Niobium & Tantalum: Inorganic & Coordination Chemistry

Abstract

The chemistry of niobium and tantalum ranges from oxidation states +V to −III with no species of oxidation state −II known so far. The chemistry of those oxophilic metals is dominated by their highest oxidation states, V and IV. Pentahalides especially chlorides are the most usual starting material. They form numerous complexes especially with hard donors and are used for access not only to pentavalent metal alkoxides, amides, thiolates, and so on but also to most molecular species in lower oxidation states. Oxo species are formed quite easily by O‐donor abstraction reactions or adventitious hydrolysis. Pentavalent alkoxides are active in various catalytic processes and are useful precursors for access to oxide ceramics such as ferroelectrics via the formation of heterometallic species. The most relevant results in organometallic chemistry of Nb and Ta have been obtained with ancillary, bulky aryloxide or siloxide ligands. Niobium is more prone to reduction than tantalum. Chalcogenides and chalcogenohalides are often one–dimensional solids and display a rich structural and synthetic chemistry. Numerous trispyrazolylborate derivatives of Nb and Ta have been reported for oxidation states V to I. The development of the nonaqueous chemistry of those elements in intermediate oxidation states has become an active and fruitful area. Species based on MM or MM multiple bonds, the latter especially for Nb, have been characterized. The NbNb triple bond (2.20 Å) is much shorter than the MM distance in the metal. Tri‐ or divalent species are able to promote CE (E = O, S, N) cleavage and thus to generate novel ligands because of the trend of Nb and Ta to form multiple ME bonds and attain their highest oxidation states. CC coupling reactions are observed with unsaturated ligands (alkynes, nitriles, etc.). M III η 2 ‐alkyne complexes have a metallocyclopropene character; they can react with unsaturated organic molecules via stoichiometric reactions. The reservoir of electrons of the MM bond is also of interest for activation of small molecules such as N 2 , H 2 , CO, and so on. Tri or tetradentate N,N; N,P; or N,O ligands have been used to stabilize dimeric dia or paramagnetic dinitrogen complexes. The bridging, highly activate N 2 ligand is amenable to functionalization in some cases, and cleavage into nitrides has been observed. The chemistry of reduced Nb and Ta halides is rich in clusters with nonintegral oxidation states (between III and I). Hexanuclear clusters dominate the aqueous chemistry for these oxidation states. One of their potential applications could be their use as contrasting agents. The species in the lowest oxidation states are essentially based on carbonyl ligands.

Determination of distribution coefficients (Kd) of various radionuclides on UTEVA resin

“…The values of the distribution coefficients for Nb in the high HCl concentration cocktail differ from previously published data [2]. The difference of K d values could be explained by hydrolysis due to the evaporation process (e. g. NbO 3+ complexes) or by partial dissolution of the extractant in the aqueous phase [21,22].…”

Quo vadis niobium? Divergent coordination behavior of early-transition metals towards MOF-5

“…12,13 With its exible coordination sphere, large covalent radius, and extreme oxophilicity, niobium is the epitome of a difficult substrate for MOF formation and should therefore provide insight into the behavior of other early transition metals for this purpose. 14 To circumvent the expected challenges associated with the direct synthesis of Nb MOFs (vide supra), we decided to focus on post-synthetic modication (PSM), a method that has previously proven successful in altering the chemical structure of pre-formed MOFs. [15][16][17] Because PSM allows for mild reaction conditions, contrasting with the harsh solvothermal synthetic routes encountered in de novo MOF preparation, it can lead to the isolation of materials that are difficult to form otherwise.…”

Structural Characterization of a High‐Nuclearity Niobium(V) Carboxylate Cluster Based on Pivalic Acid

“…Consistent with this mechanism, some early-transition metal MOFs can be made directly from discrete isolated metal carboxylate clusters. [17,18] Niobium has a highly oxophilic character (particularly in higher oxidation states), large covalent radius, and fluxional coordination sphere, [19] rendering the controlled precipitation of well-defined crystalline solids in classical de novo fashion challenging. [20,21] Tellingly, only few instances of niobium incorporation into MOFs have been reported so far [22 -26] with only one recent example of monometallic de novo synthesized framework, which is based on deprotonated 3,6-dichloro-2,5-dihydroxybenzoquinone rather than a carboxylate linker.…”