Niobium(V) chloride

Niobium chloride, also known as niobium pentachloride, is a yellow crystalline solid. It hydrolyzes in air, and samples are often contaminated with small amounts of NbOCl₃. It is often used as a precursor to other compounds of niobium. NbCl₅ may be purified by sublimation.

Structure and properties

Niobium chloride forms chloro-bridged dimers in the solid state. Each niobium centre is six-coordinate, but the octahedral coordination is significantly distorted. The equatorial niobium–chlorine bond lengths are 225 pm and 256 pm, whilst the axial niobium-chlorine bonds are 229.2 pm and are deflected inwards to form an angle of 83.7° with the equatorial plane of the molecule. The Nb–Cl–Nb angle at the bridge is 101.3°. The Nb–Nb distance is 398.8 pm, too long for any metal-metal interaction. NbBr₅, NbI₅, TaCl₅ TaBr₅ and TaI₅ are isostructural with NbCl₅.

Preparation

Industrially, niobium pentachloride is obtained by direct chlorination of niobium metal at 300 to 350 °C:
In the laboratory, niobium pentachloride is often prepared from Nb₂O₅, the main challenge being incomplete reaction to give NbOCl₃. 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 °C.

Uses

Niobium 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, NbCl₅ 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.