
Metal carbonyls are
coordination complexes of
transition metals with
carbon monoxide ligands. Metal carbonyls are useful 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 ...
and as catalysts or catalyst precursors in
homogeneous catalysis, such as
hydroformylation and
Reppe chemistry. In the
Mond process,
nickel tetracarbonyl
Nickel carbonyl (IUPAC name: tetracarbonylnickel) is a nickel(0) organometallic compound with the formula Ni(CO)4. This colorless liquid is the principal carbonyl of nickel. It is an intermediate in the Mond process for producing very high-pur ...
is used to produce pure
nickel. In
organometallic chemistry
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 ...
, metal carbonyls serve as precursors for the preparation of other organometallic complexes.
Metal carbonyls are toxic by skin contact, inhalation or ingestion, in part because of their ability to carbonylate
hemoglobin to give
carboxyhemoglobin, which prevents the binding of
oxygen.
[
]
Nomenclature and terminology
The nomenclature of the metal carbonyls depends on the charge of the complex, the number and type of central atoms, and the number and type of ligands and their binding modes. They occur as neutral complexes, as positively-charged metal carbonyl cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s or as negatively charged metal carbonylates. The carbon monoxide ligand may be bound terminally to a single metal atom or bridging to two or more metal atoms. These complexes may be homoleptic, containing only CO ligands, such as nickel tetracarbonyl
Nickel carbonyl (IUPAC name: tetracarbonylnickel) is a nickel(0) organometallic compound with the formula Ni(CO)4. This colorless liquid is the principal carbonyl of nickel. It is an intermediate in the Mond process for producing very high-pur ...
(Ni(CO)4), but more commonly metal carbonyls are heteroleptic and contain a mixture of ligands.
Mononuclear metal carbonyls contain only one metal atom as the central atom. Except vanadium hexacarbonyl
Vanadium hexacarbonyl is the inorganic compound with the formula V(CO)6. It is a blue-black volatile solid. This highly reactive species is noteworthy from theoretical perspectives as a rare isolable homoleptic metal carbonyl that is paramagneti ...
, only metals with even atomic number, such as 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 ...
, iron, nickel, and their homologs, build neutral mononuclear complexes. Polynuclear metal carbonyls are formed from metals with odd atomic numbers and contain a metal–metal bond
In inorganic chemistry, metal–metal bonds describe attractive interactions between metal centers. The simplest examples are found in bimetallic complexes. Metal–metal bonds can be "supported", i.e. be accompanied by one or more bridging ligan ...
. Complexes with different metals but only one type of ligand are called isoleptic.
Carbon monoxide has distinct binding modes in metal carbonyls. They differ in terms of their hapticity, denoted ''η'', and their bridging mode. In ''η''2-CO complexes, both the carbon and oxygen are bonded to the metal. More commonly only carbon is bonded, in which case the hapticity is not mentioned.
The carbonyl ligand engages in a wide range of bonding modes in metal carbonyl dimers and clusters. In the most common bridging mode, denoted ''μ''2 or simply ''μ'', the CO ligand bridges a pair of metals. This bonding mode is observed in the commonly available metal carbonyls: Co2(CO)8, Fe2(CO)9, Fe3(CO)12, and Co4(CO)12.[ In certain higher nuclearity clusters, CO bridges between three or even four metals. These ligands are denoted ''μ''3-CO and ''μ''4-CO. Less common are bonding modes in which both C and O bond to the metal, such as ''μ''3''η''2.
:
]
Structure and bonding
Carbon monoxide bonds to transition metals using "synergistic pi* back-bonding". The M-C bonding has three components, giving rise to a partial triple bond. A sigma (σ) bond arises from overlap of the nonbonding (or weakly anti-bonding) sp-hybridized electron pair on carbon with a blend of d-, s-, and p-orbitals on the metal. A pair of pi (π) bonds arises from overlap of filled d-orbitals on the metal with a pair of π*-antibonding
In chemical bonding theory, an antibonding orbital is a type of molecular orbital that weakens the chemical bond between two atoms and helps to raise the energy of the molecule relative to the separated atoms. Such an orbital has one or more no ...
orbitals projecting from the carbon atom of the CO. The latter kind of binding requires that the metal have d-electrons, and that the metal is in a relatively low oxidation state (0 or +1) which makes the back-donation of electron density favorable. As electrons from the metal fill the π-antibonding orbital of CO, they weaken the carbon–oxygen bond compared with free carbon monoxide, while the metal–carbon bond is strengthened. Because of the multiple bond character of the M–CO linkage, the distance between the metal and carbon atom is relatively short, often less than 1.8 Å, about 0.2 Å shorter than a metal– alkyl bond. The M-CO and MC-O distance are sensitive to other ligands on the metal. Illustrative of these effects are the following data for Mo-C and C-O distances in Mo(CO)6 and Mo(CO)3(4-methylpyridine)3: 2.06 vs 1.90 and 1.11 vs 1.18 Å.
Infrared spectroscopy is a sensitive probe for the presence of bridging carbonyl ligands. For compounds with doubly bridging CO ligands, denoted ''μ''2-CO or often just ''μ''-CO, the bond stretching frequency ''ν''CO is usually shifted by 100–200 cm−1 to lower energy compared to the signatures of terminal CO, which are in the region 1800 cm−1. Bands for face capping (''μ''3) CO ligands appear at even lower energies. In addition to symmetrical bridging modes, CO can be found to bridge asymmetrically or through donation from a metal d orbital to the π* orbital of CO. The increased π-bonding due to back-donation from multiple metal centers results in further weakening of the C–O bond.
Physical characteristics
Most mononuclear carbonyl complexes are colorless or pale yellow volatile liquids or solids that are flammable and toxic. Vanadium hexacarbonyl
Vanadium hexacarbonyl is the inorganic compound with the formula V(CO)6. It is a blue-black volatile solid. This highly reactive species is noteworthy from theoretical perspectives as a rare isolable homoleptic metal carbonyl that is paramagneti ...
