Molybdenum hexacarbonyl (also called molybdenum carbonyl) is the
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
with the formula Mo(CO)
6. This colorless solid, like its
chromium
Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal.
Chromium ...
,
tungsten
Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...
, and
seaborgium
Seaborgium is a synthetic chemical element; it has symbol Sg and atomic number 106. It is named after the American nuclear chemist Glenn T. Seaborg. As a synthetic element, it can be created in a laboratory but is not found in nature. It is als ...
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
In chemistry, octahedral molecular geometry, also called square bipyramidal, describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The oc ...
consisting of six rod-like CO
ligand
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
s 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.
Like many
metal carbonyl
A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. These properties are all associated with having electrons available at the Fermi level, as against n ...
s, Mo(CO)
6 is generally prepared by "reductive carbonylation", which involves reduction of a metal halide with under an atmosphere of
carbon monoxide
Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
. 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 (CrCl
3, MoCl
5 or WCl
6) with magnesium, zinc or aluminium powders... under CO pressures".
Occurrence
Mo(CO)
6 has been detected in
landfill
A landfill is a site for the disposal of waste materials. It is the oldest and most common form of waste disposal, although the systematic burial of waste with daily, intermediate and final covers only began in the 1940s. In the past, waste was ...
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 sewerage, sewer system. Sewage consists of wastewater discharged fro ...
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
catalyst
Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
s in
organic synthesis
Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...
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 . This colorless solid is an important isomer of the bipyridine family. It is a bidentate chelation, chelating ligand, forming complexes with many transition meta ...
to afford Mo(CO)
4(bipy). UV-photolysis of a
THF 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
Piperidine is an organic compound with the molecular formula (CH2)5NH. This heterocyclic amine consists of a six-membered ring containing five methylene bridges (–CH2–) and one amine bridge (–NH–). It is a colorless liquid with an odor de ...
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 in
boiling
Boiling or ebullition is the rapid phase transition from liquid to gas or vapor, vapour; the reverse of boiling is condensation. Boiling occurs when a liquid is heated to its boiling point, so that the vapour pressure of the liquid is equal to ...
dichloromethane
Dichloromethane (DCM, methylene chloride, or methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odor 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 formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...
izes in
toluene
Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon with the chemical formula , often abbreviated as , where Ph stands for the phenyl group. It is a colorless, water
Water is an inorganic compound with the c ...
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 pin ...
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.
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
Organomolybdenum compounds
Octahedral compounds