Bifluoride

Bifluoride
Other names Hydrogen(difluoride) Bifluoride anion Hydrogen difluoride anion
Identifiers
CAS number	18130-74-0
Jmol-3D images	Image 1
SMILES F[H][F-]
Properties
Molecular formula	HF2
Molar mass	39 g mol−1
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

The bifluoride, or hydrogen(difluoride) ion is the species HF₂⁻. This centrosymmetric triatomic anion features the strongest known hydrogen bond, with an F−H length of 114 pm^[1] and a bond strength of >155 kJ mol⁻¹.^[2] A molecular orbital diagram reveals the atoms to be held together by a 3-center 4-electron bond.^[3] Hydrogen(difluoride) is written as one word because it is an anion. Hydrogen difluoride would imply an electrically neutral compound, HF₂, which does not exist. It is isoelectronic with the hypothetical compound helium difluoride, HeF₂, which also does not exist, and the fluoroheliate anion, FHeO⁻, whose existence is suspected.

Salts

Some HF₂⁻ salts are common, examples include potassium hydrogen fluoride, KHF₂, and [NH₄][HF₂]. Many salts claimed to be anhydrous sources of fluoride (e.g. tetra-n-butylammonium fluoride) can decompose yielding bifluoride.

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 $\rightleftharpoons$ H⁺ + F⁻

However, both the H⁺ and F⁻ ions are solvated by HF, so a better descriptive equation is

3HF $\rightleftharpoons$ H₂F⁺_(HF) + HF₂⁻_(HF)

See also

• Ammonium bifluoride
• Potassium bifluoride

References

1. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0080379419. 
2. ^ Emsley, J., "Very Strong Hydrogen Bonds", Chemical Society Reviews, 1980, 9, 91-124.
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