, a uniquely stable 17-electron metal carbonyl, is a blue-black solid.[ Dimetallic and polymetallic carbonyls tend to be more deeply colored. Triiron dodecacarbonyl (Fe3(CO)12) forms deep green crystals. The crystalline metal carbonyls often are sublimable in vacuum, although this process is often accompanied by degradation. Metal carbonyls are soluble in nonpolar and polar organic solvents such as benzene, diethyl ether, acetone, glacial acetic acid, and ]carbon tetrachloride
Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also IUPAC nomenclature of inorganic chemistry, recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVAC ...
. Some salts of cationic and anionic metal carbonyls are soluble in water or lower alcohols.
Analytical characterization
Apart from X-ray crystallography, important analytical techniques for the characterization of metal carbonyls are infrared spectroscopy and 13C NMR spectroscopy. These two techniques provide structural information on two very different time scales. Infrared-active vibrational modes, such as CO-stretching vibrations, are often fast compared to intramolecular processes, whereas NMR transitions occur at lower frequencies and thus sample structures on a time scale that, it turns out, is comparable to the rate of intramolecular ligand exchange processes. NMR data provide information on "time-averaged structures", whereas IR is an instant "snapshot". Illustrative of the differing time scales, investigation of dicobalt octacarbonyl (Co2(CO)8) by means of infrared spectroscopy provides 13 ''ν''CO bands, far more than expected for a single compound. This complexity reflects the presence of isomers with and without bridging CO ligands. The 13C NMR spectrum of the same substance exhibits only a single signal at a chemical shift of 204 ppm. This simplicity indicates that the isomers quickly (on the NMR timescale) interconvert.
Iron pentacarbonyl exhibits only a single 13C NMR signal owing to rapid exchange of the axial and equatorial CO ligands by Berry pseudorotation.
Infrared spectra
An important technique for characterizing metal carbonyls is infrared spectroscopy. The C–O vibration, typically denoted ''ν''CO, occurs at 2143 cm−1 for carbon monoxide gas. The energies of the ''ν''CO band for the metal carbonyls correlates with the strength of the carbon–oxygen bond, and inversely correlated with the strength of the π-backbonding
In chemistry, π backbonding, also called π backdonation, is when electrons move from an atomic orbital on one atom to an appropriate symmetry antibonding orbital on a ''π-acceptor ligand''. It is especially common in the organometallic chemi ...
between the metal and the carbon. The π-basicity of the metal center depends on a lot of factors; in the isoelectronic series ( titanium to iron) at the bottom of this section, the hexacarbonyls show decreasing π-backbonding as one increases (makes more positive) the charge on the metal. π-Basic ligands increase π-electron density at the metal, and improved backbonding reduces νCO. The Tolman electronic parameter uses the Ni(CO)3 fragment to order ligands by their π-donating abilities.
The number of vibrational modes of a metal carbonyl complex can be determined by group theory. Only vibrational modes that transform as the electric dipole operator will have nonzero direct product
In mathematics, one can often define a direct product of objects already known, giving a new one. This generalizes the Cartesian product of the underlying sets, together with a suitably defined structure on the product set. More abstractly, one ta ...
s and are observed. The number of observable IR transitions (but not their energies) can thus be predicted. For example, the CO ligands of octahedral complexes, such as Cr(CO)6, transform as ''a''1g, ''e''g, and ''t''1u, but only the ''t''1u mode (antisymmetric stretch of the apical carbonyl ligands) is IR-allowed. Thus, only a single ''ν''CO band is observed in the IR spectra of the octahedral metal hexacarbonyls. Spectra for complexes of lower symmetry are more complex. For example, the IR spectrum of Fe2(CO)9 displays CO bands at 2082, 2019 and 1829 cm−1. The number of IR-observable vibrational modes for some metal carbonyls are shown in the table. Exhaustive tabulations are available.[ These rules apply to metal carbonyls in solution or the gas phase. Low-]polarity
Polarity may refer to:
Science
*Electrical polarity, direction of electrical current
*Polarity (mutual inductance), the relationship between components such as transformer windings
* Polarity (projective geometry), in mathematics, a duality of ord ...
solvents are ideal for high resolution. For measurements on solid samples of metal carbonyls, the number of bands can increase owing in part to site symmetry.
Nuclear magnetic resonance spectroscopy
Metal carbonyls are often characterized by 13C NMR spectroscopy. To improve the sensitivity of this technique, complexes are often enriched with 13CO. Typical chemical shift range for terminally bound ligands is 150 to 220 ppm. Bridging ligands resonate between 230 and 280 ppm. The 13C signals shift toward higher fields with an increasing atomic number of the central metal.
NMR spectroscopy can be used for experimental determination of the fluxionality
In chemistry and molecular physics, fluxional (or non-rigid) molecules are molecules that undergo dynamics such that some or all of their atoms interchange between symmetry-equivalent positions. Because virtually all molecules are fluxional in so ...
.
The activation energy of ligand exchange processes can be determined by the temperature dependence of the line broadening.
Mass spectrometry
Mass spectrometry
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...
provides information about the structure and composition of the complexes. Spectra for metal polycarbonyls are often easily interpretable, because the dominant fragmentation process is the loss of carbonyl ligands ( ''m''/''z'' = 28).
: → + CO
Electron ionization
Electron ionization (EI, formerly known as electron impact ionization and electron bombardment ionization) is an ionization method in which energetic electrons interact with solid or gas phase atoms or molecules to produce ions. EI was one of th ...
is the most common technique for characterizing the neutral metal carbonyls. Neutral metal carbonyls can be converted to charged species by derivatization
Derivatization is a technique used in chemistry which converts a chemical compound into a product (the reaction's derivate) of similar chemical structure, called a derivative.
Generally, a specific functional group of the compound participates ...
