Niobium, coordination compounds

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

Holloway CE, Melnik M (1985) Niobium coordination compounds classification and analysis... 

Although trialkyl- and triarylbismuthines are much weaker donors than the corresponding phosphoms, arsenic, and antimony compounds, they have nevertheless been employed to a considerable extent as ligands in transition metal complexes. The metals coordinated to the bismuth in these complexes include chromium (72—77), cobalt (78,79), iridium (80), iron (77,81,82), manganese (83,84), molybdenum (72,75—77,85—89), nickel (75,79,90,91), niobium (92), rhodium (93,94), silver (95—97), tungsten (72,75—77,87,89), uranium (98), and vanadium (99). The coordination compounds formed from tertiary bismuthines are less stable than those formed from tertiary phosphines, arsines, or stibines. 

The steric similarity of oxygen and fluorine ions enables the formation of coordination-type structures in some tantalum and niobium oxyfluoride compounds. 

Tantalum and niobium fluoride compounds that crystallize in coordination-type structures also seem to be perspective candidates for the investigation of ferroelectric properties. Ravez and Mogus-Milancovic [404] showed that some fluoride and oxyfluoride compounds with crystal structures similar to the Re03 type exhibit ferroelastic properties. For instance, ferroelastic properties were found in some solid solutions based on Nb02F and Ta02F [405,406]. 

The number of authenticated niobium and tantalum coordination compounds in oxidation state +1 is very limited. They were generally obtained through reduction of [MCl4(dmpe)2], [TaCl2(dmpe)2] or Nb(OC6H3Me2-3,5)2(dmpe)2.731a... 

Compounds containing niobium or tantalum in negative formal oxidation states -I and -III are mainly metal carbonyl anions. Although these are organometallic derivatives, the report of efficient procedures for the synthesis of [M(CO)6] since the review of Labinger8 merits mention, as it can be anticipated that these highly reduced and reactive species will be important precursors of a large variety of new coordination compounds and metal clusters. 

Winter and co-workers used the proton NMR spectra of niobium(v) complexes with 5-phenyltetrazolate ligand recorded at temperatures between —80 and +40°C in toluene-structural analysis <2001IC6451>. One- and two-dimensional 111 NMR methods in solution and solid state were used for analyzing the structure of irbesartan 83 <1998J(P2)475> as well as for the analysis of the structure of coordination compounds of ruthenium(n) and iron(n) with 5-aryltetrazolates <2002OM3774, 2003JOM(669)135, 2006IC695>. 

The number of niobium and tantalum coordination compounds in oxidation states lower than II is limited, and they often are organometallic derivatives involving carbonyl, cyclopentadienyl, or alkenic ligands. 

No derivative in which niobium or tantalum displays an oxidation state of —II has been reported so far, while carbonyl complexes derived from [M(CO)s] represent the only coordination compounds for these metals in oxidation state —III. 

This account would not be complete without mention of distortional isomerism, a term Joe Chatt was to coin in describing two isomeric forms of the six-coordinate compounds, [Mo(0)Cl2(PMe2Ph)3] (35, Figure 12) in a short communication published in Chem. Commun. in 1971, but which later was to spark an intense debate into the reality or otherwise of the phenomenon of bond stretch isomerism. We followed the debate with some interest, since we had inadvertently stumbled across seven-coordinate oxo and sulfido compounds of niobium of the type [Nb(E)Cl3(PMe3)3] (E = O or S 36) which seemed to show a closely related effect. 

Table 5. Nb-Nb interatomic distances (in pm) in compounds with condensed clusters. Nb " denotes niobium coordinated with 4 oxygen atoms, Nbf- denotes niobium coordinated with 5 oxygen atoms, and Nb f denotes niobium coordinated with 6 oxygen atoms. 

Niobium and tantalum halides also react with isocyanides to give migration products [equation (12.51)]. Azide complexes react with CNR to afford coordination compounds with tetrazolyl [equation (12.52)]. Codimerization and cooligomerization of... 

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