Bifluoride
The bifluoride, or hydrogen(difluoride), ion is the species HF2−. This centrosymmetric triatomic anion features 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, and with the hypothetical compound helium difluoride, HeF2. "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).
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