Bifluoride

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]⁻ ↽ − ⇀ HF + F⁻

Bifluoride protonates to give hydrogen fluoride:

HF−
2 + H⁺ ↽ − ⇀ 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 ↽ − ⇀ 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 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. It is isoelectronic with the fluoroheliate anion, FHeO⁻, whose existence is suspected 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 ⇌ H⁺ + F⁻

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

3HF ⇌ H₂F⁺_(HF) + HF₂⁻_(HF)