Molybdenum hexacarbonyl (also called molybdenum carbonyl) is the chemical compound with the formula Mo(CO)6. This colorless solid, like its chromium, tungsten, and seaborgium analogues, is noteworthy as a volatile, air-stable derivative of a metal in its zero oxidation state.
Structure and properties 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 have a C–H bond. Like many metal carbonyls, Mo(CO)6 is generally prepared by reductive carbonylation, which involves reduction of a metal halide with under an atmosphere of carbon monoxide. As described in a 2023 survey of methods "most cost-effective routes for the synthesis of group 6 hexacarbonyls are based on the reduction of the metal chlorides (CrCl3, MoCl5 or WCl6) with magnesium, zinc or aluminium powders... under CO pressures".
Occurrence Mo(CO)6 has been detected in landfills and sewage plants, the reducing, anaerobic environment being conducive to formation of Mo(CO)6.
Inorganic and organometallic derivatives Molybdenum hexacarbonyl is a popular reagent in research. One or more CO ligands can be displaced by other ligands. Examples:
Mo(CO)6 + bipy → Mo(CO)4(bipy) + 2 CO (bipy = 2,2′-bipyridine) Mo(CO)6 + thf → Mo(CO)5(thf) + CO (thf = tetrahydrofuran), this reaction requires UV radiation Mo(CO)6 + 2 pip → Mo(CO)4(pip)2 + 2 CO (pip = piperidine) Mo(CO)6 + 3 MeCN → Mo(CO)3(MeCN)3 + 3 CO (MeCN = acetonitrile) Some of these species are suited for substitution. For instance, the reaction of [Mo(CO)4(piperidine)2] with triphenyl phosphine gives cis- and trans-[Mo(CO)4(PPh3)2]. [Mo(CO)3(MeCN)3 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 and sodium cyclopentadienide gives [MoTp(CO)3]− and [MoCp(CO)3]− anions, respectively. These anions react with a variety of electrophiles. A related source of Mo(CO)3 is cycloheptatrienemolybdenum tricarbonyl.
Organic synthesis Mo(CO)6, [Mo(CO)3(MeCN)3], and related derivatives are employed as catalysts or reagents in organic synthesis for example, alkyne metathesis and the Pauson–Khand reaction. The hexacarbonyl can serve as a source of CO.
Source of Mo atoms Molybdenum hexacarbonyl is widely used in electron beam-induced deposition technique - it is easily vaporized and decomposed by the electron beam providing a convenient source of molybdenum atoms.
Safety and handling Like all metal carbonyls, Mo(CO)6 is a dangerous source of volatile metal as well as CO.
References
Further reading Marradi, M. (2005). "Synlett Spotlight 119: Molybdenum Hexacarbonyl [Mo(CO)6]" (PDF). Synlett. 2005 (7): 1195–1196. doi:10.1055/s-2005-865206. Feldmann, J.; Cullen, W. R. (1997). "Occurrence of Volatile Transition Metal Compounds in Landfill Gas: Synthesis of Molybdenum and Tungsten Carbonyls in the Environment". Environ. Sci. Technol. 31 (7): 2125–2129. Bibcode:1997EnST...31.2125F. doi:10.1021/es960952y. Feldmann, J.; Grümping, R.; Hirner, A. V. (1994). "Determination of Volatile Metal and Metalloid Compounds in Gases from Domestic Waste Deposits with GC/ICP-MS". Fresenius' J. Anal. Chem. 350 (4–5): 228–234. doi:10.1007/BF00322474. S2CID 95405500.
