Molybdenum hexacarbonyl

Molybdenum hexacarbonyl (also called molybdenum carbonyl) is the chemical compound with the formula Mo(CO)6. 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.

Preparation, properties, and structure
Mo(CO)6 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.

Mo(CO)6 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 a C-H bond. By this strict definition, Mo(CO)6 is not organometallic.

Applications in inorganic and organometallic synthesis
Mo(CO)6 is a popular reagent in organometallic synthesis because one or more CO ligands can be displaced by other donor ligands. For example, Mo(CO)6 reacts with 2,2'-bipyridine to afford Mo(CO)4(bipy). UV-photolysis of a THF solution of Mo(CO)6 gives Mo(CO)5(THF). Many metal carbonyls are similarly photo-activatable.

[Mo(CO)4(piperidine)2]
The thermal reaction of Mo(CO)6 with piperidine affords Mo(CO)4(piperidine)2. 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 [Mo(CO)4(piperidine)2] with triphenyl phosphine in boiling dichloromethane (b.p. ca. 40 °C) gives cis-[Mo(CO)4(PPh3)2], this cis complex isomerizes in toluene to trans-[Mo(CO)4(PPh3)2].

[Mo(CO)3(MeCN)3]
Upon heating in a solution of acetonitrile, Mo(CO)6 converts to its tris(acetonitrile) derivative. The resulting compound serves as a source of "Mo(CO)3". For instance treatment with allyl chloride gives [MoCl(allyl)(CO)2(MeCN)2], whereas treatment with KTp andNacyclopentadienyl gives [MoTp(CO)3]- and [MoCp(CO)3]- anions. These anions can be reacted with electrophiles to form a wide range of products.

Applications in organic synthesis
Mo(CO)6, [Mo(CO)3(MeCN)3], and related derivatives are employed as catalysts in organic synthesis. For example, these catalysts can be used for alkyne metathesis and the Pauson–Khand reaction.

Occurrence in nature
Mo(CO)6 has been detected in landfills and sewage plants, the reducing, anaerobic environment being conducive to formation of Mo(CO)6.

Safety and handling
Like all metal carbonyls, Mo(CO)6 is dangerous source of volatile metal as well as CO. It diffuses readily into plastic stoppers.

Other reading

 * Marradi, "Synlett spotlight 119", SYNLETT 2005, No. 7, pp 1195-1196
 * J. Feldmann, W.R. Cullen, Occurrence of volatile transition metal compounds in landfill gas: synthesis of molybdenum and tungsten carbonyls in the environment, Environ. Sci. Technol. 1997, 31, 2125-2129.
 * J. Feldmann, Determination of Ni(CO)4, Fe(CO)5, Mo(CO)6 and W(CO)6­ in sewage gas by using cryotrapping gas chromatography inductively coupled plasma mass spectrometry, J. Environm. Mon., 1999, 1, 33-37.
 * J. Feldmann, R. Grümping, A.V. Hirner, Determination of volatile metal and metalloid compounds in gases from domestic waste deposits with GC-ICP-MS, Fresenius J. Anal. Chem., 1994, 350, 228-235.