Structures of fluorides

NMR, Raman and IR spectroscopy are most frequently used to investigate the complex structures of fluoride solutions containing tantalum and niobium. Most investigations of such solutions were performed on the liquid-liquid extraction of tantalum and niobium, with the objective of describing the mechanism of the process. These publications will be discussed separately. 

Three conceptual steps can be discerned in the definition of the ionic structure of fluoride melts containing tantalum or niobium. Based on the very first thermodynamic calculations and melting diagram analysis, it was initially believed that the coordination numbers of tantalum and niobium, in a molten system containing alkali metal fluorides, increase up to 8. 

Jahn-Teller distortions of octahedra are expected to be much smaller than in the cases mentioned above, if the orbital degeneracy occurs in the non-bonding d e-orbitals. Indeed no structures of fluorides are known, with the possible exception of FeF2 (24,130), in which distortions without doubt are effected by this degeneracy of rfs-orbitals. Weinstock and Claassen [322, 323) reported evidence of d5mamic Jahn-Teller effects on the vibrational properties of hexafluoride molecules, however. 

The first condition concerns the electronic configuration of the cations, the second one the arrangement of ions in the lattice, the crystal structure. Depending on the combination of those different factors, superexchange may occur in 3, 2 or 1 dimensions or fail to appear. Examples of all these four cases wiU be discussed in the following in context with the magnetic structures of fluorides. 

The use of H NMR to investigate the structure of fluoridated apatite appears to be of crucial importance, particularly for determining the active surface sites, suggested to be calcium hydroxyl (Ca-OH) and phosphorus (POH) groups [51],... 

Currently, it is fair to say that asymmetric synthesis using chiral quaternary ammonium fluorides remains an underdeveloped field, and the various useful stereoselective carbon-carbon bond-forming processes described in this chapter are only the start of an exploration of the vast synthetic potential of this process, particularly when combined with current knowledge of organosilicon chemistry. It seems that the key issue to be addressed is the rational molecular design of chiral quaternary ammonium cations with appropriate steric and electronic properties. These are expected to be readily tunable to impart not only a sufficient reactivity but also an ideal chiral environment to the requisite nucleophile involved in a desired chemical transformation. Clearly, the continuous accumulation of information related to the structure of fluoride salts and their reactivity and selectivity should create a solid basis for this field, offering - in time - a unique yet reliable tool for sophisticated bond construction events with rigorous stereocontrol, under mild conditions. 

At least in compounds with metals of lower oxidation states up to three or four, the metal-fluorine bond is almost purely ionic and the small, least polarizable fluoride ion behaves as a small hard sphere with an ioitic radius which varies, depending on the coordination number, between 1.285 (CN 2) and 1.320 A (CN 6). The structures of fluorides are therefore determined mainly by simple geometrical and electrostatic principles, such as sphere packing, the rigid sphere concept, which allows derivation of the coordination number around... 

As Figure 1 shows, the inversion process is visualized as angle deformation motions and called the edge inversion. The T-shaped Cj, transition structure (3) arises from the pyramidal structure of fluorides by simultaneous opening of the F—M—F and F—M—lone pair angles. Therefore, there are three identical reaction channels. The... 

Edwards, S.F., T.F. Poulos, and J. Kraut. 1984. The crystal structure of fluoride-inhibited cytochrome c peroxidase.. Biol. Chem. 259 12984—12988. 

The structures of fluorides, oxides, and oxyfluorides of xenon discussed in other sections can also be rationalized on the basis of VSEPR theory. Figure 2.22... 

A. Pedone, T. Charpentier, M.C. Menziani, The structure of fluoride-containing bioactive glasses new insights from first-principles calculations and solid state NMR spectroscopy. J. Mater. Chem. 22,12599-12608 (2012)... 

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