Hydrogen difluoride
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The bifluoride anion

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.

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 hydrofluoric acid, 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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References

1. ^ Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 
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