Niobium is very similar to tantalum and zirconium in many ways. It reacts with fluorine at room temperature, chlorine and hydrogen at 200°C, and nitrogen at 400°C, and the products are generally interstitial inhomogeneous compounds. Niobium metal oxidizes in air at 200°C and is resistant to molten alkalis and acids, including aqua regia, hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid. However, hydrofluoric acid and a mixture of hydrofluoric and nitric acids can attack niobium.
Although niobium can form a variety of compounds in the oxidation state of +5 to −1, it is most commonly in the +5 oxidation state. Niobium compounds with an oxidation state below +5 all contain niobium-niobium bonds.
Oxides and sulfides
Niobium oxides can have the following oxidation states: +5 (Nb2O5), +4 (NbO2) and +3 (Nb2O3) and, rarely, +2 (NbO). Niobium pentoxide is the most common niobium oxide, and the preparation of niobium metal and all niobium compounds should start with it. To make niobate, niobium pentoxide can be dissolved in an alkaline hydroxide solution, or melted in an alkali metal oxide. Lithium niobate (LiNbO3) has a perovskite-type metatrigonal crystal structure, while lanthanum niobate contains isolated NbO3− ions. Other known compounds include niobium sulfide (NbS2), which forms a layered structure.
A thin layer of niobium pentoxide can be added to the surface of the material by chemical vapor deposition or atomic layer deposition, and both methods use the principle that niobium (V) ethanol will decompose at more than 350 °C.
Halide
Niobium can form halides with +5 and +4 oxidation states, as well as various substoichiometric compounds. Niobium penthalide (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 with a melting point of 203.4 °C. Both can be hydrolyzed to form oxides and halide oxides, such as NbOCl3. Niobium pentachloride is also a volatile reagent that can be used in the synthesis of various organometallic compounds, including dichlorodiniobene ((C5H5)2NbCl2). Niobium tetrahalides (NbX4) are dark polymers with niobium-niobium bonds, such as black, hygroscopic niobium tetrafluoride (NbF4) and brown niobium tetrachloride (NbCl4).
Niobium halide aninations are also present because the pentahalides of niobium are all Lewis acids. One of the most important is [NbF7], which is an intermediate compound in the mineral separation of niobium and tantalum. It is more easily converted to oxypentafluoride than its tantalum counterpart. Other halogenated complexes include [NbCl6]:
Nb2Cl10+ 2Cl→ 2 [NbCl6]
Niobium also forms a variety of reduced halide clusters, such as [Nb6Cl18].
Nitrides and carbides
Niobium nitride (NbN) turns into a superconductor at low temperatures and is used in infrared detectors. The main niobium carbide is NbC, which is extremely hard and is a refractory ceramic material that can be used as a cutting tool bit material.