The
bifluoride, or
hydrogen(difluoride), ion is the species HF
2−. This centrosymmetric triatomic anion features the strongest known
hydrogen bondA hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, like nitrogen, oxygen or fluorine . The hydrogen must be covalently bonded to another electronegative atom to create the bond...
, with an
FFluorine is the chemical element with atomic number 9, represented by the symbol F. Fluorine forms a single bond with itself in elemental form, resulting in the diatomic F
2 molecule. F
2 is a supremely reactive, poisonous, pale, yellowish brown gas. Elemental fluorine is the...
−
HHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
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, HF
2, which does not exist.
Salts
Some HF
2− salts are common, examples include potassium hydrogen fluoride, KHF
2, and [NH
4][HF
2]. Many salts claimed to be anhydrous sources of fluoride (
e.g. tetra-n-butylammonium fluorideTetra-n-butylammonium fluoride or TBAF is a quaternary ammonium salt with the chemical formula 4N+F-...
) can decompose yielding bifluoride.
Autodissociation of pure HF
The bifluoride ion also contributes to the unusually high auto-protolysis constant of liquid anhydrous
hydrogen fluorideHydrogen fluoride is a chemical compound with the formula HF. It is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the petrochemical...
, which
autodissociatesDissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner...
in a manner similar to the
self-ionization of waterThe self-ionization of water is the chemical reaction in which two water molecules react to produce a hydronium ion and a hydroxide ion :...
. This
equilibriumIn a chemical process, chemical equilibrium is the state in which the chemical activities or concentrations of the reactants and products have no net change over time. Usually, this would be the state that results when the forward chemical process proceeds at the same rate as their reverse reaction...
can be denoted as
- HF H+ + F−
However, both the H
+ and F
− ions are
solvatedSolvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...
by HF, so a better descriptive equation is
- 3HF H2F+(HF) + HF2−(HF)