Niobium nitrogen ligands

Niobium and tantalum halides form adducts with various nitrogen donor ligands including aliphatic and aromatic amines nitriles, Schiffs bases and imidazoles (Table 5). The reactions of MXS with pyridine and related ligands such as bipy or phen depend critically on the reaction conditions. With py at low temperature MX5 (X = Cl, Br) yielded 1 1 adducts that are rapidly reduced to [MX4(py)2] on increasing the temperature, with formation of l-(4-pyridyl)pyridinium halide. Similarly, bipy and phen reduced the metal in MeCN to oxidation state +IV and formed monoadducts of type [MX bipy)] at room temperature, while at 0°C the same reactions yielded [NbCls(bipy)(MeCN)] and [TaX5(bipy)(MeCN)J (X = C1 or Br). NbBrs and Tals formed [MX5(bipy)2], which were formulated as the eight-coordinate [MX4(bipy)2]X.1 Reduction of the metal can however be prevented, even at room temperature,... 

Numerous complexes of niobium(IV) and niobium(V) halides with various nitrogen-donor ligands have been reported in the literature. The products obtained from these reactions are critically dependent upon the reaction conditions. " Tetrakis(isothiocyanato)bis(2,2 -bipyridine)niobium(IV) can be prepared directly from the hexakis(isothiocyanato)niobate(IV) complex or by the reduction of the hexakis(isothiocyanato)niobate(V) ion according to the published method. This method can also be extended to the preparation of other analogous complexes. 

In the solid state at room temperature the compound is stable for only about 1 hr, but it can be stored indefinitely under nitrogen at —20°. The compound should always be handled under an inert atmosphere as it is air-sensitive. Its infrared spectrum consists of strong bands at 1010 and 815 cm-i arising from vibrations of the //-cyclopentadienyl ligand and bands at 1100, 750, 700, and 490 cm-i due to vibrations of coordinated dimethylphenylphosphine. Bands at 1050 and 530 cm i in spectra of the tetrafluoroborate salt or at 830 cm i in those of the hexafluorophosphate salt arise from vibrations of the anion a band at 1740 cm- is due to a niobium-hydrogen stretching vibration. The nmr spectrum (TMS internal standard) at —36° of an acetone-dg solution of the tetrafluoroborate salt is composed of a complex band centered at 72.4, a doublet with Jph = 2.0 Hz centered at t4.42, a doublet with Jph = 7.1 Hz centered at r8.52, and a doublet with Jph = 31.5 Hz centered at t13.96. 

Even fewer complexes with nitrogen donor ligands have been reported and all are methyl cyanide adducts (Tables X and XI). Protactinium pentabromide forms a soluble 1 3 complex in contrast to the 1 1 complexes formed by niobium and tantalum pentahalides (46). Other actinide pentahalide-methyl cyanide complexes are still unknown. Protactinium tetrachloride, tetrabromide, and tetraiodide react with anhydrous, oxygen-free methyl cyanide to form slightly soluble 1 4 complexes (44, 48) which are isostructural with their actinide tetrahalide analogs. 

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