== Thank you for the reason of the structure of the bifluoride anion: HF2- == 

 Smokefoot, Thank you very much for your very judicious comment on the structure of the bifluoride anion in the ammonium bifluoride and the potassium bifluoride. It is true that HF is a very weak acid, simply because the fluoride mono-atomic anions is very small and strongly attracts the proton. As you very judiciously mentioned: fluoride is a strong base. I totally overlooked the text on top of the ammonium bifluoride page, simply because all my attention was drawn by the problematic systematic names in the diff view. It is the picture on Pubchem with the non-dissociated HF molecule that let me think more into detail to the structure of this compound and of the bifluoride anion. Quite logically, the considerable strength of the hydrogen bond between a small fluoride anion and a HF molecule explains the formation of the difluoride anion: F- + HF —> HF2-. It is also well explained on the discussion page of potassium bifluoride. -) Best wishes and a happy new year,