Hexafluoroarsenate

The hexafluoroarsenate 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:
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, which abstracts fluoride from common donors, such as hydrogen fluoride or cis-difluorodiazine. 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.

Like its pnictogen congeners, hexafluoroarsenate is a noncoordinating anion, a counterion used to stably store extremely reactive cations. Through the appropriate choice of fluorine donor, the synthesis of hexafluoroarsenate can also double as preparation of an exotic cation. The resulting salts are typically stable to metathesis with silver, 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. The more commonly encountered hydrate is isostructural with the hydrates of hexafluorophosphoric acid and hexafluoroantimonic acid. These salts contain MF₆⁻, 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.