Niobium(V) chloride

Niobium(V) chloride, also known as niobium pentachloride, is a yellow crystalline solid often used as a starting material in niobium chemistry. It is prepared by heating niobium metal in chlorine. It is often contaminated with small amounts of niobium(V) oxychloride, NbOCl₃, formed by hydrolysis or from traces of oxygen during the preparation. NbCl₅ may be purified by sublimation.

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Structure and properties

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 niobium–chlorine bond lengths are 225 pm (terminal) and 256 pm (bridging), while the Nb–Cl–Nb angle at the bridge is 101.3°. The Nb–Nb distance is 395.1 pm, too long for any metal-metal interaction. NbBr₅, TaCl₅ and TaBr₅ are isostructural with NbCl₅, but NbI₅ and TaI₅ have different structures.

Uses

Niobium(V) chloride is used in organic chemistry as a 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^[1].

Bibliography

• Cotton, F. Albert; Wilkinson, Geoffrey (1980). Advanced Inorganic Chemistry (4th Edn.). New York:Wiley. ISBN 0-471-02775-8. 

References

1. ^  Andrade, C. K. Z.; Rocha, R. O.; Russowsky, D.; & Godoy, M. N. (2005). Studies on the Niobium Pentachloride-Mediated Nucleophilic Additions to an Enantiopure Cyclic N-acyliminium Ion Derived from (S)-malic acid. J. Braz. Chem. Soc. 16: 535–539. Online Article (.PDF file)