Niobium Compounds

Niobium is in many ways very similar to tantalum and zirconium. It will react with fluorine at room temperature, with chlorine and hydrogen at 200 °C, and with nitrogen at 400 °C, the products generally being interstitial non-integer ratio compounds. Niobium metal oxidizes in air at 200°C and is resistant to molten bases and a variety of acids, including aqua regia, hydrochloric, sulfuric, nitric, and phosphoric acids. However, hydrofluoric acid and mixtures of hydrofluoric and nitric acids can attack niobium.
Although niobium can form a variety of compounds with oxidation states ranging from +5 to -1, it is most commonly found in the +5 oxidation state. Niobium compounds with oxidation states below +5 contain niobium-niobium bonds.

Oxides and sulfides
Oxides of niobium can have the following oxidation states: +5 (Nb2O5), +4 (NbO2), and +3 (Nb2O3), plus the rarer +2 state (NbO). Niobium pentoxide is the most common niobium oxide from which niobium metal and all niobium compounds are prepared. To make niobates, niobium pentoxide can be dissolved in an alkaline hydroxide solution or melted in an alkali metal oxide. Lithium niobate (LiNbO3) has a calcite-type biotriangular crystal system structure, while lanthanum niobate contains isolated NbO3- ions. Other known compounds include niobium sulfide (NbS2), which forms a layered structure.
Thin layers of niobium pentoxide can be added to the surface of the material using chemical vapor deposition or atomic layer deposition, both of which use the principle that ethanol niobium(V) thermally decomposes above 350 °C.
Halides
Niobium can form halides with +5 and +4 oxidation states, as well as various sub-stoichiometric compounds. Niobium pentahalide (NbX5) contains an octahedral niobium central atom. Niobium pentafluoride (NbF5) is a white solid with a melting point of 79.0°C, while niobium pentachloride (NbCl5) is yellow in color with a melting point of 203.4°C. Both can be hydrolyzed to form oxides and halogenated oxides such as NbOCl3. Niobium pentachloride is also a volatile reagent used in the synthesis of a variety of organometallic compounds, including niobium dichlorocene ((C5H5)2NbCl2). Organometallic compounds. The tetrahalides of niobium (NbX4) are dark polymers containing niobium-niobium bonds, such as the black, hygroscopic niobium tetrafluoride (NbF4) and the brown niobium tetrachloride (NbCl4).
Negative halide ions of niobium are also present, due to the fact that the pentahalides of niobium are all Lewis acids. The most important one is [NbF7], which is an intermediate compound in the process of separating the minerals of niobium and tantalum. It is more readily converted to the oxygen pentahalide than the corresponding tantalum compound. Other halogenated complexes include [NbCl6]:
Nb2Cl10+ 2Cl→ 2 [NbCl6].
Niobium also forms a variety of reduced halide atom clusters, such as [Nb6Cl18].
Nitrides and carbides
Niobium nitride (NbN) becomes a superconductor at low temperatures and is used in infrared detectors. The predominant niobium carbide is NbC, which is extremely hard, a refractory ceramic material, and is used as cutting tool bit material.