Bifluoride ion

Corrosion. Ammonium bifluoride dissolves in aqueous solutions to yield the acidic bifluoride ion the pH of a 5% solution is 3.5. In most cases, NH4HF2 solutions react readily with surface oxide coatings on metals thus NH4HF2 is used in pickling solutions (see Metal surface treatments). Many plastics, such as polyethylene, polypropylene, unplasticized PVC, and carbon brick, are resistant to attack by ammonium bifluoride. 

Properties. Other names for potassium bifluoride are potassium hydrogen difluoride and potassium acid fluoride. This white crystalline salt is a soft, waxy soHd. The crystal forms of potassium bifluoride are tetragonal and cubic (21). The bifluoride ion in KHF2 averages 0.2292 nm between fluoride... 

Figure 3.84 An illustration of the Pimentel-Rundle three-center MO model of hypervalency, showing equilibrium valence AO (xa-/b-Xc) overlap patterns for (a) 2pF—2pF—2pF NAOs of the trifluoride ion, F3 and (b) 2pF—lsp—2pF NAOs of the bifluoride ion, FHF-.

The bifluoride ion, HFj 297 Strong hydrogen bonding in p-diketones 309 Keto/enol equilibrium 310 Structures 312 Hydrogen-bond energies 314 Vibrational modes 315 Nmr spectroscopy 317... 

Although there may be other very strong hydrogen-bond systems, it is still the case that the bifluoride ion represents the upper limit of hydrogen-bond strength. (If any system has both H and D above the internal energy barrier. 

Table 8 Experimental, empirical and theoretical estimates of the hydrogen-bond energy of the bifluoride ion, A [HF(g) -F - FHF,g) ]/kJ mol". ...

Since the proton is centred in the bifluoride ion, it is possible to redefine the hydrogen-bond energy in a way comparable to the mean bond energies... 

The interpretation is that in any of these bifluoride ions, the potential-energy well is of the flattened type shown in Fig 5. When the anion is at a symmetrical site, the minimum will be exactly at the centre as in curve (d). This occurs in NaHF2 and KHF2. When there is no symmetry, the curve will be tilted, as in curve (/), so that the minimum is considerably displaced to one side. (A close analogy would be a ball-bearing in an egg-cup or in a saucer, corresponding to curves (e) or (d). A 100 tilt of the egg-cup would cause only a small shift in the rest-position of the ball a 10° tilt of the saucer would cause a large shift. A big shift is therefore evidence of a saucer rather than an egg-cup.)... 

There are also examples of molecules whose existence is beyond question, but for which no satisfactory Lewis structures can be written. Two examples are the triiodide ion I3 , and the bifluoride ion HF2 . The triiodide ion is a well known species found in aqueous solutions containing iodine and iodide ions ... 

The bifluoride ion is formed in a rather similar way in hydrofluoric acid solutions containing fluoride ion ... 

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