, which enables the use of electrospray ionization (ESI), instrumentation for which is often widely available. For example, treatment of a metal carbonyl with alkoxide
In chemistry, an alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They are written as , where R is the organic substituent. Alkoxides are strong bases and, whe ...
generates an anionic metallaformate that is amenable to analysis by ESI-MS:
:L''n''M(CO) + RO− → ''n''M−C(=O)OR">''n''M−C(=O)ORsup>−
Some metal carbonyls react with azide
In chemistry, azide is a linear, polyatomic anion with the formula and structure . It is the conjugate base of hydrazoic acid . Organic azides are organic compounds with the formula , containing the azide functional group. The dominant applic ...
to give isocyanato complexes with release of nitrogen. By adjusting the cone voltage or temperature, the degree of fragmentation can be controlled. The molar mass of the parent complex can be determined, as well as information about structural rearrangements involving loss of carbonyl ligands under ESI-MS conditions.
Mass spectrometry
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...
combined with infrared photodissociation spectroscopy can provide vibrational informations for ionic carbonyl complexes in gas phase.
Occurrence in nature
In the investigation of the infrared spectrum of the Galactic Center of the Milky Way, monoxide vibrations of iron carbonyls in interstellar dust clouds were detected. Iron carbonyl clusters were also observed in Jiange H5 chondrites identified by infrared spectroscopy. Four infrared stretching frequencies were found for the terminal and bridging carbon monoxide ligands.
In the oxygen-rich atmosphere of the Earth, metal carbonyls are subject to oxidation to the metal oxides. It is discussed whether in the reducing hydrothermal environments of the prebiotic prehistory such complexes were formed and could have been available as catalysts for the synthesis of critical biochemical compounds
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology an ...
such as pyruvic acid
Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell.
Pyruvic aci ...
. Traces of the carbonyls of iron, nickel, and tungsten were found in the gaseous emanations from the sewage sludge of municipal treatment plants.
The hydrogenase enzymes contain CO bound to iron. It is thought that the CO stabilizes low oxidation states, which facilitates the binding of hydrogen. The enzymes carbon monoxide dehydrogenase
In enzymology, carbon monoxide dehydrogenase (CODH) () is an enzyme that catalyzes the chemical reaction
:CO + H2O + A \rightleftharpoons CO2 + AH2
The chemical process catalyzed by carbon monoxide dehydrogenase is similar to the water-gas shif ...
and acetyl-CoA synthase
Acetyl-CoA synthase (ACS), not to be confused with Acetyl-CoA synthetase or Acetate-CoA ligase (ADP forming), is a nickel-containing enzyme involved in the metabolic processes of cells. Together with Carbon monoxide dehydrogenase (CODH), it forms ...
also are involved in bioprocessing of CO. Carbon monoxide containing complexes are invoked for the toxicity of CO and signaling.
Synthesis
The synthesis of metal carbonyls is a widely-studied subject of organometallic research. Since the work of Mond and then Hieber, many procedures have been developed for the preparation of mononuclear metal carbonyls as well as homo- and heterometallic carbonyl clusters.
Direct reaction of metal with carbon monoxide
Nickel tetracarbonyl
Nickel carbonyl (IUPAC name: tetracarbonylnickel) is a nickel(0) organometallic compound with the formula Ni(CO)4. This colorless liquid is the principal carbonyl of nickel. It is an intermediate in the Mond process for producing very high-pur ...
and iron pentacarbonyl can be prepared according to the following equations by reaction of finely divided metal with carbon monoxide:
:Ni + 4 CO → Ni(CO)4 (1 bar, 55 °C)
:Fe + 5 CO → Fe(CO)5 (100 bar, 175 °C)
Nickel tetracarbonyl is formed with carbon monoxide already at 80 °C and atmospheric pressure, finely divided iron reacts at temperatures between 150 and 200 °C and a carbon monoxide pressure of 50–200 bar. Other metal carbonyls are prepared by less direct methods.
Reduction of metal salts and oxides
Some metal carbonyls are prepared by the reduction of metal halides in the presence of high pressure of carbon monoxide. A variety of reducing agents are employed, including copper, aluminum, hydrogen, as well as metal alkyls such as triethylaluminium. Illustrative is the formation of chromium hexacarbonyl from anhydrous chromium(III) chloride in benzene with aluminum as a reducing agent, and aluminum chloride as the catalyst:
:CrCl3 + Al + 6 CO → Cr(CO)6 + AlCl3
The use of metal alkyls, such as triethylaluminium and diethylzinc, as the reducing agent leads to the oxidative coupling of the alkyl radical to form the dimer alkane
In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...
:
:WCl6 + 6 CO + 2 Al(C2H5)3 → W(CO)6 + 2 AlCl3 + 3 C4H10
Tungsten, 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 ...
, manganese, and rhodium salts may be reduced with lithium aluminum hydride. Vanadium hexacarbonyl
Vanadium hexacarbonyl is the inorganic compound with the formula V(CO)6. It is a blue-black volatile solid. This highly reactive species is noteworthy from theoretical perspectives as a rare isolable homoleptic metal carbonyl that is paramagneti ...
is prepared with sodium as a reducing agent in chelating solvents such as diglyme.
:VCl3 + 4 Na + 6 CO + 2 diglyme → Na(diglyme)2 6">(CO)6+ 3 NaCl
: 6">(CO)6sup>− + H+ → H 6">(CO)6→ H2 + V(CO)6
In the aqueous phase, nickel or cobalt salts can be reduced, for example by sodium dithionite. In the presence of carbon monoxide, cobalt salts are quantitatively converted to the tetracarbonylcobalt(−1) anion:
:Co2+ + + 6 OH− + 4 CO → + 3 + 3 H2O
Some metal carbonyls are prepared using CO directly as the reducing agent. In this way, Hieber and Fuchs first prepared dirhenium decacarbonyl
Dirhenium decacarbonyl is the inorganic compound with the chemical formula Re2(CO)10 . Commercially available, it is used as a starting point for the synthesis of many rhenium carbonyl complexes. It was first reported in 1941 by Walter Hieber, who ...
from the oxide:
:Re2O7 + 17 CO → Re2(CO)10 + 7 CO2
If metal oxides are used carbon dioxide is formed as a reaction product. In the reduction of metal chlorides with carbon monoxide phosgene
Phosgene is the organic chemical compound with the formula COCl2. It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. Phosgene is a valued and important industrial building block, espe ...
is formed, as in the preparation of osmium carbonyl chloride from the chloride salts. Carbon monoxide is also suitable for the reduction of sulfide
Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...
s, where carbonyl sulfide is the byproduct.
