Acid-base behaviour in liquid HF

Using the solvent-oriented definition (Section 9.4), a species that produces [H2F] ions in liquid HF is an acid, and one that produces [HF2] is abase. 

Many organic compounds are soluble in liquid HF, and in the cases of, for example, amines and carboxylic acids, protonation of the organic species accompanies dissolution (eq. 9.38). Proteins react immediately with liquid HF, and it produces very serious skin bums. 

Most inorganic salts are converted to the corresponding fluorides when dissolved in liquid HF, but only a few of these are soluble. Huorides of the 5-block metals, silver and thallium(l) dissolve to give salts such as K[HF2] and K[H2F3], and thus exhibit basic character. The single crystal stractures of a number of salts containing [HF2] or [DF2] (i.e. deuterated species) have been determined by X-ray or neutron diffraction techniques these include [NH4][HF2], Na[HF2], K[HF2], Rb[HF2], Cs[HF2] and [HFJ. 

The [HF2] anion is linear (structure 9.13), and its formation is a consequence of the H and F atoms being involved in strong hydrogen bonding. In the solid state stracmres 

Ammonium fluoride is basic in liquid HF. Studies of the Me4NF—HF system over a range of compositions and 

The single crystal structures of a number of salts containing [HF2] or [DF2] (i.e. deuterated species) have been determined by X-ray or neutron diffraction techniques these include [NH4][HF2], NapFj], KpFj], RbpFj], CspFj] and TlpFj]. 

Few protic acids are able to exhibit acidic behaviour in liquid HF, on account of the competition between HF and the solute as H donors perchloric acid and fluorosulfonic acid (equation 8.43) do act as acids. 

Among molecular fluorides, CF4 and SiF4 are insoluble in liquid HF, but F acceptors such as ASF5 and SbF5 dissolve 

VM Fig. 9.4 The structures of the anions (a) [H2F3] and (b) [H3F4], determined by low-temperature X-ray diffraction for the [Me N] salts. The distances given are the average values for like internuclear separations the experimental error on each distance is 3-6 pm [D. Mootz et al. (1987) Z. Anorg. Allg. Chem., vol. 544, p. 159]. Colour code F, green H, white. 

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