Molybdenum(V) chloride

Molybdenum(V) chloride
IUPAC name Molybdenum(V) chloride Molybdenum pentachloride
Identifiers
CAS number	10241-05-1 Y
RTECS number	QA4690000
Properties
Molecular formula	Cl10Mo2
Molar mass	273.21 g/mol
Appearance	dark-green solid hygroscopic paramagnetic
Density	2.928 g/cm3
Melting point	194 °C
Boiling point	268 °C
Solubility in water	hydrolyzes
Solubility	soluble in dry ether, dry alcohol, organic solvents
Structure
Crystal structure	monoclinic
Coordination geometry	edge-shared bioctahedron
Hazards
EU Index	Not listed
Main hazards	oxidizer, hydrolyzes to release HCl
Flash point	Non-flammable
Related compounds
Other anions	Molybdenum(V) fluoride Molybdenum(IV) bromide Molybdenum(III) iodide
Other cations	Chromium(IV) chloride Tungsten(V) chloride
Related molybdenum chlorides	Molybdenum(II) chloride Molybdenum(III) chloride Molybdenum(IV) chloride
Y chloride (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Molybdenum(V) chloride is the inorganic compound with the formula [MoCl₅]₂. 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 pentachlorides of W, Nb and Ta. 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.

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.^[1]

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 versatile reagent in the chemistry of molybdenum.^[2] MoCl₅ is reduced by HBr to form MoBr₄. The reaction is conducted in ethylbromide at -50 C:

2 MoCl₅ + 10 HBr → 2 MoBr₄ + 10 HCl + Br₂

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

MoCl₅ is a good Lewis acid toward non-oxidizable ligands. It forms an adduct with chloride to form [MoCl₆]^-. In organic synthesis, the compound finds occasional use in chlorinations and deoxygenation reactions.^[4]

Safety considerations

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

References

1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. ^ 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.
3. ^ Calderazzo, F.; Maichle-Mössmer, C.; Pampaloni, G.; 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.doi:10.1039/DT9930000655
4. ^ T. Kauffmann, S. Torii, T. Inokuchi “Molybdenum(V) Chloride” in Encyclopedia of Reagents for Organic Synthesis 2004, J. Wiley & Sons, New York. doi:10.1002/047084289.