Photolysis and thermolysis
Photolysis
Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...
or thermolysis of mononuclear carbonyls generates di- and polymetallic carbonyls such as diiron nonacarbonyl (Fe2(CO)9). On further heating, the products decompose eventually into the metal and carbon monoxide.
:2 Fe(CO)5 → Fe2(CO)9 + CO
The thermal decomposition of triosmium dodecacarbonyl (Os3(CO)12) provides higher-nuclear osmium carbonyl clusters such as Os4(CO)13, Os6(CO)18 up to Os8(CO)23.
Mixed ligand carbonyls of ruthenium, osmium
Osmium (from Greek grc, ὀσμή, osme, smell, label=none) is a chemical element with the symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mos ...
, rhodium, and iridium are often generated by abstraction of CO from solvents such as dimethylformamide (DMF) and 2-methoxyethanol
2-Methoxyethanol, or methyl cellosolve, is an organic compound with formula that is used mainly as a solvent. It is a clear, colorless liquid with an ether-like odor. It is in a class of solvents known as glycol ethers which are notable for the ...
. Typical is the synthesis of IrCl(CO)(PPh3)2 from the reaction of iridium(III) chloride
Iridium(III) chloride is the inorganic compound with the formula IrCl3. The anhydrous compound is relatively rare, but the related hydrate is useful for preparing other iridium compounds. The anhydrous salt is a dark green crystalline solid. ...
and triphenylphosphine in boiling DMF solution.
Salt metathesis
Salt metathesis reaction of salts such as KCo(CO)4 with 3Cl2">u(CO)3Cl2sub>2 leads selectively to mixed-metal carbonyls such as RuCo2(CO)11.
:4 KCo(CO)4 + 3Cl2">u(CO)3Cl2sub>2 → 2 RuCo2(CO)11 + 4 KCl + 11 CO
Metal carbonyl cations and carbonylates
The synthesis of ionic carbonyl complexes is possible by oxidation or reduction of the neutral complexes. Anionic metal carbonylates can be obtained for example by reduction of dinuclear complexes with sodium. A familiar example is the sodium salt of iron tetracarbonylate (Na2Fe(CO)4, ''Collman's reagent
Disodium tetracarbonylferrate is the organoiron compound with the formula Na2 e(CO)4 It is always used as a solvate, e.g., with tetrahydrofuran or dimethoxyethane, which bind to the sodium cation. An oxygen-sensitive colourless solid, it is a r ...
''), which is used in organic synthesis.
The cationic hexacarbonyl salts of manganese, technetium and rhenium can be prepared from the carbonyl halides under carbon monoxide pressure by reaction with a Lewis acid
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
.
:Mn(CO)5Cl + AlCl3 + CO → [][]
The use of strong acids succeeded in preparing gold carbonyl cations such as [Au(CO)2]+, which is used as a catalyst for the carbonylation of alkenes. The cationic platinum carbonyl complex 4">t(CO)4sup>2+ can be prepared by working in so-called superacid
In chemistry, a superacid (according to the classical definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (), which has a Hammett acidity function (''H''0) of −12. According to the modern definition, a superacid ...
s such as antimony pentafluoride
Antimony pentafluoride is the inorganic compound with the formula Sb F5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, formed when mixing liquid HF with liquid SbF5 in a 2:1 ratio. ...
. Although CO is considered generally as a ligand for low-valent metal ions, the tetravalent iron complex 2Fe">p*2Fesup>2+ (16-valence electron complex) quantitatively binds CO to give the diamagnetic Fe(IV)-carbonyl 2FeCO">p*2FeCOsup>2+ (18-valence electron complex).
Reactions
Metal carbonyls are important precursors for the synthesis of other organometallic complexes. Common reactions are the substitution
Substitution may refer to:
Arts and media
*Chord substitution, in music, swapping one chord for a related one within a chord progression
* Substitution (poetry), a variation in poetic scansion
* "Substitution" (song), a 2009 song by Silversun Pi ...
of carbon monoxide by other ligands, the oxidation or reduction reactions of the metal center, and reactions at the carbon monoxide ligand.[
]
CO substitution
The substitution of CO ligands can be induced thermally or photochemically by donor ligands. The range of ligands is large, and includes phosphines, cyanide
Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms.
In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of a ...
(CN−), nitrogen donors, and even ethers, especially chelating ones. Alkenes, especially dienes, are effective ligands that afford synthetically useful derivatives. Substitution of 18-electron complexes generally follows a dissociative mechanism, involving 16-electron intermediates.
Substitution proceeds via a dissociative mechanism:
:M(CO)''n'' → M(CO)''n''−1 + CO
:M(CO)''n''−1 + L → M(CO)''n''−1L
The dissociation energy
The bond-dissociation energy (BDE, ''D''0, or ''DH°'') is one measure of the strength of a chemical bond . It can be defined as the standard enthalpy change when is cleaved by homolysis to give fragments A and B, which are usually radical s ...
is for nickel tetracarbonyl
Nickel carbonyl (IUPAC name: tetracarbonylnickel) is a nickel(0) organometallic compound with the formula Ni(CO)4. This colorless liquid is the principal carbonyl of nickel. It is an intermediate in the Mond process for producing very high-pur ...
and for chromium hexacarbonyl.[
Substitution in 17-electron complexes, which are rare, proceeds via associative mechanisms with a 19-electron intermediates.
:M(CO)''n'' + L → M(CO)''n''L
:M(CO)''n''L → M(CO)''n''−1L + CO
The rate of substitution in 18-electron complexes is sometimes catalysed by catalytic amounts of oxidants, via electron transfer.
