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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 FH 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]. 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 <math>\rightleftharpoons</math> H+ + F

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

3HF <math>\rightleftharpoons </math> H2F+(HF) + HF2(HF)


  1. Template:Greenwood&Earnshaw
  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

it:Idrogenodifluoruro Template:WH Template:WS