Fluorine ion by a process of hydrogen bonding forms the anion [HF2]-. This tendency is missing with other halides for lack of possibility of such strong intermolecular attraction.
F is much more electronegative than Cl hence has much stronger hydrogen bonding, which stabilizes the structure with anion F-.
A mixture of molten KF and HF is hence chemically K+ [HF2]- and is used in moisson method to extract F2 which was chemically almost impossible.
Further, [HF2]- involves strongest symmetric 3c-4e hydrogen bonding.
Note- H2F2 exists as dimeric molecule due to H–bonding and thus shows dibasic nature. Hence it gives two series of salts KHF2 and KF.
