Molybdenum(V) chloride

Molybdenum pentachloride is the chemical compound with the formula MoCl₅. This dark volatile solid is an important starting reagent in the preparation of molybdenum compounds. In the solid state molybdenum pentachloride exists as a dimer with the formula Mo₂Cl₁₀, with a structure similar to that of the corresponding Nb and Ta pentachlorides. In the gas phase, it is a monomeric pentahalide. It is paramagnetic, with one unpaired electron per Mo center, reflecting the fact that the formal oxidation state is 5+, leaving one valence electron.

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Preparation and properties

MoCl₅ is prepared by chlorination of Mo metal but also chlorination of MoO₃. Notice that MoCl₆ cannot be prepared under normal conditions. In contrast, the heavier analogues WCl₆ is well known.

MoCl₅ is a strong oxidant, as indicated by its non-reactivity toward Cl₂. It is easily reduced by even MeCN to afford the adduct MoCl₄(MeCN)₂, which in turn reacts with THF to give MoCl₄(THF)₂, a very versatile reagent in the chemistry of molybdenum. MoCl₅ is reduced by HBr to form MoBr₄. The reaction is conducted in ethylbromide at -50 C:

2 MCl₅ + 10 HBr → 2 MoBr₄ + 10 HCl + Br₂

MoBr₄ reacts with THF to give the Mo(III) species mer-MoBr₃(THF)₃.

MoCl₅ is a good Lewis acid toward non-oxidizable ligands. It forms an adduct with chloride to form [MoCl₆]^-.

Safety considerations

MoCl₅ is an aggressive oxidant and readily hydrolyzes to release HCl.

References

• T. Kauffmann, S. Torii, T. Inokuchi “Molybdenum(V) Chloride” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
• Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
• Dilworth, J. R.; Richards, R. L. "The Synthesis of Molybdenum and Tungsten Dinitrogen Compounds" Inorganic Syntheses, 1990, volume 28, pages 33-45. ISBN 0-471-52619-3
• Calderazzo, F.; Maichle-Mossmer, C.; Pampaloni, G. and Strähle, J., "Low-temperature Syntheses of Vanadium(III) and Molybdenum(IV) Bromides by Halide Exchange", Journal of the Chemical Society, Dalton Transactions, 1993, pages 655-8.