:Iron(II) fluoride

Anhydrous FeF₂ adopts the TiO₂ rutile structure. As such, the iron cations are octahedral and fluoride anions are trigonal planar.{{cite journal|last=Stout|first=J.|author2=Stanley A. Reed|title=The Crystal Structure of MnF₂, FeF₂, CoF₂, NiF₂ and ZnF₂|journal=J. Am. Chem. Soc.|year=1954|volume=76 |issue=21|pages=5279–5281|doi=10.1021/ja01650a005}}{{Cite journal|last1=M.J.M.|first1=de Almeida|last2=M.M.R.|first2=Costa|last3=J.A.|first3=Paixão|date=1989-12-01|title=Charge density of FeF2|journal=Acta Crystallographica Section B|language=en|volume=45|issue=6|pages=549–555|doi=10.1107/S0108768189007664|issn=0108-7681}}

The tetrahydrate can exist in two structures, or polymorphs. One form is rhombohedral and the other is hexagonal, the former having a disorder.{{cite journal | last1 = Penfold | first1 = B. R. | last2 = Taylor | first2 = M. R. | year = 1960 | title = The crystal structure of a disordered form of iron(II) fluoride tetrahydrate | journal = Acta Crystallographica | volume = 13 | issue = 11| pages = 953–956 | doi = 10.1107/S0365110X60002302 | doi-access = free }}

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.{{cite journal |last=Erickson| first=R. |title=Neutron Diffraction Studies of Antiferromagnetism in Manganous Fluoride and Some Isomorphous Compounds|journal=Physical Review|date=June 1953|volume=90|issue=5|pages=779–785|doi=10.1103/PhysRev.90.779| bibcode=1953PhRv...90..779E }} Heat capacity measurements reveal an event at 78.3 K corresponding to ordering of antiferromagnetic state.{{cite journal |last=Stout |first=J. |author2=Edward Catalano |title=Thermal Anomalies Associated with the Antiferromagnetic Ordering of FeF₂, CoF₃, and NiF₂|journal=Physical Review|date=December 1953|volume=92|issue=6|pages=1575|doi=10.1103/PhysRev.92.1575|bibcode=1953PhRv...92.1575S }}

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.{{Cite journal|last1=Bardi|first1=Gianpiero|last2=Brunetti|first2=Bruno|last3=Piacente|first3=Vincenzo|date=1996-01-01|title=Vapor Pressure and Standard Enthalpies of Sublimation of Iron Difluoride, Iron Dichloride, and Iron Dibromide|journal=Journal of Chemical & Engineering Data|volume=41|issue=1|pages=14–20|doi=10.1021/je950115w|issn=0021-9568}}

The following reaction is proposed in order to calculate the atomization energy for Fe⁺:{{cite journal|last=Kent|first=Richard|date=November 1965|title=Mass Spectrometric Studies at High Temperatures. VIII. The Sublimation Pressure of Iron(II) Fluoride|journal=Journal of the American Chemical Society|volume=87|issue=21|pages=4754–4756|doi=10.1021/ja00949a016|author2=John L. Margrave}}

:FeF₂ + e → Fe⁺ + F₂ (or 2F) + 2e

The anhydrous salt can be prepared by reaction of ferrous chloride with anhydrous hydrogen fluoride.W. Kwasnik "Iron(II) Fluoride" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 266. It is slightly soluble in water (with solubility product K_sp = 2.36×10⁻⁶ at 25 °C){{Cite web|url=http://www4.ncsu.edu/~franzen/public_html/CH201/data/Solubility_Product_Constants.pdf|title=SOLUBILITY PRODUCT CONSTANTS|access-date=2016-11-07|archive-date=2018-07-12|archive-url=https://web.archive.org/web/20180712140117/http://www4.ncsu.edu/~franzen/public_html/CH201/data/Solubility_Product_Constants.pdf|url-status=dead}} as well as dilute hydrofluoric acid, giving a pale green solution. It is insoluble in organic solvents.

