Niobium(V) chloride, also known as niobium pentachloride, is a yellow crystalline solid. It hydrolyzes in air, and samples are often contaminated with small amounts of NbOCl<sub>3</sub>. It is often used as a precursor to other compounds of niobium. NbCl<sub>5</sub> may be purified by sublimation.
Niobium(V) chloride forms chloro-bridged dimers in the solid state (see figure). Each niobium centre is six-coordinate, but the octahedral coordination is significantly distorted. The equatorial niobiumâÂÂchlorine bond lengths are 225 pm (terminal) and 256 pm (bridging), whilst the axial niobium-chlorine bonds are 229.2 pm and are deflected inwards to form an angle of 83.7ð with the equatorial plane of the molecule. The NbâÂÂClâÂÂNb angle at the bridge is 101.3ð. The NbâÂÂNb distance is 398.8 pm, too long for any metal-metal interaction. NbBr<sub>5</sub>, NbI<sub>5</sub>, TaCl<sub>5</sub> TaBr<sub>5</sub> and TaI<sub>5</sub> are isostructural with NbCl<sub>5</sub>.
Industrially, niobium pentachloride is obtained by direct chlorination of niobium metal at 300 to 350 ðC:
In the laboratory, niobium pentachloride is often prepared from Nb<sub>2</sub>O<sub>5</sub>, the main challenge being incomplete reaction to give NbOCl<sub>3</sub>. The conversion can be effected with thionyl chloride: It also can be prepared by chlorination of niobium pentoxide in the presence of carbon at 300 ðC.
Niobium(V) chloride is the main precursor to the alkoxides of niobium, which find uses in sol-gel processing. It is also the precursor to many other Nb-containing reagents, including most organoniobium compounds.
In organic synthesis, NbCl<sub>5</sub> is a very specialized Lewis acid in activating alkenes for the carbonyl-ene reaction and the Diels-Alder reaction. Niobium chloride can also generate N-acyliminium compounds from certain pyrrolidines which are substrates for nucleophiles such as allyltrimethylsilane, indole, or the silyl enol ether of benzophenone.