Hydrogen difluoride
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Hydrogen(difluoride)
Bifluoride anion Hydrogen difluoride anion |
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3D model (Jmol)
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| Molar mass | 39.00 g·mol−1 | ||
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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.
Bifluoride undergoes the typicalchemical 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:
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).
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