Niobium //-complexes

Among the different families of tantalum and niobium complex fluorides and oxyfluorides, the family of compounds with an X Me ratio equal to 6 is the largest. Table 22 presents the main structural characteristics of hexafluoroniobates and hexafluorotantalates. All known cases of niobium- and tantalum-containing formulary analogs have the same crystal structure type, at least at ambient temperature. 

The fact that tantalum and niobium complexes form in fluoride solutions not only supplements fundamental data on the coordination chemistry of fluoride compounds, but also has a broad practical importance. This type of solution is widely used in the technology of tantalum and niobium compounds in raw material digestion, liquid-liquid extraction, precipitation and re-pulping of hydroxides, and in the crystallization and re-crystallization of K-salts and other complex fluoride compounds. 

Stefanovich, Leonov and Venevtsev [417] describe a typical procedure of SHG measurement. The scheme of the SHG equipment enables to perform measurements at different temperatures, as shown in Fig. 96. SHG measurements of some tantalum and niobium complex fluoride and oxyfluoride compounds in the powdered form were reported in [206, 211] and some results are presented in Table 58. 

The process of separating the intermediate products from the purified solutions, in the form of solid complex fluoride salts or hydroxides, is also related to the behavior of tantalum and niobium complexes in solutions of different compositions. The precipitation of complex fluoride compounds must be performed under conditions that prevent hydrolysis, whereas the precipitation of hydroxides is intended to be performed along with hydrolysis in order to reduce contamination of the oxide material by fluorine. 

This particular difference in the Lewis acidity of tantalum and niobium complexes provides the possibility of an effective separation between the elements using liquid-liquid extraction. It is obvious that tantalum will extract into the organic phase at a lower acidity of the aqueous solution, whereas niobium will require a higher level of acidity in order to be extracted. The stripping of the elements from the organic phase into the aqueous phase will take place in reverse order. 

Extraction and stripping (re-extraction) of tantalum and niobium complex acids are shown schematically in Fig. 124. 

Niobium, tris(diethyldilhiocarbamato)oxy-stereochemistry, 1,82 structure, 1, 83 Niobium, tris(oxa ato)oxy-stereochcmistry, 1, 82 Niobium, tris(phcnylcncdirhio)-structure, 1, 63 Niobium alanate, 3, 685 Niobium complexes alkyl alkoxy reactions, 2, 358 amides, 2,164 properties, 2, 168 synthesis, 2, 165 applications, 6,1014 carbamicacid, 2, 450 clusters, 3, 672,673,675 hexamethylbenzene ligands, 3, 669 cyanides synthesis, 2, 9 p-dinitrogen, 3, 418 fluoro... 

Using an electron-gun source, tungsten atoms were reacted with benzene, toluene, or mesitylene at 77 K, to form the expected (arene)2W complex (42) in a yield of 30%, compared with the —2% yield from the previously published, bis(benzene)W synthesis (32). These arene complexes are reversibly protonated, to give the appropriate [(T7-arene)2WH] species. By using the same technique, the analogous, niobium complexes were isolated (43). 

The first complexes of a-keto ylides and group 5 early transition metals have only recently been obtained by reaction of Nb(III) derivatives [[NbCl3(dme) (R C=CR")] with 25 (R = thiazolyl) (Scheme 16). The chelation of the ylide occurs through an N,0-coordination to the metal center and in presence of MeLi a deprotonation of a phenyl ring takes place with the loss of alkyne, leading to the formation of a new orfho-metallated binuclear compound 32. The two ylides involved in the complexation behave as tridentate anionic ligands and are mutually in a trans disposition in order to minimize the steric hindrance [71,72]. Another binuclear niobium complex 33 has been obtained from 25 (R = Me, Ph) with this time an 0-coordinated a-keto ylide [68]. 

Niobium complexes are known, with little direct siHca-silsesquioxane analogy [28]. 

Oxidation-Reduction Titrations. The extent of reduction resulting from reaction of niobium (V) chloride and bromide with pyridine was determined by indirect titration of crude reaction mixtures with standard ammonium tetrasulfato-cerate(IV) solution. Samples were stirred overnight in a stoppered flask with an excess of iron (III) ammonium sulfate. Any iron (II) formed by reaction with the niobium complex mixture was then titrated with the standard tetrasulfato-cerate(IV) solution using ferroin as indicator. Results of these determinations are given in Table III. 

The corresponding chemistry of analogous niobium complexes was inhibited by the requirement of a more complicated synthetic approach for the isolation of the niobaziridine hydride. The use of the isopropyl substitued —N(Pr )Ar amido ligand proved unsuitable for the stabilization of [Nb(H) N(P )Ar 2(ri -Me2CNAr)] because of insertion into the Nb—H bond. °2 These difficulties were overcome with use of the N(CH2Bu )Ar substituent and a synthetic approach based upon [Nb(O) N(CH2Bu )Ar 3]3 which enabled the isolation of [Nb(H) N(CH2Bu )Ar 2( ri -CH(Bu )=NAr)] via reduction. The synthesis of this species has opened routes to some unusual chemistry as shown in Scheme 6.5. ... 

Hydrogenation of aromatic rings will not be described in detail in this review since it is mostly concerned with isolated double bonds. This is however an area of active research, as recently demonstrated by the use of catalytic niobium complexes for the selective hydrogenation of aryl phosphines (Scheme 3)56. 

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