Bifluoride

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:

Bifluoride protonates to give hydrogen fluoride:

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.

Bifluoride salts arise by treatment of hydrogen fluoride with base:

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 an anion; "hydrogen difluoride" would instead imply the electrically neutral compound HF₂ (CAS number 12528-21-1). It is classified as a weak base.