Al-F bonds

This molecule (type AB3) has a trigonal planar electronic geometry and trigonal planar molecular geometry. The Al-F bonds are polar, but since the molecule is symmetrical, the bond dipoles cancel to give a nonpolar molecule (Section 8-6). 

Aside from its color, trivalent chromium differs most markedly from Al(III) in its complexing ability. There are no analogs in aluminum chemistry for many of the octahedral complexes formed by Cr(III) with CN, SCN, NOj, NH3, and with a number of organic amines. The octahedral complexes of aluminum are confined to those having Al—0 or Al—F bonds. In contrasting the complexes derived from the two... 

In the structure of [PSH][TFA] (Fig. 3) [50] the expected tetrahedral anion was confirmed. Not surprisingly, the F-Al-F bond angles were approximately 109°, with equivalent Al-F distances of around 1.62 A. The closest contact between the coordinated fluoride and the chelated proton of the cation was 2.77 A, which could be considered a long hydrogen bond. The crystal structures of [Ph4P][TFA], [Ph4As][TFA], and [n-Bu4N][TFA] (Fig. 4) have also been reported and are similar to that of [PSH][TFA] with discrete cations and anions [8]. 

In the complex anion of a 1 1 complex, one Lewis acid R3AI is coordinated to the Lewis base, i. e., the fluoride anion. In the complex anion of a 1 2 complex on the other hand, two R3AI units are coordinated to the fluoride anion. The crystal structures of two of the 1 2 complexes of KF with R3AI are known, with R = Me and Et [5, 153). In both compounds, the fluorine atom is bound to two A1 atoms, the Al-F-Al angle equals 180°, and the Al-F bond length is 1.782 A in the methyl complex [118] and 1.820 A in the ethyl compound. The coordination geometry of A1 in both cases is tetrahedral, see Fig. 2. 

The waters of the hydration sphere are usually ignored in the equilibrium constant, because excess water is present in aqueous solutions, and the energy of the Al-F bond must be greater than that of the ion-water bonds for the complex to form. The concentration of the AIF2+ complex ion increases with increasing concentrations of Al3+ andF-. 

Finally, NMR has been used to characterize the structural features of surface-fluorinated aluminas and amorphous aluminosilicates. Both the F NMR chemical shifts and spin-spin relation times differentiate sensitively between Si-F and Al-F bonds. Lineshape and spin echo decay data suggest that the homonuclear F- F dipolar interactions are much weaker than in AlFj-hydrate, indicating that in samples containing up to 5 wt% fluorine, most of the Al-F species are isolated from each other [78]. 

Other examples of additives are hydrogen fluoride (see also Equations 17.31-17.34), forming AIF3, or perfluorinated inorganic anions [260], producing compounds with several Li-F or Al-F bonds. 

Y. Liu, J. Tossell, Possible Al-F bonding environment in fluorine-bearing sodium aluminosilicate glasses from calculation of NMR shifts, J. Phys. Chem. B, 107, 11280-11289 (2003). 

It is important to note that the phase diagram is only based on thermodynamic considerations. The kinetic barriers to phase transitions are not considered. For instance, there is likely to be a considerable barrier to the transition from the (1 x l)tothe( 2x y 2) structure as it requires the cleavage and formation of several Al-F bonds. a-AlFa is usually synthesized at elevated temperatures, at which the ( 2 x 2) 3F termination will dominate. It may be that the transition to the (1 x 1) 3F+H2O termination upon cooling to room temperature is kinetically hindered. Conversely, catalytically active HS-AIF3 is synthesized using sol-gel methods that proceed at lower temperatures [32,33]. Under these conditions it is speculated that structures that are similar to those found on the (1 x 1) 3F termination form. 

The success of the method is due to the presence of large numbers of Al—OH sites in allophane and the much greater stability of the Al—F bond relative to Al—OH. If allowed to proceed far enough, aluminum is removed from the allophane structure and converted into A1F ions, which may form cryolite. 

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