Molybdenum tetrachloride is the inorganic compound with the empirical formula MoCl<sub>4</sub>. The material exists as two polymorphs, both being dark-colored paramagnetic solids. These compounds are mainly of interest as precursors to other molybdenum complexes.
The ñ polymorph is a polymer. The ò polymorph is a hexamer. In both polymorph, the Mo center is octahedral with two terminal chloride ligands and four doubly bridging ligands. In addition to these two binary phases, a number of adducts are known with the formula where L is a Lewis base.
ñ-Molybdenum tetrachloride can be prepared from by dechlorination of molybdenum pentachloride using tetrachloroethene:
Heating ñ-molybdenum tetrachloride in a sealed container in the presence of molybdenum pentachloride induces conversion to the ò polymorph.
When heated in an open container, molybdenum tetrachloride evolves chlorine, giving molybdenum trichloride;
The acetonitrile complex adduct can be prepared by reduction of the pentachloride with acetonitrile:
The MeCN ligands can be exchanged with other ligands:
The pentachloride can be reduced to the ether complex MoCl<sub>4</sub>(Et<sub>2</sub>O)<sub>2</sub> using tin powder. It is a beige, paramagnetic solid.