in the Journal of the American Chemical Society, DOI [https://pubs.acs.org/doi/10.1021/ja01564a018 10.1021/ja01564a018].{{cite journal|journal=Inorganic Chemistry|volume=10|number=5|publication-date=1971|title=Lithium Hexafluoroarsenate and Hexafluoroarsenic Acid|first1=Edward W.|last1=Lawless|first2=C. J. Wesley|last2=Wiegand|first3=Yukio|last3=Mizumoto|first4=Constance|last4=Weis|date=July 28, 1970|pages=1084–1086|doi=10.1021/ic50099a048 }}

Like its pnictogen congeners, hexafluoroarsenate is a noncoordinating anion, a counterion used to stably store extremely reactive cations.{{cite journal|journal=Journal of the Less Common Metals|volume=94|date=3 February 1983|pages=305–308|title=Hexafluoroarsenate as a Non-Coordinating Anion in Lanthanide Complexes with the Diphenyl Sulphoxide Ligand|last1=Maia Melo|first1=Sérgio|last2=Sousa Silveira|first2=Alexandre|issue=2 |publisher=Elsevier Sequoia|place=The Netherlands|doi=10.1016/0022-5088(83)90029-2}} Through the appropriate choice of fluorine donor, the synthesis of hexafluoroarsenate can also double as preparation of an exotic cation.{{cite journal|journal=Journal of Fluorine Chemistry|volume=25|publication-date=1984|pages=387–394|date=December 5, 1983|title=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|first1=Darryl D.|last1=Desmarteau|first2=William Y.|last2=Lam|first3=Brian A.|last3=O'Brien|author4=Shi-Ching Chang|issue=3 |publisher=Elsevier Sequoia S.A.|place=The Netherlands|doi=10.1016/S0022-1139(00)81212-9}} 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.{{cite journal|title=The Existence of Hexafluoroarsenic(V) Acid|journal= Angewandte Chemie International Edition|doi=10.1002/anie.201308023|first1=Joachim|last1=Axhausen|first2=Karin|last2=Lux|first3=Andreas|last3=Kornath|year=2014|publisher=Wiley|volume=53|issue=14 |pages=3720–3721|pmid=24446235 }} The more commonly encountered hydrate is isostructural with the hydrates of hexafluorophosphoric acid and hexafluoroantimonic acid.{{cite journal|title=Hydrogen Fluoride Containing Isostructural Hydrates of Hexafluoro­phosphoric, Hexafluoro­arsenic, and Hexafluoro­antimonic Acids|last1=Davidson|first1=D. W.|last2=Calvert|first2=L. D.|last3=Lee|first3=F.|last4=Ripmeester|first4=J. A.|date=31 July 1980|journal=Inorg. Chem.|publication-date=1981|volume=20|pages=2013–2016|doi=10.1021/ic50221a016}} Also published as NRCC 18823. These salts contain MF₆⁻ (M = P, As, Sb), HF, and water.

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.{{cite journal|journal=Materials Science and Engineering|volume=31|year=1977|pages=261–265|publisher=Elsevier Sequoia S.A.|place=The Netherlands/Lausanne, Switzerland|title=High Electrical Conductivity in Graphite Intercalated with Acid Fluorides|first1=F. L.|last1=Vogel|first2=G. M. T.|last2=Foley|first3=C.|last3=Zeller|first4=E. R.|last4=Falardeau|first5=J.|last5=Gan|doi=10.1016/0025-5416(77)90043-X}}{{cite journal|title=Ionic Salt Limit in Graphite–Fluoroarsenate Intercalation Compounds|journal=J. Chem. Phys.|volume=78|pages=5800–5808|doi=10.1063/1.445423|date=1 May 1983|publication-date=31 August 1998|first1=J. W.|last1=Milliken|first2=J. E.|last2=Fischer|number=9|bibcode=1983JChPh..78.5800M }}

• {{annotated link|Superacid}}
• Hexafluorophosphate — phosphorus analogue
• {{annotated link|Arsenic pentafluoride}}
• {{annotated link|Hexafluoroantimonate}}

{{Reflist}}

Category:Superacids

Category:Arsenic(V) compounds

Category:Fluorine compounds