Niobium(V) chloride

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 NbOCl₃. It is often used as a precursor to other compounds of niobium. NbCl₅ may be purified by sublimation.^[1]

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 niobium–chlorine 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 83.7° 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.^[2] NbBr₅, NbI₅, TaCl₅ TaBr₅ and TaI₅ are isostructural with NbCl₅.

Industrially, niobium pentachloride is obtained by direct chlorination of niobium metal at 300 to 350 °C:^[3]

2 Nb + 5 Cl₂ → 2 NbCl₅

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:^[4] It also can be prepared by chlorination of niobium pentoxide in the presence of carbon at 300 °C.

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.

有機合成において、NbCl _5はカルボニル-エン反応およびディールス-アルダー反応においてアルケンを活性化する非常に特殊なルイス酸である。塩化ニオブは、アリルジメチルトリメチルシラン、インドール、またはベンゾフェノンのシリルエノールエーテルなどの求核剤の基質となる特定のピロリジンから N-アシルイミニウム化合物を生成することもできる。^{[ 5 ]}

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