Bifluoride

Bifluoride

Names
Other names Hydrogen(difluoride) Bifluoride anion Hydrogen difluoride anion
Identifiers
CAS Number	18130-74-0
3D model (Jmol)	Interactive image
ChemSpider	35308425
InChI InChI=1S/F2H/c1-3-2/q-1 Key: LJRMFMQHZAVYNS-UHFFFAOYSA-N
SMILES F[H][F-]
Properties
Chemical formula	F2H
Molar mass	39.00 g·mol^−1
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

Bifluoride is an inorganic anion with the chemical formula HF−
2 (also written [HF₂]⁻). It contributes no color to bifluoride salts. Salts of bifluoride are used to etch glass.

Acid-base properties and production

Bifluoride undergoes the typical chemical reactions of a weak acid. Upon treatment with a standard acid, it converts to hydrofluoric acid and a metal salt. Oxidation of bifluoride gives fluorine. When heated, bifluoride salts decompose to produce fluoride salts and hydrogen fluoride:

[HF₂]⁻ ${\displaystyle {\ce {<=>>}}}$ HF + F⁻

Bifluoride protonates to give hydrogen fluoride:

HF−
2 + H⁺ ${\displaystyle {\ce {<=>>}}}$ 2 HF

Because of this capture of a proton (H⁺), bifluoride has basic character. Its conjugate acid is the reactive intermediate, μ-fluoro-fluorodihydrogen (H₂F₂), which subsequently dissociates to become hydrogen fluoride. In solution, most bifluoride ions are dissociated.

HF−
2 + H
2O ${\displaystyle {\ce {<=>>}}}$ 2 HF + HO−

Bifluoride salts arise by treatment of hydrogen fluoride with base:

HF + F⁻ → [HF₂]⁻

Structure

This centrosymmetric triatomic anion features a symmetric hydrogen bond, the strongest known hydrogen bond, with an F−H 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] It is isoelectronic with the fluoroheliate anion, FHeO⁻, whose existence is suspected^[4] but not confirmed. "Hydrogen(difluoride)" is written as one word because it is a unified (covalent) anion; "hydrogen difluoride" would instead imply the electrically neutral compound HF₂ (CAS number 12528-21-1).

Salts

Some HF₂⁻ salts are common, examples include potassium bifluoride (KHF₂, also called potassium hydrogen fluoride) and ammonium bifluoride ([NH₄][HF₂]). Many salts claimed to be anhydrous sources of simple fluoride (F⁻) ions, for example, tetra-n-butylammonium fluoride, can decompose to yield bifluoride instead.

Autodissociation of pure HF

The bifluoride ion also contributes to the unusually high auto-protolysis constant of liquid anhydrous hydrogen fluoride, which autodissociates in a manner similar to the self-ionization of water. This equilibrium can be denoted as

HF $\rightleftharpoons$ H⁺ + F⁻

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

3HF $\rightleftharpoons$ H₂F⁺_(HF) + HF₂⁻_(HF)

References

1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
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
4. "Collapse of helium's chemical nobility predicted by Polish chemist" (PDF). Retrieved 2009-05-15.