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

FeF₂ is used to catalyze some organic reactions.{{Ullmann | doi = 10.1002/14356007.a14_591 | title = Iron Compounds | year = 2000 | last1 = Wildermuth | first1 = Egon | last2 = Stark | first2 = Hans | last3 = Friedrich | first3 = Gabriele | last4 = Ebenhöch | first4 = Franz Ludwig | last5 = Kühborth | first5 = Brigitte | last6 = Silver | first6 = Jack | last7 = Rituper | first7 = Rafael }}

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.{{Cite journal |last1=Nikitin |first1=M. I. |last2=Chilingarov |first2=N. S. |last3=Alikhanyan |first3=A. S. |date=2021-01-01 |title=Thermal Stability of Mixed Fluorides of 3d Elements |url=https://link.springer.com/article/10.1134/S0036023621010058 |journal=Russian Journal of Inorganic Chemistry |language=en |volume=66 |issue=1 |pages=89–95 |doi=10.1134/S0036023621010058 |issn=1531-8613|url-access=subscription }} Stability not only enhances safety and lowers the risk of thermal runaway.{{Cite journal |last1=Viswanathan |first1=Venkatasubramanian |last2=Epstein |first2=Alan H. |last3=Chiang |first3=Yet-Ming |last4=Takeuchi |first4=Esther |last5=Bradley |first5=Marty |last6=Langford |first6=John |last7=Winter |first7=Michael |date=March 2022 |title=Author Correction: The challenges and opportunities of battery-powered flight |url=https://www.nature.com/articles/s41586-022-04612-5 |journal=Nature |language=en |volume=603 |issue=7903 |pages=E30 |doi=10.1038/s41586-022-04612-5 |pmid=35293393 |issn=1476-4687}}

FeF_X exhibits distinctive phase evolution, intermediate phases, and morphological transformations during lithiation and delithiation.{{Cite journal |last1=Badway |first1=F. |last2=Cosandey |first2=F. |last3=Pereira |first3=N. |last4=Amatucci |first4=G. G. |date=2003 |title=Carbon Metal Fluoride Nanocomposites |url=https://iopscience.iop.org/article/10.1149/1.1602454 |journal=Journal of the Electrochemical Society |language=en |volume=150 |issue=10 |pages=A1318 |doi=10.1149/1.1602454}}{{Cite journal |last1=Olbrich |first1=Lorenz F. |last2=Xiao |first2=Albert W. |last3=Pasta |first3=Mauro |date=2021-12-01 |title=Conversion-type fluoride cathodes: Current state of the art |url=https://linkinghub.elsevier.com/retrieve/pii/S2451910321000934 |journal=Current Opinion in Electrochemistry |volume=30 |pages=100779 |doi=10.1016/j.coelec.2021.100779 |issn=2451-9103}} A stable lattice of fluoride anions is maintained throughout charge and discharge cycles, consistent with high cycling reversibility.{{Cite journal |last1=Xiao |first1=Albert W. |last2=Lee |first2=Hyeon Jeong |last3=Capone |first3=Isaac |last4=Robertson |first4=Alex |last5=Wi |first5=Tae-Ung |last6=Fawdon |first6=Jack |last7=Wheeler |first7=Samuel |last8=Lee |first8=Hyun-Wook |last9=Grobert |first9=Nicole |last10=Pasta |first10=Mauro |date=June 2020 |title=Understanding the conversion mechanism and performance of monodisperse FeF2 nanocrystal cathodes |url=https://www.nature.com/articles/s41563-020-0621-z |journal=Nature Materials |language=en |volume=19 |issue=6 |pages=644–654 |doi=10.1038/s41563-020-0621-z |pmid=32094491 |issn=1476-4660}}{{Cite journal |last1=Olbrich |first1=Lorenz F. |last2=Xiao |first2=Albert W. |last3=Schart |first3=Maximilian |last4=Ihli |first4=Johannes |last5=Matthews |first5=Guillaume |last6=Sanghadasa |first6=Mohan |last7=Pasta |first7=Mauro |date=February 2024 |title=Iron fluoride-lithium metal batteries in bis(fluorosulfonyl)imide-based ionic liquid electrolytes |url=https://linkinghub.elsevier.com/retrieve/pii/S2666386424000079 |journal=Cell Reports Physical Science |language=en |volume=5 |issue=2 |pages=101787 |doi=10.1016/j.xcrp.2024.101787|doi-access=free }}

{{reflist}}

• [https://web.archive.org/web/20060116134617/http://www.npi.gov.au/database/substance-info/profiles/44.html National Pollutant Inventory - Fluoride and compounds fact sheet]

{{Iron compounds}}

{{fluorides}}

Category:Fluorides

Category:Metal halides

Category:Iron(II) compounds