Iron(II) fluoride

Iron(II) fluoride or ferrous fluoride is an inorganic compound with the molecular formula FeF₂. It forms a tetrahydrate FeF₂·4H₂O that is often referred to by the same names. The anhydrous and hydrated forms are white crystalline solids.^[1][5]

Iron(II) fluoride

Identifiers
CAS Number	7789-28-8 anhydrous Y 13940-89-1 (tetrahydrate) N
3D model (JSmol)	Interactive image
ChemSpider	74215 Y
ECHA InfoCard	100.029.232
PubChem CID	522690
UNII	NP4W87HLVO Y
CompTox Dashboard (EPA)	DTXSID50894779
InChI InChI=1S/2FH.Fe/h2*1H;/q;;+2/p-2 Y Key: FZGIHSNZYGFUGM-UHFFFAOYSA-L Y InChI=1/2FH.Fe/h2*1H;/q;;+2/p-2 Key: FZGIHSNZYGFUGM-NUQVWONBAX
SMILES [Fe+2].[F-].[F-]
Properties
Chemical formula	FeF2
Molar mass	93.84 g/mol (anhydrous) 165.902 g/mol (tetrahydrate)
Appearance	colorless transparent crystals[1]
Density	4.09 g/cm3 (anhydrous) 2.20 g/cm3 (tetrahydrate)
Melting point	970 °C (1,780 °F; 1,240 K) (anhydrous) 100 °C (tetrahydrate)[2]
Boiling point	1,100 °C (2,010 °F; 1,370 K) (anhydrous)
Solubility product (Ksp)	2.36×10−6[3]
Solubility	insoluble in ethanol, ether; dissolves in HF
Magnetic susceptibility (χ)	+9500.0·10−6 cm3/mol
Structure
Crystal structure	Rutile (tetragonal), tP6
Space group	P42/mnm, No. 136
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Causes severe skin burns & eye damage; Hazardous decomposition products formed under fire conditions- Iron oxides[4]
GHS labelling:
Pictograms
NFPA 704 (fire diamond)	4 0 2 COR
Flash point	not applicable[4]
Related compounds
Other anions	Iron(II) chloride Iron(II) bromide Iron(II) iodide Iron(II) oxide
Other cations	Manganese(II) fluoride Cobalt(II) fluoride
Related compounds	Iron(III) fluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Structure and bonding

Anhydrous FeF₂ adopts the TiO₂ rutile structure. As such, the iron cations are octahedral and fluoride anions are trigonal planar.^[6][7]

The tetrahydrate can exist in two structures, or polymorphs. One form is rhombohedral and the other is hexagonal, the former having a disorder.^[1]

Like most fluoride compounds, the anhydrous and hydrated forms of iron(II) fluoride feature high spin metal center. Low temperature neutron diffraction studies show that the FeF₂ is antiferromagnetic.^[8] Heat capacity measurements reveal an event at 78.3 K corresponding to ordering of antiferromagnetic state.^[9]

Selected physical properties

FeF₂ sublimes between 958 and 1178 K. Using Torsion and Knudsen methods, the heat of sublimation was experimentally determined and averaged to be 271 ± 2 kJ mole⁻¹.^[10]

The following reaction is proposed in order to calculate the atomization energy for Fe⁺:^[11]

FeF₂ + e → Fe⁺ + F₂ (or 2F) + 2e

Synthesis and reactions

The anhydrous salt can be prepared by reaction of ferrous chloride with anhydrous hydrogen fluoride.^[12] It is slightly soluble in water (with solubility product K_sp = 2.36×10⁻⁶ at 25 °C)^[13] as well as dilute hydrofluoric acid, giving a pale green solution.^[1] It is insoluble in organic solvents.^[5]

The tetrahydrate can be prepared by dissolving iron in warm hydrated hydrofluoric acid and precipitating the result by addition of ethanol.^[1] It oxidizes in moist air to give, inter alia, a hydrate of iron(III) fluoride, (FeF₃)₂·9H₂O.^[1]

Uses

FeF₂ is used to catalyze some organic reactions.^[14]

Battery research

FeF₂ has been investigated as a cathode material for both lithium-ion and fluoride-ion batteries. Unlike conventional metal oxides, which rely on an intercalation-based lithium storage mechanism, FeF_X (x = 2, 3) operates via a complex conversion mechanism, resulting in higher energy density. Fluoride cathodes are stable up to 1000°C.^[15] Stability not only enhances safety and lowers the risk of thermal runaway.^[16]

FeF_X exhibits distinctive phase evolution, intermediate phases, and morphological transformations during lithiation and delithiation.^[17][18] A stable lattice of fluoride anions is maintained throughout charge and discharge cycles, consistent with high cycling reversibility.^[19][20]