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 cannot exist.
Ammonium Bifluoride
Ammonium hydrogen fluoride is a compound formed from ammonium fluoride and hydrogen fluoride, with hydrogen fluoride molecules occupying spaces in the ammonium fluoride crystal lattice. It is the major constituent in glass-etching fluid (though sometimes replaced by potassium hydrogen fluoride), etching occurring due to the fluorination of silicon:
The structure of [NH4][HF2] contains tetrahedral [NH4]+ cations and linear [HF2]− anions. Each cation is surrounded by four fluorine atroms in a tetrahedron, with hydrogen - fluorine hydrogen bonds present between the hydrogen atoms of the ammonium ion and the fluorine.
Potassium Bifluoride
Potassium hydrogen fluoride is a compound formed from Potassium fluoride and hydrogen fluoride, with hydrogen fluoride molecules occupying spaces in the ammonium fluoride crystal lattice. It is the major constituent in glass-etching fluid (though sometimes replaced by potassium hydrogen fluoride), etching occurring due to the fluorination of silicon.
Sodium Bifluoride
Sodium hydrogen fluoride is a compound formed from Sodium fluoride and hydrogen fluoride, with hydrogen fluoride molecules occupying spaces in the ammonium fluoride crystal lattice. It is the major constituent in glass-etching fluid (though sometimes replaced by Sodium hydrogen fluoride), etching occurring due to the fluorination of silicon.
