Bifluoride

The bifluoride ion is an inorganic anion with the chemical formula HF−
2.

Bifluoride
Names
Other names
Hydrogen(difluoride)
Bifluoride anion
Hydrogen difluoride anion
Identifiers
3D model (JSmol)
ChemSpider
Properties
HF2−
Conjugate acid Hydrogen fluoride
Conjugate base Fluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Structure and bonding

The bifluoride ion has a linear, centrosymmetric structure (D∞h symmetry), with an F−H bond length of 114 pm.[1] The bond strength is estimated to be greater than 155 kJ/mol.[2] In molecular orbital theory, the atoms are modeled to be held together by a 3-center 4-electron bond.[3]

Acid-base properties and salts

In the reaction

HF−
2 + H+ ⇌ 2 HF

the ion is acting as a Brønsted–Lowry base. This reaction can occur in non-aqueous solution. On the other hand, in the reaction

HF−
2 + H2O ⇌ 2 F− + H3O+

it is acting as an acid. Salts, such as potassium bifluoride and ammonium bifluoride can be made in the direct reaction

MF + HF → M(HF2)   M = K+ or NH+
4

References
  1. Denne, W.A.; Mackay, M.F. (1971). "Crystal structure of p-toluidinium bifluoride". Journal of Crystal and Molecular Structure. 1 (5): 311–318. doi:10.1007/BF01200805.
  2. Emsley, J. (1980). "Very Strong Hydrogen Bonds". Chemical Society Reviews. 9: 91–124. doi:10.1039/CS9800900091.
  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
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