Bifluoride

Structure and bonding

The bifluoride ion has a linear, centrosymmetric structure (D∞h symmetry), with an F−H bond length of 114 pm. The bond strength is estimated to be greater than 155 kJ/mol. In molecular orbital theory, the atoms are modeled to be held together by a 3-center 4-electron bond (symmetrical hydrogen bond), in a sort of hybrid between a hydrogen bond and a covalent bond.

Reactions

Salts, such as potassium bifluoride and ammonium bifluoride are produced by treating fluoride salts with hydrofluoric acid: :, where = or Potassium bifluoride binds a second equivalent of HF: : Heating these salts releases anhydrous HF.

The bifluoride anion is present in solutions of HF and buffered oxide etch, used in microfabrication etching. In these processes, bifluoride breaks down silicon oxides, doing more effectively than HF (~4.5 times faster). :

References

References

1. (1971). "Crystal structure of p-toluidinium bifluoride". Journal of Crystal and Molecular Structure.
2. (1980). "Very Strong Hydrogen Bonds". Chemical Society Reviews.
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
4. (July 28, 2021). "Nature of the Bonding in the Bifluoride Anion, FHF–". The Journal of Physical Chemistry Letters.
5. (2000). "Fluorine Compounds, Inorganic".
6. (Dec 1996). "Etch rates for micromachining processing". Journal of Microelectromechanical Systems.