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.
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.
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
H+ + F−However, both the H+ and F− ions are solvated by HF, so a better descriptive equation is
H2F+(HF) + HF2−(HF) |
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