]
Reduction
Metal carbonyls react with reducing agents such as metallic sodium or sodium amalgam to give carbonylmetalate (or carbonylate) anions:
:Mn2(CO)10 + 2 Na → 2 Na 5">n(CO)5
For iron pentacarbonyl, one obtains the tetracarbonylferrate with loss of CO:
:Fe(CO)5 + 2 Na → Na2 4">e(CO)4+ CO
Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
can insert into the metal–metal bonds of some polynuclear metal carbonyls:
:Co2(CO)8 + Hg → (CO)4Co−Hg−Co(CO)4
Nucleophilic attack at CO
The CO ligand is often susceptible to attack by nucleophile
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
s. For example, trimethylamine oxide
Trimethylamine ''N''-oxide (TMAO) is an organic compound with the formula (CH3)3NO. It is in the class of amine oxides. Although the anhydrous compound is known, trimethylamine ''N''-oxide is usually encountered as the dihydrate. Both the anhydro ...
and potassium bis(trimethylsilyl)amide convert CO ligands to CO2 and CN−, respectively. In the " Hieber base reaction", hydroxide ion
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...
attacks the CO ligand to give a metallacarboxylic acid, followed by the release of carbon dioxide and the formation of metal hydrides or carbonylmetalates. A well-studied example of this nucleophilic addition
In organic chemistry, a nucleophilic addition reaction is an addition reaction where a chemical compound with an electrophilic double or triple bond reacts with a nucleophile, such that the double or triple bond is broken. Nucleophilic additions di ...
is the conversion of iron pentacarbonyl to hydridoiron tetracarbonyl anion:
:Fe(CO)5 + NaOH → Na 4CO2H">e(CO)4CO2H:Na 4COOH">e(CO)4COOH+ NaOH → Na 4">Fe(CO)4+ NaHCO3
Hydride reagents also attack CO ligands, especially in cationic metal complexes, to give the formyl derivative:
: 6">e(CO)6sup>+ + H− → Re(CO)5CHO
Organolithium reagents add with metal carbonyls to acylmetal carbonyl anions. ''O''- Alkylation of these anions, such as with Meerwein salts, affords Fischer carbenes.
With electrophiles
Despite being in low formal oxidation states, metal carbonyls are relatively unreactive toward many electrophiles. For example, they resist attack by alkylating agents, mild acids, and mild oxidizing agents. Most metal carbonyls do undergo halogenation
In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers, ...
. Iron pentacarbonyl, for example, forms ferrous carbonyl halides:
:Fe(CO)5 + X2 → Fe(CO)4X2 + CO
Metal–metal bonds are cleaved by halogens. Depending on the electron-counting scheme used, this can be regarded as an oxidation of the metal atoms:
:Mn2(CO)10 + Cl2 → 2 Mn(CO)5Cl
Compounds
Most metal carbonyl complexes contain a mixture of ligands. Examples include the historically important IrCl(CO)(P(C6H5)3)2 and the antiknock agent (CH3C5H4)Mn(CO)3. The parent compounds for many of these mixed ligand complexes are the binary carbonyls, those species of the formula ''x''(CO)''n''">''x''(CO)''n''sup>''z'', many of which are commercially available. The formulae of many metal carbonyls can be inferred from the 18-electron rule.
Charge-neutral binary metal carbonyls
*Group 2 elements
The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are al ...
calcium, strontium
Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...
, and barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element.
Th ...
can all form octacarbonyl complexes M(CO)8 (M = Ca, Sr, Ba). The compounds were characterized in cryogenic matrices by vibrational spectroscopy and in gas phase by mass spectrometry.
* Group 4 elements with 4 valence electrons are expected to form heptacarbonyls; while these are extremely rare, substituted derivatives of Ti(CO)7 are known.
* Group 5 elements with 5 valence electrons, again are subject to steric effects that prevent the formation of M–M bonded species such as V2(CO)12, which is unknown. The 17-VE V(CO)6 is however well known.
* Group 6 elements with 6 valence electrons form hexacarbonyls Cr(CO)6, Mo(CO)6, W(CO)6, and Sg(CO)6. Group 6 elements (as well as group 7) are also well known for exhibiting the ''cis'' effect (the labilization of CO in the cis position) in organometallic synthesis.
* Group 7 elements with 7 valence electrons form pentacarbonyl dimers Mn2(CO)10, Tc2(CO)10, and Re2(CO)10.
* Group 8 elements with 8 valence electrons form pentacarbonyls Fe(CO)5, Ru(CO)5 and Os(CO)5. The heavier two members are unstable, tending to decarbonylate to give Ru3(CO)12, and Os3(CO)12. The two other principal iron carbonyls are Fe3(CO)12 and Fe2(CO)9.
*Group 9 element
Group 9, by modern IUPAC numbering, is a group (column) of chemical elements in the periodic table. Members of Group 9 include cobalt (Co), rhodium (Rh), iridium (Ir) and meitnerium (Mt).Leigh, G. J. ''Nomenclature of Inorganic Chemistry: Recomme ...
s with 9 valence electrons and are expected to form tetracarbonyl dimers M2(CO)8. In fact the cobalt derivative of this octacarbonyl is the only stable member, but all three tetramers are well known: Co4(CO)12, Rh4(CO)12, Rh6(CO)16, and Ir4(CO)12. Co2(CO)8 unlike the majority of the other 18 VE transition metal carbonyls is sensitive to oxygen.
* Group 10 elements with 10 valence electrons form tetracarbonyls such as Ni(CO)4. Curiously Pd(CO)4 and Pt(CO)4 are not stable.
Anionic binary metal carbonyls
* Group 3 elements scandium and yttrium form monoanions, 8">(CO)8sup>− (M = Sc, Y) which are 20-electron carbonyls, as does the lanthanide lanthanum.
* Group 4 elements as dianions resemble neutral group 6 derivatives: 6">i(CO)6sup>2−.
