The bifluoride anion

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 \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)

References

1. ↑
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.