Molybdenum hexacarbonyl (also called molybdenum carbonyl) is the
chemical compound with the formula Mo(CO)
6. This colorless solid, like its
chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hardne ...
and
tungsten 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
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
" 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.
Mo(CO)
6 is prepared by the
reduction of
molybdenum
Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...
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
Mo(CO)
6 has been detected in
landfill
A landfill site, also known as a tip, dump, rubbish dump, garbage dump, or dumping ground, is a site for the disposal of waste materials. Landfill is the oldest and most common form of waste disposal, although the systematic burial of the waste ...
s and
sewage
Sewage (or domestic sewage, domestic wastewater, municipal wastewater) is a type of wastewater that is produced by a community of people. It is typically transported through a sewer system. Sewage consists of wastewater discharged from residenc ...
plants, the reducing, anaerobic environment being conducive to formation of Mo(CO)
6.
Inorganic and organometallic research
Molybdenum hexacarbonyl is a popular reagent in academic research.
One or more CO ligands can be displaced by other ligands. Mo(CO)
6,
o(CO)3(MeCN)3 and related derivatives are employed as
catalysts in
organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
for example,
alkyne metathesis and the
Pauson–Khand reaction.
Mo(CO)
6 reacts with
2,2′-bipyridine
2,2′-Bipyridine (bipy or bpy, pronounced ) is an organic compound with the formula C10H8N2. This colorless solid is an important isomer of the bipyridine family. It is a bidentate chelating ligand, forming complexes with many transition metals ...
to afford Mo(CO)
4(bipy). UV-photolysis of a
THF
Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water-miscible organic liquid with low viscosity. It is ma ...
solution of Mo(CO)
6 gives Mo(CO)
5(THF).
o(CO)4(piperidine)2/h2>
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
o(CO)4(piperidine)2with
triphenyl phosphine
Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to P Ph3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists ...
in
boiling
Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Th ...
dichloromethane
Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible with ...
(b.p. ca. 40 °C) gives
''cis''-
o(CO)4(PPh3)2 This ''cis-'' complex
isomer
In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers.
Iso ...
izes in
toluene to
''trans''-
o(CO)4(PPh3)2
o(CO)3(MeCN)3/h2>
Mo(CO)
6 also can be converted to its tris(acetonitrile) derivative. The compound serves as a source of "Mo(CO)
3". For instance treatment with allyl chloride gives
oCl(allyl)(CO)2(MeCN)2 whereas treatment with
KTp and
sodium cyclopentadienide
Sodium cyclopentadienide is an organosodium compound with the formula C5H5Na. The compound is often abbreviated as NaCp, where Cp− is the cyclopentadienide anion. Sodium cyclopentadienide is a colorless solid, although samples often are pi ...
gives
oTp(CO)3sup>− and
oCp(CO)3sup>− anions, respectively. These anions react with a variety of electrophiles.
A related source of Mo(CO)
3 is
cycloheptatrienemolybdenum tricarbonyl
Cycloheptatrienemolybdenum tricarbonyl is the organomolybdenum compound with the formula (C7H8)Mo(CO)3. It is a red-orange solid that is soluble in nonpolar organic solvents. The compound has no practical value but is a prototypical complex of c ...
.
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
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{{Molybdenum compounds
Molybdenum(0) compounds
Carbonyl complexes
Octahedral compounds
Organomolybdenum compounds