Molybdenum(V) chloride

Molybdenum(V) chloride

Names
IUPAC names Molybdenum(V) chloride Molybdenum pentachloride
Identifiers
CAS Number	10241-05-1 MoCl5 26814-39-1 Mo2Cl10
EC Number	233-575-3
PubChem	61497
RTECS number	QA4690000
Properties
Chemical formula	Cl10Mo2
Molar mass	273.21 g/mol
Appearance	dark-green solid hygroscopic paramagnetic
Density	2.928 g/cm^3
Melting point	194 °C (381 °F; 467 K)
Boiling point	268 °C (514 °F; 541 K)
Solubility in water	hydrolyzes
Solubility	soluble in dry ether, dry alcohol, organic solvents
Structure
Crystal structure	monoclinic
Coordination geometry	edge-shared bioctahedron
Hazards
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
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Molybdenum(V) chloride is the inorganic compound with the formula [MoCl₅]₂. This dark volatile solid is used in research to prepare other molybdenum compounds. it is moisture-sensitive and soluble in chlorinated solvents. Usually called molybdenum pentachloride, it is in fact a dimer with the formula Mo₂Cl₁₀.^[1]

Structure

Each molybdenum has local octahedral symmetry and two chlorides bridge between the molybdenum centers.^[2] A similar structure is also found for the pentachlorides of W, Nb and Ta.^[3] In the gas phase and partly in solution, the dimers partially dissociates to give a monomeric pentahalide.^[4] The monomer is paramagnetic, with one unpaired electron per Mo center, reflecting the fact that the formal oxidation state is +5, leaving one valence electron on the metal center.

Preparation and properties

MoCl₅ is prepared by chlorination of Mo metal but also chlorination of MoO₃. The unstable hexachloride MoCl₆ is not produced in this way.^[5]

MoCl₅ is a strong oxidant. It is reduced by MeCN to afford an orange complex, MoCl₄(MeCN)₂, which in turn reacts with THF to give MoCl₄(THF)₂, a precursor to other molybdenum-containing complexes.^[6] MoCl₅ is reduced by HBr to form MoBr₄. The bromination reaction is conducted in ethylbromide at −50 °C, forming MoBr₅ as an intermediate. Upon warming to 20 °C, Br₂ is produced and the formal oxidation state of molybdenum changes from +5 to +4. The net transformation is shown in the equation:

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

MoBr₄ reacts with THF to give the Mo(III) species mer-MoBr₃(THF)₃.^[7]

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, deoxygenation, and oxidative coupling reactions.^[8]

Safety considerations

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

References

1. Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego, CA: Academic Press. ISBN 0-12-352651-5.
2. Beck, J.; Wolf, F. (1997). "Three New Polymorphic Forms of Molybdenum Pentachloride". Acta Crystallogr. B53: 895–903. doi:10.1107/S0108768197008331.
3. Wells, A. E. (1984). Structural Inorganic Chemistry (5th ed.). Oxford: Clarendon Press.
4. Brunvoll, J.; Ischenko, A. A.; Spiridonov, V. P.; Strand, T. G. (1984). Acta Chem. Scand. A38: 115–120. Missing or empty |title= (help)
5. Tamadon, Farhad; Seppelt, Konrad (2013). "The Elusive Halides VCl₅, MoCl₆, and ReCl₆". Angew. Chem. Int. Ed. 52: 767–769. doi:10.1002/anie.201207552.
6. Dilworth, J. R.; Richards, R. L. (1990). "The Synthesis of Molybdenum and Tungsten Dinitrogen Compounds". Inorg. Synth. 28: 33–45. ISBN 0-471-52619-3.
7. Calderazzo, F.; Maichle-Mössmer, C.; Pampaloni, G.; Strähle, J. (1993). "Low-temperature Syntheses of Vanadium(III) and Molybdenum(IV) Bromides by Halide Exchange". Dalton Trans. 1993: 655–658. doi:10.1039/DT9930000655.
8. Kauffmann, T.; Torii, S.; Inokuchi, T. (2004). "Molybdenum(V) Chloride". Encyclopedia of Reagents for Organic Synthesis. New York, NY: J. Wiley & Sons. doi:10.1002/047084289.