* Group 5 elements as monoanions resemble again neutral group 6 derivatives: 6">(CO)6sup>−.
* Group 7 elements as monoanions resemble neutral group 8 derivatives: 5">(CO)5sup>− (M = Mn, Tc, Re).
* Group 8 elements as dianaions resemble neutral group 10 derivatives: 4sup>2− (M = Fe, Ru, Os)">(CO)4sup>2− (M = Fe, Ru, Os). Condensed derivatives are also known.
*Group 9 element
Group 9, by modern IUPAC numbering, is a group (column) of chemical elements in the periodic table. Members of Group 9 include cobalt (Co), rhodium (Rh), iridium (Ir) and meitnerium (Mt).Leigh, G. J. ''Nomenclature of Inorganic Chemistry: Recomme ...
s as monoanions resemble neutral group 10 metal carbonyl. 4">o(CO)4sup>− is the best studied member.
Large anionic clusters of nickel, palladium, and platinum are also well known. Many metal carbonyl anions can be protonated to give metal carbonyl hydrides.
Cationic binary metal carbonyls
*Group 2 elements
The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are al ...
form 8">(CO)8sup>+ (M = Ca, Sr, Ba), characterized in gas phase by mass spectrometry and vibrational spectroscopy.
* Group 3 elements form 7">c(CO)7sup>+ and 8">(CO)8sup>+ in gas phase.
* Group 7 elements as monocations resemble neutral group 6 derivative 6">(CO)6sup>+ (M = Mn, Tc, Re).
* Group 8 elements as dications also resemble neutral group 6 derivatives 6">(CO)6sup>2+ (M = Fe, Ru, Os).
Nonclassical carbonyl complexes
Nonclassical describes those carbonyl complexes where νCO is higher than that for free carbon monoxide. In nonclassical CO complexes, the C-O distance is shorter than free CO (113.7 pm). The structure of 6">e(CO)6sup>2+, with dC-O = 112.9 pm, illustrates this effect. These complexes are usually cationic, sometimes dicationic.
Applications
Metallurgical uses
Metal carbonyls are used in several industrial processes. Perhaps the earliest application was the extraction and purification of nickel via nickel tetracarbonyl
Nickel carbonyl (IUPAC name: tetracarbonylnickel) is a nickel(0) organometallic compound with the formula Ni(CO)4. This colorless liquid is the principal carbonyl of nickel. It is an intermediate in the Mond process for producing very high-pur ...
by the Mond process (see also carbonyl metallurgy
Carbonyl metallurgy is used to manufacture products of iron, nickel, steel, and other metals. Coatings are produced by vapor plating using metal carbonyl vapors. These are metal-ligand complexes where carbon monoxide is bonded to individual atoms ...
).
By a similar process carbonyl iron Carbonyl iron is a highly pure (97.5% for grade S, 99.5+% for grade R) iron, prepared by chemical decomposition of purified iron pentacarbonyl. It usually has the appearance of grey powder, composed of spherical microparticles. Most of the impuritie ...
, a highly pure metal powder, is prepared by thermal decomposition of iron pentacarbonyl. Carbonyl iron is used inter alia for the preparation of inductors, pigments, as dietary supplement
A dietary supplement is a manufactured product intended to supplement one's diet by taking a pill, capsule, tablet, powder, or liquid. A supplement can provide nutrients either extracted from food sources or that are synthetic in order ...
s, in the production of radar-absorbing materials in the stealth technology
Stealth technology, also termed low observable technology (LO technology), is a sub-discipline of military tactics and passive and active electronic countermeasures, which covers a range of methods used to make personnel, aircraft, ships, subm ...
, and in thermal spraying.
Catalysis
Metal carbonyls are used in a number of industrially important carbonylation reactions. In the oxo process, an alkene, hydrogen gas, and carbon monoxide react together with a catalyst (such as dicobalt octacarbonyl) to give aldehydes. Illustrative is the production of butyraldehyde from propylene:
:CH3CH=CH2 + H2 + CO → CH3CH2CH2CHO
Butyraldehyde is converted on an industrial scale to 2-ethylhexanol
2-Ethylhexanol (abbreviated 2-EH) is an organic compound with formula CHO. It is a branched, eight-carbon chiral alcohol (chemistry), alcohol. It is a colorless liquid that is poorly soluble in water but soluble in most organic solvents. It is prod ...
, a precursor to PVC plasticizers, by aldol condensation, followed by hydrogenation of the resulting hydroxyaldehyde. The "oxo aldehydes" resulting from hydroformylation are used for large-scale synthesis of fatty alcohols, which are precursors to detergent
A detergent is a surfactant or a mixture of surfactants with cleansing properties when in dilute solutions. There are a large variety of detergents, a common family being the alkylbenzene sulfonates, which are soap-like compounds that are more ...
s. The hydroformylation is a reaction with high atom economy, especially if the reaction proceeds with high regioselectivity.
:
Another important reaction catalyzed by metal carbonyls is the hydrocarboxylation. The example below is for the synthesis of acrylic acid and acrylic acid esters:
:
:
Also the cyclization of acetylene to cyclooctatetraene uses metal carbonyl catalysts:
In the Monsanto and Cativa processes, acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...
is produced from methanol, carbon monoxide, and water using hydrogen iodide
Hydrogen iodide () is a diatomic molecule and hydrogen halide. Aqueous solutions of HI are known as hydroiodic acid or hydriodic acid, a strong acid. Hydrogen iodide and hydroiodic acid are, however, different in that the former is a gas under sta ...
as well as rhodium and iridium carbonyl catalysts, respectively. Related carbonylation reactions afford acetic anhydride
Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH3CO)2O. Commonly abbreviated Ac2O, it is the simplest isolable anhydride of a carboxylic acid and is widely used as a reagent in organic synthesis. It is a col ...
.
