Niobium complexes coordination numbers

An increase in the Me F ratio leads to an increase in the acidity of the initial solution, whereas the acidity of alkali metals increases according to their molecular weight, from Li to Cs. Therefore the additives of fluorides of alkali metals having higher atomic weight provide formation of complex fluorides with lower coordination number of tantalum or niobium. 

Since the coordination number of tantalum or niobium in fluoride and oxyfluoride compounds cannot be lower than 6 due to steric limitations, further decrease of the X Me ratio (lower than 6) leads to linkage between complex ions in order to achieve coordination saturation by sharing of ligands between different central atoms of the complexes. The resulting compounds have X Me ratios between 6 and 4, and form crystals with a chain-type structure. 

S Tantalum and niobium are present in the crystal structure in the form of complex ions. The lowest coordination number, 6, corresponds to the formation of slightly distorted octahedrons. The linking and packaging of the octahedrons depends on the X Me ratio, where X is the total number of oxygen and fluorine atoms, and Me is the total number of tantalum or niobium ions as well as other metals that can replace tantalum or niobium in the octahedral polyhedron. The crystal structure type can be defined based on the X Me ratio, as follows ... 

The results of solid state reactions of protactinium dioxide and pentoxide with other metal oxides (89, 93-96) support the view that the oxide systems of protactinium resemble those of other actinide elements rather than those of niobium and tantalum. However, when assessing results of this type one must always bear in mind the relative ionic radii of the respective M " and M + ions since they obviously play a large part in determining the structures of the complex phases. This comment applies equally well, of course, to the structural properties of other types of compound and in particular to the high coordination numbers exhibited by protactinium(V) in its chloro and nitrato complexes. 

The nbo or niobium oxide net is built from perpendicular oriented square planar units, sec Figure 5.11. This is the second most common four-connected net, and as the square planar geometry is mostly restricted to the metal ions Co(I), Rh(I), Ir(I), Ni(II), Pd(Il) and Pt(II) this may seem a bit surprising. However, the connectivity at the node is not necessarily the same as the coordination number of the metal atom (if this is a metal ion centred net), and thus this net is frequently encountered for five-coordinated Cu(II) in distorted square-planar pyramidal geometries, [27-301 or in octahedral complexes with two non-bridging axial ligands as in our example shown in Figure 5.12. 

The greatest effect is seen with nitride (A 0.390-0.330 A) and is slightly greater in six versus seven coordination. It can also be seen (Table V) that addition of Lewis acids to the nitride results in a reduction in the trans influence, while as seen from the niobium complexes, [NbE(S2CNEt2)3] (Table IV) the effect diminishes in the order O > S > NAr. In a number of instances, two... 

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