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.

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.