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|Molar mass||39.00 g mol−1|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
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Bifluoride (systematically named difluorohydrogenuide and difluoridohydrogenate(1−)), also named hydrogen(difluoride) is an inorganic anion with the chemical formula HF−
2 (also written [HF2]−). It contributes no color to bifluoride salts.
The fluorohydrogenate group (-HF−) in anions such as bifluoride can assimilate a proton by recombination:
2 + H+ → H2F2 → 2 HF
Because of this capture of a proton (H+), bifluoride has basic character. Its conjugate acid is the reactive intermediate, μ-fluoro-fluorodihydrogen (H2F2), which subsequently dissociates to become hydrogen fluoride. In solution, most bifluoride ions are dissociated.
2 + 3 H2O 2 H3O+ + F− + HO−
This centrosymmetric triatomic anion features a symmetric hydrogen bond, the strongest known hydrogen bond, with an F−H length of 114 pm and a bond strength of >155 kJ mol−1. A molecular orbital diagram reveals the atoms to be held together by a 3-center 4-electron bond. It is isoelectronic with the fluoroheliate anion, FHeO−, whose existence is suspected but not confirmed, and with the hypothetical compound helium difluoride, HeF2. "Hydrogen(difluoride)" is written as one word because it is an anion; "hydrogen difluoride" would instead imply the electrically neutral compound HF2 (CAS number 12528-21-1). It is classified as a weak base.
Bifluoride undergoes the typical chemical reactions of a fluorohydronate. Upon treatment with a standard acid, it converts to hydrofluoric acid and a metal salt. Oxidation of bifluoride gives fluorine. When heated to a high temperature, bifluoride salts decompose to produce fluoride salts and hydrogen fluoride:
- [HF2]− → HF + F−
Bifluoride is produced by fluorination of hydrogen fluoride. Hydrogen fluoride and fluoride anions react to produce bifluoride according to the following reaction:
- HF + F− → [HF2]−
This process involves μ-fluoro-fluorodihydrogen as an intermediate, and occurs in two steps. No catalyst is needed for fluorination (step 2).
- 2 HF → [H2F2]
- [H2F2] + F− → [HF2]− + HF
Catalytic amounts of hydrogen fluoride are used for the process.
Some HF2− salts are common, examples include potassium bifluoride (KHF2, also called potassium hydrogen fluoride) and ammonium bifluoride ([NH4][HF2]). Many salts claimed to be anhydrous sources of simple fluoride (F–) ions, for example, tetra-n-butylammonium fluoride, can decompose to yield bifluoride instead.
Autodissociation of pure HF
The bifluoride ion also contributes to the unusually high auto-protolysis constant of liquid anhydrous hydrogen fluoride, which autodissociates in a manner similar to the self-ionization of water. This equilibrium can be denoted as
- HF H+ + F−
However, both the H+ and F− ions are solvated by HF, so a better descriptive equation is
- 3HF H2F+(HF) + HF2−(HF)
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- Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419.
- Emsley, J., "Very Strong Hydrogen Bonds", Chemical Society Reviews, 1980, 9, 91-124.
- 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
- "Collapse of helium’s chemical nobility predicted by Polish chemist". Retrieved 2009-05-15.