Tantalum tetrahalides

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...

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... 

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 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. 

Thermal cycling will be lessened if the control (but not the measuring) thermocouple is placed between the liner and the furnace wall. It is highly advisable that the fused silica jacket about the welded container be sufficiently long to extend from the end of the furnace to permit condensation of the tetrahalide, should a leak develop. Unless a thermal gradient is required, the tantalum tube itself should be centered in the furnace. Although the explosion hazard is very slight if these procedures are followed, the open ends of the furnace should still be shielded from workers. 

The weighed tetrahalide is first transferred into the cap-welded end of a tantalum tube of size noted above (tube volume 1.5 cm3/g ZrX). The appropriate weight of turnings ( 0.1%) is then packed into the tube. Reactants are loaded in this order to prevent welding difficulties from the volatile tetrahalides. 

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