CO-releasing molecules (CO-RMs)
Carbon monoxide-releasing molecules are metal carbonyl complexes that are being developed as potential drugs to release CO. At low concentrations, CO functions as a vasodilatory and an anti-inflammatory agent. CO-RMs have been conceived as a pharmacological strategic approach to carry and deliver controlled amounts of CO to tissues and organs.
Related compounds
Many ligands are known to form homoleptic and mixed ligand complexes that are analogous to the metal carbonyls.
Nitrosyl complexes
Metal nitrosyls, compounds featuring NO ligands, are numerous. In contrast to metal carbonyls, however, homoleptic metal nitrosyls are rare. NO is a stronger π-acceptor than CO. Well known nitrosyl carbonyls include CoNO(CO)3 and Fe(NO)2(CO)2, which are analogues of Ni(CO)4.
Thiocarbonyl complexes
Complexes containing CS are known but uncommon. The rarity of such complexes is partly attributable to the fact that the obvious source material, carbon monosulfide
Carbon monosulfide is a chemical compound with the formula CS. This diatomic molecule is the sulfur analogue of carbon monoxide, and is unstable as a solid or a liquid, but it has been observed as a gas both in the laboratory and in the interstel ...
, is unstable. Thus, the synthesis of thiocarbonyl complexes requires indirect routes, such as the reaction of disodium tetracarbonylferrate with thiophosgene
Thiophosgene is a red liquid with the formula . It is a molecule with trigonal planar geometry. There are two reactive C–Cl bonds that allow it to be used in diverse organic syntheses.
Preparation
is prepared in a two-step process from carbo ...
:
:Na2Fe(CO)4 + CSCl2 → Fe(CO)4CS + 2 NaCl
Complexes of CSe
CSE may refer to:
Education Examinations
* Certificate of Secondary Education, a secondary school qualification in the United Kingdom, replaced by the GCSE
* Civil Services Examination, an examination to qualify for government service in India
Fi ...
and CTe have been characterized.
Isocyanide complexes
Isocyanides also form extensive families of complexes that are related to the metal carbonyls. Typical isocyanide ligands are methyl isocyanide and ''t''-butyl isocyanide (Me3CNC). A special case is CF3NC, an unstable molecule that forms stable complexes whose behavior closely parallels that of the metal carbonyls.
Toxicology
The toxicity of metal carbonyls is due to toxicity of carbon monoxide, the metal, and because of the volatility and instability of the complexes, any inherent toxicity of the metal is generally made much more severe due to ease of exposure. Exposure occurs by inhalation, or for liquid metal carbonyls by ingestion or due to the good fat solubility by skin resorption. Most clinical experience were gained from toxicological poisoning with nickel tetracarbonyl
Nickel carbonyl (IUPAC name: tetracarbonylnickel) is a nickel(0) organometallic compound with the formula Ni(CO)4. This colorless liquid is the principal carbonyl of nickel. It is an intermediate in the Mond process for producing very high-pur ...
and iron pentacarbonyl due to their use in industry. Nickel tetracarbonyl is considered as one of the strongest inhalation poisons.
Inhalation of nickel tetracarbonyl
Nickel carbonyl (IUPAC name: tetracarbonylnickel) is a nickel(0) organometallic compound with the formula Ni(CO)4. This colorless liquid is the principal carbonyl of nickel. It is an intermediate in the Mond process for producing very high-pur ...
causes acute non-specific symptoms similar to a carbon monoxide poisoning, such as nausea, cough, headache
Headache is the symptom of pain in the face, head, or neck. It can occur as a migraine, tension-type headache, or cluster headache. There is an increased risk of depression in those with severe headaches.
Headaches can occur as a result ...
, fever, and dizziness. After some time, severe pulmonary symptoms such as cough, tachycardia, and cyanosis
Cyanosis is the change of body tissue color to a bluish-purple hue as a result of having decreased amounts of oxygen bound to the hemoglobin in the red blood cells of the capillary bed. Body tissues that show cyanosis are usually in locations ...
, or problems in the gastrointestinal tract
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...
occur. In addition to pathological alterations of the lung, such as by metalation of the alveoli, damages are observed in the brain, liver, kidneys, adrenal glands, and spleen. A metal carbonyl poisoning often necessitates a lengthy recovery.
Chronic exposure by inhalation of low concentrations of nickel tetracarbonyl can cause neurological symptoms such as insomnia, headaches, dizziness and memory loss. Nickel tetracarbonyl is considered carcinogenic, but it can take 20 to 30 years from the start of exposure to the clinical manifestation of cancer.
History
Initial experiments on the reaction of carbon monoxide with metals were carried out by Justus von Liebig
Justus Freiherr von Liebig (12 May 1803 – 20 April 1873) was a German scientist who made major contributions to agricultural and biological chemistry, and is considered one of the principal founders of organic chemistry. As a professor at t ...
in 1834. By passing carbon monoxide over molten potassium he prepared a substance having the empirical formula KCO, which he called ''Kohlenoxidkalium''. As demonstrated later, the compound was not a carbonyl, but the potassium salt of benzenehexol (K6C6O6) and the potassium salt of acetylenediol (K2C2O2).
The synthesis of the first true heteroleptic metal carbonyl complex was performed by Paul Schützenberger in 1868 by passing chlorine and carbon monoxide over platinum black, where dicarbonyldichloroplatinum (Pt(CO)2Cl2) was formed.
Ludwig Mond, one of the founders of Imperial Chemical Industries
Imperial Chemical Industries (ICI) was a British chemical company. It was, for much of its history, the largest manufacturer in Britain.
It was formed by the merger of four leading British chemical companies in 1926.
Its headquarters were at M ...
, investigated in the 1890s with Carl Langer and Friedrich Quincke various processes for the recovery of chlorine which was lost in the Solvay process by nickel metals, oxides, and salts. As part of their experiments the group treated nickel with carbon monoxide. They found that the resulting gas colored the gas flame of a burner in a greenish-yellowish color; when heated in a glass tube it formed a nickel mirror. The gas could be condensed to a colorless, water-clear liquid with a boiling point of 43 °C. Thus, Mond and his coworker had discovered the first pure, homoleptic metal carbonyl, nickel tetracarbonyl
Nickel carbonyl (IUPAC name: tetracarbonylnickel) is a nickel(0) organometallic compound with the formula Ni(CO)4. This colorless liquid is the principal carbonyl of nickel. It is an intermediate in the Mond process for producing very high-pur ...
