Molybdenum hexacarbonyl
Encyclopedia
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.
s 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, Mo(CO)6 is not organometallic.
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.
s and sewage
plants, the reducing, anaerobic environment being conducive to formation of Mo(CO)6.
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.
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 isomer
izes in toluene
to trans-[Mo(CO)4(PPh3)2].
, Mo(CO)6 converts to its tris(acetonitrile) derivative. The resulting air-sensitive 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
and sodium cyclopentadienide
gives [MoTp(CO)3]- and [MoCp(CO)3]- anions respectively. These anions can be reacted with electrophiles to form a wide range of products.
. For example, these catalysts can be used for alkyne metathesis
and the Pauson–Khand reaction
.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
with the formula Mo(CO)6. This colorless solid, like its chromium and tungsten
Tungsten hexacarbonyl
Tungsten hexacarbonyl is the chemical compound with the formula W6. This complex gave rise to the first example of a dihydrogen complex....
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 ligandLigand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s 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, Mo(CO)6 is not organometallic.
Preparation
Mo(CO)6 is prepared by the reductionRedox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
of molybdenum chlorides or oxides under a pressure of carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
, 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 landfillLandfill
A landfill site , is a site for the disposal of waste materials by burial and is the oldest form of waste treatment...
s and sewage
Sewage
Sewage is water-carried waste, in solution or suspension, that is intended to be removed from a community. Also known as wastewater, it is more than 99% water and is characterized by volume or rate of flow, physical condition, chemical constituents and the bacteriological organisms that it contains...
plants, the reducing, anaerobic environment being conducive to formation of Mo(CO)6.
Applications in inorganic and organometallic synthesis
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. Mo(CO)6 is also 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'-bipyridine2,2'-Bipyridine
2,2'-Bipyridine is a organic compound with the formula . This colorless solid, commonly abbreviated bipy or bpy , 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
ThF
Follicular B helper T cells , are antigen-experienced CD4+ T cells found in the B cell follicles of secondary lymphoid organs such as lymph nodes, spleens and Peyer's patches, and are identified by their constitutive expression of the B cell follicle homing receptor CXCR5...
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 piperidinePiperidine
Piperidine is an organic compound with the molecular formula 5NH. This heterocyclic amine consists of a six-membered ring containing five methylene units and one nitrogen atom...
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
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 vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure. While below the boiling point a liquid...
dichloromethane
Dichloromethane
Dichloromethane is an organic compound with the formula CH2Cl2. This colorless, volatile liquid with a moderately sweet aroma is widely used as a solvent. Although it is not miscible with water, it is miscible with many organic solvents...
(b.p. ca. 40 °C) gives cis-[Mo(CO)4(PPh3)2]. This cis- complex isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...
izes in toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...
to trans-[Mo(CO)4(PPh3)2].
[Mo(CO)3(MeCN)3]
Upon refluxing in a solution of acetonitrileAcetonitrile
Acetonitrile is the chemical compound with formula . This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture...
, Mo(CO)6 converts to its tris(acetonitrile) derivative. The resulting air-sensitive 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
Scorpionate ligand
The term scorpionate ligand refers to a tridentate ligand which would bind to a metal in a fac manner. The most popular class of scorpionates are the hydrotrisborates or Tp ligands. These were also the first to become popular. These ligands first appeared in journals in 1966 from the then...
and sodium cyclopentadienide
Sodium cyclopentadienide
Sodium cyclopentadienide is an organosodium compound with the formula C5H5Na. The compound is often abbreviated as NaCp or CpNa, where Cp− is the cyclopentadienide anion. Cp is also used as an abbreviation for the cyclopentadienyl ligand in coordination chemistry.-Preparation:Sodium...
gives [MoTp(CO)3]- and [MoCp(CO)3]- anions respectively. 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 synthesisOrganic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
. For example, these catalysts can be used for alkyne metathesis
Alkyne metathesis
Alkyne metathesis is an organic reaction involving the redistribution of alkyne chemical bonds. This reaction is closely related to olefin metathesis. Metal-catalyzed alkyne metathesis was first described in 1968 by Bailey, et al. The Bailey system utilized a mixture of tungsten and silicon oxides...
and the Pauson–Khand reaction
Pauson–Khand reaction
The Pauson–Khand reaction is a chemical reaction described as a [2+2+1] cycloaddition between an alkyne, an alkene and carbon monoxide to form a α,β-cyclopentenone...
.
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.Further 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, 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.