Bifluoride

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 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. 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]. In fact many salts claimed to be anhydrous sources of fluoride (e.g. tetra-n-butylammonium fluoride) contain this anion.

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)