Niobium structural data

Niobium coordination compounds classification and analysis of crystallographic and structural data. C. E. Holloway and M. Melnik, Rev. Inorg. Chem., 1985,7,162 (198). 

Virtually all of the reported structural data on titanium alloy hydrides and deuterides indicate that the solute atoms occupy tetrahedral interstitial sites in the metal lattice. Neutron diffraction data obtained for deuterium in Ti/34 atom % Zr and in Ti/34 atom % Nb (17) indicate tetrahedral site occupancy in the bcc /3-phase. Similarly, data reported for deuterium in Ti/19 atom % V and in Ti/67 atom % Nb (18) indicate tetrahedral site occupancy in the fee 7-phase. Crystallographic examination of the 7-phase Ti-Nb-H system (19) reveals that increasing niobium content linearly increases the lattice parameter of the fee 7-phase for Nb contents ranging from 0 to 70.2 atom %. Vanadium, on the other hand, exerts the opposite effect (6) at H/M = 1.85, the 7-phase lattice parameter decreases with increasing vanadium contents. 

Table 44 Structural Data for Various Niobium and Tantalum Ousters of Type (M6X12)4+...

The crystal structures of both (PPN)[M(CO)6] derivatives have been determined.717 The coordination polyhedron is octahedral (Nb—C 2.098(5) A CNbC 89.2(2)°). The PPN moiety is constrained to be centrosymmetric, and thus linear. These compounds correspond to the lowest oxidation state of niobium and tantalum for which structural data are available. A single v(CO) is found in the IR (1854 and 1852 cm-1 for Nb and Ta respectively). Comparable spectra are observed for Na[M(CO)6] in pyridine, but in solvents of lower dielectric constants such as tetrahydrofuran, additional bands attributed to distortion of the anion by the countercation are observed. [Nb(CO)6] appears to be the most labile carbonyl of the group VA analogs. 

Tetranuclear monoadducts with asymmetrical M- O M bridges related to those of [M2OCI9] have been characterized but most structural data concern monomeric NbOCls bis adducts. In such derivatives, the metal is octahedrally surrounded with the neutral Ugands cis to each other, one being trans to the 0x0 bond, which is short (typical Nb=0 Bond Length 1.70 A). The coordination polyhedron is distorted as a result of the niobium-oxygen multiple bond (see Valence Shell Electron Pair Repulsion Model). ... 

The major difference in the dithiocarhamato chemistry of tantalum and niobium is the existence of the Ta(V) species Ta(R2C tc)5, whereas the comparable niobium compound is unknown. The preparative methods for Ta(R2C tc)5 involve either a CS2 insertion in Ta(NR2)j (R = Me )) or a reaction between TaClj and sodium or ammonium dithiocarbamate (R = Et (7), R2 = ( 112)4 ( ))- The structure of this interesting diamagnetic compound is not elucidated, but from infrared data the presence of one or more unidentate ligands is suggested 11,15). 

In contrast with the difluorides, the distribution of trifluorides extends to the third series of the transition metals, where iridium and gold trifluorides are fully characterized. In the second series, trifluorides are known for the elements from niobium to rhodium, with the exception of technetium, and in the first series, from titanium to cobalt. All the trifluorides have been characterized structurally, with earlier reports based on X-ray powder-diffraction data, since the compounds were not prepared in single-crystal form until more recently, when high-temperature, crystal-growth techniques became available. 

Chemical shift data have been obtained by photoelectron spectroscopy501" for the core electrons of niobium and tantalum compounds. The similarity in chemical shifts between structurally similar compounds of these two elements is in accord with the similarity in their ionic radii. 

Hutchings (170) plotted (Figure 31) the activity against the surface area for a number of promoted catalysts and deduced that most of the catalysts conform to a linear correlation. The only enhancement of the specific activity was observed for the cerium-promoted catalyst. This result shows that care must be taken in the interpretation of the catalyst performance data, particularly when catalysts prepared by different methods are compared. In a separate study, Hutchings and Higgins (171) found that chromium, niobium, palladium, antimony, ruthenium, thorium, zinc, and zirconium each had very little effect on the specific activity of (VO)2P207. A significant increase in surface area was observed with zirconium, zinc, and chromium, which could be of use as structural promoters. Iron-, cesium-, and silver-doped catalysts decreased the specific activity, and cobalt and molybdenum were the only promoters found to increase the specific activity. 

Crystallographic Shear (C5) Phases.—The CS phases are the best-known group of materials which appear to be intolerant of point-defect populations. There are three major families those based upon tungsten trioxide, WO3, upon rutile, Ti02, and upon niobium pentoxide, Nb205. These and other less studied systems have been described in some considerable detail in two previous review articles in this series and elsewhere and the fundamental principles underlying their structures will not be repeated here. In this section some of the results found in the tungsten trioxide and rutile-related systems will be outlined. Older results, covered in the earlier reviews, will merely be sketched in where relevant, and emphasis will be upon newer data or else on a re-examination of earlier results from the point of view of this article. 

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