Ferric Fluoride

Iron(III) fluoride, also known as ferric fluoride, are inorganic compounds with the formula FeF₃(H₂O)_x where x = 0 or 3. They are mainly of interest by researchers, unlike the related iron(III) chloride. Anhydrous iron(III) fluoride is white, whereas the hydrated forms are light pink.Housecroft, Catherine E.; Sharpe, Alan G. (2008) ''Inorganic Chemistry'' (3rd ed.), Pearson: Prentice Hall. .

Chemical and physical properties

Iron(III) fluoride is a thermally robust, antiferromagnetic solid consisting of high spin Fe(III) centers, which is consistent with the pale colors of all forms of this material. Both anhydrous iron(III) fluoride as well as its hydrates are hygroscopic.

Structure

The anhydrous form adopts a simple structure with octahedral Fe(III)F₆ centres interconnected by linear Fe-F-Fe linkages. In the language of crystallography, the crystals are classified as rhombohedral with an R-3c space group. The structural motif is similar to that seen in ReO₃. Although the solid is nonvolatile, it evaporates at high temperatures, the gas at 987  °C consists of FeF₃, a planar molecule of D_3h symmetry with three equal Fe-F bonds, each of length 176.3 pm. At very high temperatures, it decomposes to give FeF₂ and F₂.

Two crystalline forms—or more technically, polymorphs—of FeF₃·3H₂O are known, the α and β forms. These are prepared by evaporation of an HF solution containing Fe³⁺ at room temperature (α form) and above 50 °C (β form). The space group of the β form is P4/m, and the α form maintains a P4/m space group with a J6 substructure. The solid α form is unstable and converts to the β form within days. The two forms are distinguished by their difference in quadrupole splitting from their Mössbauer spectra.

Preparation, occurrence, reactions

Anhydrous iron(III) fluoride is prepared by treating virtually any anhydrous iron compound with fluorine. More practically and like most metal fluorides, it is prepared by treating the corresponding chloride with hydrogen fluoride:
:FeCl₃ + 3 HF → FeF₃ + 3 HCl

It also forms as a passivating film upon contact between iron (and steel) and hydrogen fluoride.J. Aigueperse, P. Mollard, D. Devilliers, M. Chemla, R. Faron, R. Romano, J. P. Cuer, "Fluorine Compounds, Inorganic" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH, Weinheim, 2005. The hydrates crystallize from aqueous hydrofluoric acid.

The material is a fluoride acceptor. With xenon hexafluoride it forms eF₄XeF₅].

Pure FeF₃ is not yet known among minerals. However, hydrated form is known as the very rare fumarolic mineral topsøeite. Generally a trihydrate, its chemistry is slightly more complex: FeF _0.5(H₂O)_0.5sub>4·H₂O.

Applications

The primary commercial use of iron(III) fluoride in the production of ceramics."Ferric Fluoride." CAMEO Chemicals. National Oceanic and Atmospheric Administration. Web. 7 Apr. 2010.

Some cross coupling reaction are catalyzed by ferric fluoride-based compounds. Specifically the coupling of biaryl compounds are catalyzed by hydrated iron(II) fluoride complexes of N-heterocyclic carbene ligands. Other metal fluorides also catalyse similar reactions.
Iron(III) fluoride has also been shown to catalyze chemoselective addition of cyanide to aldehydes to give the cyanohydrins.

Safety

The anhydrous material is a powerful dehydrating agent. The formation of ferric fluoride may have been responsible for the explosion of a cylinder of hydrogen fluoride gas.

References

External links

National Pollutant Inventory—Fluoride and compounds fact sheetCAMEO Chemicals: Database of Hazardous Materials

{{DEFAULTSORT:Iron(III) Fluoride

Fluorides
Metal halides
Iron(III) compounds