(Ni(CO)4). The unusual high volatility of the metal compound nickel tetracarbonyl led Kelvin to the statement that Mond had "given wings to the heavy metals".
The following year, Mond and Marcellin Berthelot independently discovered iron pentacarbonyl, which is produced by a similar procedure as nickel tetracarbonyl. Mond recognized the economic potential of this class of compounds, which he commercially used in the Mond process and financed more research on related compounds. Heinrich Hirtz and his colleague M. Dalton Cowap synthesized metal carbonyls of cobalt, 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 ...
, ruthenium, and diiron nonacarbonyl. In 1906 James Dewar and H. O. Jones were able to determine the structure of diiron nonacarbonyl, which is produced from iron pentacarbonyl by the action of sunlight. After Mond, who died in 1909, the chemistry of metal carbonyls fell for several years in oblivion. BASF started in 1924 the industrial production of iron pentacarbonyl by a process which was developed by Alwin Mittasch
Paul Alwin Mittasch (Sorbian languages, sorbian: ''Pawoł Alwin Mitaš'') (born 27 December 1869 in Großdehsa/Dažin, today to Löbau, Germany; died 4 June 1953 in Heidelberg, Germany) was a German chemist and scientific historian of Sorbs, Sorbia ...
. The iron pentacarbonyl was used for the production of high-purity iron, so-called carbonyl iron Carbonyl iron is a highly pure (97.5% for grade S, 99.5+% for grade R) iron, prepared by chemical decomposition of purified iron pentacarbonyl. It usually has the appearance of grey powder, composed of spherical microparticles. Most of the impuritie ...
, and iron oxide pigment. Not until 1927 did A. Job and A. Cassal succeed in the preparation of chromium hexacarbonyl and tungsten hexacarbonyl
Tungsten hexacarbonyl (also called tungsten carbonyl) is the chemical compound with the formula W(CO)6. This complex gave rise to the first example of a dihydrogen complex.Kubas, G. J., Metal Dihydrogen and σ-Bond Complexes, Kluwer Academic/Plenu ...
, the first synthesis of other homoleptic metal carbonyls.
Walter Hieber played in the years following 1928 a decisive role in the development of metal carbonyl chemistry. He systematically investigated and discovered, among other things, the Hieber base reaction, the first known route to metal carbonyl hydrides and synthetic pathways leading to metal carbonyls such as dirhenium decacarbonyl
Dirhenium decacarbonyl is the inorganic compound with the chemical formula Re2(CO)10 . Commercially available, it is used as a starting point for the synthesis of many rhenium carbonyl complexes. It was first reported in 1941 by Walter Hieber, who ...
. Hieber, who was since 1934 the Director of the Institute of Inorganic Chemistry at the Technical University Munich published in four decades 249 papers on metal carbonyl chemistry.
Also in the 1930s Walter Reppe, an industrial chemist and later board member of BASF, discovered a number of homogeneous catalytic processes, such as the hydrocarboxylation, in which olefins or alkyne
\ce
\ce
Acetylene
\ce
\ce
\ce
Propyne
\ce
\ce
\ce
\ce
1-Butyne
In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...
s react with carbon monoxide and water to form products such as unsaturated acid
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
s and their derivatives. In these reactions, for example, nickel tetracarbonyl or cobalt carbonyls act as catalysts. Reppe also discovered the cyclotrimerization
In organic chemistry, an alkyne trimerisation is a +2+2nbsp;cycloaddition reaction in which three alkyne units () react to form a benzene ring. The reaction requires a metal catalyst. The process is of historic interest as well as being applica ...
and tetramerization of acetylene
Acetylene (systematic name: ethyne) is the chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure ...
and its derivatives to benzene and benzene derivatives with metal carbonyls as catalysts. BASF built in the 1960s a production facility for acrylic acid
Acrylic acid (IUPAC: propenoic acid) is an organic compound with the formula CH2=CHCOOH. It is the simplest unsaturated carboxylic acid, consisting of a vinyl group connected directly to a carboxylic acid terminus. This colorless liquid has a ...
by the Reppe process, which was only superseded in 1996 by more modern methods based on the catalytic propylene oxidation.
For the rational design of new complexes the concept of the isolobal analogy has been found useful. Roald Hoffmann was awarded the Nobel Prize in chemistry for the development of the concept. This describes metal carbonyl fragments of M(CO)''n'' as parts of octahedral building blocks in analogy to the tetrahedral CH3–, CH2– or CH– fragments in organic chemistry. In example dimanganese decacarbonyl is formed in terms of the isolobal analogy of two ''d''7 Mn(CO)5 fragments, that are isolobal to the methyl radical . In analogy to how methyl radicals combine to form ethane, these can combine to dimanganese decacarbonyl. The presence of isolobal analog fragments does not mean that the desired structures can be synthesized. In his Nobel Prize lecture Hoffmann emphasized that the isolobal analogy is a useful but simple model, and in some cases does not lead to success.
The economic benefits of metal-catalysed carbonylations, such as Reppe chemistry and hydroformylation, led to growth of the area. Metal carbonyl compounds were discovered in the active sites of three naturally occurring enzymes.
See also
*
*
*Alkaline earth octacarbonyl complex
Alkaline earth octacarbonyl complexes are a class of neutral compounds that have the general formula M(CO)8 where M is a heavy Group 2 element ( Ca, Sr, or Ba). The metal center has a formal oxidation state of 0 and the complex has a high level ...
References
External links
metal carbonyls at Louisiana State University
{{DEFAULTSORT:Metal, carbonyl
Organometallic chemistry
Transition metals
Carbon monoxide