Bifluoride

Bifluoride
Names
	Other names Hydrogen(difluoride); Bifluoride anion; Hydrogen difluoride anion
Identifiers
CAS Number	18130-74-0 ;
3D model (JSmol)	Interactive image;
ChemSpider	35308425;
CompTox Dashboard (EPA)	DTXSID70274195 ;
	InChI InChI=1S/F2H/c1-3-2/q-1 Key: LJRMFMQHZAVYNS-UHFFFAOYSA-N;
	SMILES F[H][F-];
Properties
Chemical formula	F2H
Molar mass	39.005 g·mol−1
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). verify (what is ?) 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^{[clarification needed]} 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.^{[citation needed]} When heated, bifluoride salts decompose to produce fluoride salts and hydrogen fluoride:

[HF₂]⁻

{\ce {<=>>}}

HF + F⁻

Bifluoride protonates to give hydrogen fluoride:

HF⁻
₂ + H⁺

{\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

{\ce {<=>>}}

2 HF + HO⁻

Bifluoride salts arise by treatment of hydrogen fluoride with base^{[clarification needed]}:

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.^{[citation needed]}

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

^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
^ Emsley, J., "Very Strong Hydrogen Bonds", Chemical Society Reviews, 1980, 9, 91-124.
^ 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
^ "Collapse of helium's chemical nobility predicted by Polish chemist" (PDF). Retrieved 2009-05-15.

[1] Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.

[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.

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