Bifluoride

The bifluoride ion is an inorganic anion with the chemical formula HF−
2. The anion is colorless. Salts of bifluoride are commonly encountered in the reactions of fluoride salts with hydrofluoric acid. The commercial production of fluorine involves electrolysis of bifluoride salts.

Bifluoride
Names
Other names
Hydrogen(difluoride)
Bifluoride anion
Hydrogen difluoride anion
Identifiers
3D model (JSmol)
ChemSpider
Properties
HF2−
Conjugate acid Hydrogen fluoride
Conjugate base Fluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Structure and bonding

The bifluoride ion has a linear, centrosymmetric structure (D∞h symmetry), with an F−H bond length of 114 pm.[1] The bond strength is estimated to be greater than 155 kJ/mol.[2] In molecular orbital theory, the atoms are modeled to be held together by a 3-center 4-electron bond.[3]

Reactions

Salts, such as potassium bifluoride and ammonium bifluoride are produced by treating fluoride salts with hydrofluoric acid:

MF + HF → M(HF2)   M = K+ or NH+
4

Potassium bifluoride binds a second equivalent of HF:[4]

K(HF2) + HF → K(H2F3)  

Heating these salts releases anhydrous HF.

References
  1. Denne, W.A.; Mackay, M.F. (1971). "Crystal structure of p-toluidinium bifluoride". Journal of Crystal and Molecular Structure. 1 (5): 311–318. doi:10.1007/BF01200805.
  2. Emsley, J. (1980). "Very Strong Hydrogen Bonds". Chemical Society Reviews. 9: 91–124. doi:10.1039/CS9800900091.
  3. Pimentel, G. C. The Bonding of Trihalide and Bifluoride Ions by the Molecular Orbital Method. J. Chem. Phys. 1951, 19, 446-448. doi:10.1063/1.1748245
  4. Aigueperse, Jean; Mollard, Paul; Devilliers, Didier; Chemla, Marius; Faron, Robert; Romano, René; Cuer, Jean Pierre (2000). "Fluorine Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a11_307.
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