Bifluoride ion, FHF

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 linear bifluoride ion (FHF) has one of the strongest hydrogen bonds known. The asymmetric FHF stretching vibration absorbs broadly at 1700-1400 cm" and the FHF deformation absorbs at 1260-1200 cm ... 

In the 1980s, the DuPont Company developed and patented [10] a new type of polymerization that mechanistically is similar to anionic polymerization. Group-transfer polymerization (GTP) has been defined as polymerization of a,/3-unsaturated esters, ketones, nitriles, or amides, initiated by silyl ketene acetals [11]. It has most commonly been used to polymerize acrylate and methacrylate monomers with the aid of anionic catalysts (they are true catalysts here), such as the bifluoride ion, [FHF] , or bioxyanions. GTP is illustrated below for the polymerization of methyl methacrylate (MMA) with silyl ketene acetal (SKA) ... 

A simple and provocative example of such strange association complexes is provided by the bifluoride ion (FHF ). This species can be formulated perfectly well as the Lewis-compliant HF molecule and F fluoride anion. 

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

The best, and longest known, case is that of the bifluoride anion in NaHF2 and KHF2. In each the FHF ion lies across a centre of symmetry, and careful neutron-diffraction study finds the proton at the centre. However, neutron diffraction also shows, or appears to show, that the proton vibrates with a particularly large amplitude along the F H F bond. So there is a possibility that this may be partly due to disorder — qualitatively like that in ice (see also section 12), but now with the alternative proton sites not more than 0.1 A from the centre. The neutron-diffraction measurements can be equally well explained by two models the proton vibrates... 

Figure 12.7 presents the molecular graph of CLOH—HBeH complex where the intermolecular O-H—H-Be DHB exists. It was found that this structure corresponds to the stable energetic minimum [35], This is the shortest ever found theoretically intermolecular contact (H—H) since it amounts to only 1.049 A. On the other hand, such a short contact represents the proton-acceptor distance.9 It is worth mentioning that the other shortest contact was revealed for bifluoride (FHF)-ion. For this system, the H—F equivalent distances amount to 1.1—1.2 A. The exact value depends on the type of experimental measurements or the applied method of calculations [36]. 

If there are only two molecules of formula MN j in the unit cell, each molecule must be centrosymmetric. Atom M must lie at a centre of inversion, which could be taken as the origin (0,0,0) whilst an N-atom at X, y, z must be matched by a second atom, symbolized by M, at —x, —y, —z. The cell must also contain an identical MN molecule at another centre of inversion. A s)nnmetrical NMll molecule, with M in a special position, is a formal consequence of the crystal symmetry. Such a situation, involving two molecules of MHXj in the cell, implies a S3mimetrical [XHX] ion, with the two X-moieties linked by an 0--H--0 bond across a S5unmetry element of the crystal. These are the Type A acid salts, the oxygen-analogues of the well known bifluorides such as K+ [FHF]-. 

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