Molybdenum hexacarbonyl


Molybdenum hexacarbonyl is the chemical compound with the formula Mo6. This colorless solid, like its chromium and tungsten analogues, is noteworthy as a volatile, air-stable derivative of a metal in its zero oxidation state.

Structure and properties

Mo6 adopts an octahedral geometry consisting of six rod-like CO ligands radiating from the central Mo atom. A recurring minor debate in some chemical circles concerns the definition of an "organometallic" compound. Usually, organometallic indicates the presence of a metal directly bonded via a M–C bond to an organic fragment, which must in turn have a C–H bond. By this strict definition, Mo6 is not organometallic.

Preparation

Mo6 is prepared by the reduction of molybdenum chlorides or oxides under a pressure of carbon monoxide, although it would be unusual to prepare this inexpensive compound in the laboratory. The compound is somewhat air-stable and sparingly soluble in nonpolar organic solvents.

Occurrence

Mo6 has been detected in landfills and sewage plants, the reducing, anaerobic environment being conducive to formation of Mo6.

Intermediate in inorganic and organometallic synthesis

Mo6 is a popular reagent in organometallic synthesis because one or more CO ligands can be displaced by other donor ligands. Mo6, , and related derivatives are employed as catalysts in organic synthesis for example, alkyne metathesis and the Pauson–Khand reaction.
Mo6 reacts with 2,2′-bipyridine to afford Mo4. UV-photolysis of a THF solution of Mo6 gives Mo5.

Mo(CO)4(piperidine)2

The thermal reaction of Mo6 with piperidine affords Mo42. The two piperidine ligands in this yellow-colored compound are labile, which allows other ligands to be introduced under mild conditions. For instance, the reaction of with triphenyl phosphine in boiling dichloromethane gives cis-. This cis- complex isomerizes in toluene to trans-.

Mo(CO)3(MeCN)3

Mo6 also can be converted to its tris derivative. The compound serves as a source of "Mo3". For instance treatment with allyl chloride gives , whereas treatment with KTp and sodium cyclopentadienide gives − and − anions, respectively. These anions react with a variety of electrophiles. A related source of Mo3 is cycloheptatrienemolybdenum tricarbonyl.

Safety and handling

Like all metal carbonyls, Mo6 is dangerous source of volatile metal as well as CO.