Niobium tetrahalides

The separation of niobium from tantalum tluough the gaseous chlorides is carried out at higher temperature, about 900 K, and it is therefore to be expected, as is the case, that the thermodynamic data will provide a useful guide. These metals form a number of chlorides, mainly the ui- tetra- and pentachlorides. These latter are much more volatile than the tetrahalides, and the exchange reaction at 900 K... 

The niobium and tantalum tetrahalides are all known, except TaF4, whose non-existence is consistent with the fact that [TaF5]4 should be the least reducible halide of the group VA... 

Table 30 Adducts of Niobium and Tantalum Tetrahalides with Phosphorus Donors...

The development of the chemistry of niobium and tantalum in their lower oxidation states, three or less, has long been precluded by the lack of convenient starting materials. Indeed, stoichiometric trihalides MX3 (X = Cl, Br) are only incidental compositions. They are obtained by reduction of the pentahalides or disproportionation of the tetrahalides, and are polymeric and rather inert. 

Even fewer complexes with nitrogen donor ligands have been reported and all are methyl cyanide adducts (Tables X and XI). Protactinium pentabromide forms a soluble 1 3 complex in contrast to the 1 1 complexes formed by niobium and tantalum pentahalides (46). Other actinide pentahalide-methyl cyanide complexes are still unknown. Protactinium tetrachloride, tetrabromide, and tetraiodide react with anhydrous, oxygen-free methyl cyanide to form slightly soluble 1 4 complexes (44, 48) which are isostructural with their actinide tetrahalide analogs. 

Tetrahalides and Their Derivatives. All possible tetrahalides are known except TaF4. Niobium(iv) fluoride, is a black involatile, paramagnetic solid in which each Nb atom lies at the center of an octahedron. The other six tetrahalides differ from NbF4 and resemble each other in their structures and in being diamagnetic. 

A more complex example, but one that is understandable along similar lines, is the structure of the niobium and tantalum tetrahalides. They form chains based on the sharing of opposite edges of NbXe octahedra with niobium-niobium distances which alternate along the chain (13.42). The structure is an obvious distortion of the idea] edge-sharing chain (13.41). The electronic configuration of the metals is... 

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