Molybdenum(V) chloride


Molybdenum chloride is the inorganic compound with the formula 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.

Structure

Each molybdenum has local octahedral symmetry and two chlorides bridge between the molybdenum centers. A similar structure is also found for the pentachlorides of W, Nb and Ta. In the gas phase and partly in solution, the dimers partially dissociates to give a monomeric pentahalide. 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.
MoCl5 is reduced by acetonitrile to afford an orange complex, MoCl42, which in turn reacts with THF to give MoCl42, a precursor to other molybdenum-containing complexes.
Molybdenum bromide is prepared by treatment of molybdenum chloride with hydrogen bromide:
The reaction proceeds via the unstable molybdenum bromide, which releases bromine at room temperature.
MoCl5 is a good Lewis acid toward non-oxidizable ligands. It forms an adduct with chloride to form −. In organic synthesis, the compound finds occasional use in chlorinations, deoxygenation, and oxidative coupling reactions.

Molybdenum hexachloride

is known, but it cannot be prepared by addition of chlorine to MoCl5.

Safety considerations

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