metal carbonyl
What are metal carbonyls?
How are metal carbonyls produced?
What is the structure of tetracarbonylnickel Ni(CO)4?
How are metal carbonyls used industrially?
metal carbonyl, any coordination or complex compound in which one or more carbon monoxide (CO) molecules are attached to a metal atom. These complexes have distinctive bonding features, show different patterns in how they react, and are important in inorganic chemistry. They also have industrial significance, such as being used as catalysts in chemical synthesis, and in refining and purifying metals. The CO molecules are ligandsāmolecules that donate electron pairs. Some common metal carbonyls are tetracarbonylnickel Ni(CO)4, pentacarbonyliron Fe(CO)5, and octacarbonyldicobalt Co2(CO)8. In general, the metal carbonyls are produced by direct action of carbon monoxide on the finely divided metal. They are used in the preparation of metals of exceptionally high purity and as catalysts in organic syntheses.
Structure and bonding
In 1884 German-born English chemist Ludwig Mond noticed that CO gas was corroding nickel valves in an industrial plant. While investigating the phenomenon, he reacted nickel powder with CO gas and isolated Ni(CO)4, the first known metal carbonyl. This finding led to the establishment of the Mond Nickel Company, dedicated to the large-scale refining of nickel.
Metal carbonyls can be classified based on how many metal atoms they contain, a property called nuclearity. They are mononuclear if they have a single metal atom, such as Ni(CO)4. If they have two or more metal atoms, they are polynuclear, such as Mn2(CO)10. In these compounds the atoms are bonded in specific three-dimensional arrangements, so each carbonyl complex has a characteristic molecular geometry. The geometry of these compounds depends on the metal and the number of ligands. For example, Ni(CO)4 is tetrahedral, Fe(CO)5 has a trigonal bipyramidal structure, and Cr(CO)6 is octahedral. In some cases, such as Fe(CO)5, the ligands exhibit fluxionality, where they rapidly exchange positions. Metal carbonyls commonly follow the 18-electron rule, also called the effective atomic number (EAN) rule, which states that stable complexes often have 18 valence electrons, mirroring the electron configuration of noble gases.
The bonding in metal carbonyls involves two distinct types of electron interaction. The CO molecule has a lone pair of electrons in its carbon atom that is not used in bonding. It donates this pair to an empty orbital on the metal, forming a direct bond between the ligand and the metal. This is known as sigma (Ļ) donation. At the same time, electrons from filled metal d orbitals are donated back into an empty antibonding orbital (Ļ*) on the CO molecule. This process is called pi (Ļ) backbonding, and it has two important effects: It strengthens the bond between the metal and carbon, and it weakens the internal carbon-oxygen (CāO) bond within the CO ligand by adding electron density into an orbital that opposes the CāO bond. This two-way exchange of electrons is called synergic bonding and helps stabilize the structure of the metal carbonyl complex.
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Properties
Physical properties
Most metal carbonyls are volatile compounds at room temperature. Mononuclear carbonyls tend to be pale yellow, and polynuclear species are darker in color. These compounds are generally soluble in organic solvents such as acetone, ether, benzene, and carbon tetrachloride.
Metal carbonyls have low melting points. When heated, they decomposeāthat is, they break down into simpler substancesāreleasing carbon monoxide and elemental metal. Most metal carbonyls are diamagnetic, meaning their electrons are paired and they are therefore weakly repelled by a magnetic field. However, there are exceptions, such as vanadium hexacarbonyl V(CO)6, which is paramagneticāit has one or more unpaired electrons and is consequently attracted to a magnetic field. Metal carbonyls can be dangerous because they are volatile: They evaporate easily and can release toxic carbon monoxide and metal vapors. For example, Ni(CO)4 is hazardous and is considered carcinogenic with prolonged exposure.
Chemical properties
Chemically, metal carbonyls undergo several reactions. Some of them are:
- Ligand substitution: CO ligands can be replaced by other ligands such as phosphines, isocyanides, or bidentate ligands such as diars (arsine derivatives).
- Reaction with sodium hydroxide: Carbonyls such as Fe(CO)5 react with NaOH. In this reaction the hydroxide attacks one of the carbon monoxide ligands attached to the metal. That gives a metal-carboxylate intermediate, which then typically loses carbon dioxide and produces a metal hydride complex. This series of steps is known as the Hieber base reaction, for the German chemist Walter Hieber.
- Reaction with halogens: Halogens can form carbonyl halides or decompose the carbonyl complex.
- Reaction with nitric oxide: Nitric oxide can substitute CO ligands to form carbonyl-nitrosyl complexes.
Applications
In industrial settings Ni(CO)4 is used in the Mond process for the purification of nickel by thermal decomposition. Cobalt carbonyls, such as HCo(CO)4 and Co2(CO)8, are catalysts in hydroformylation (also called oxo process), which transforms alkenes into aldehydes. Rhodium and iridium carbonyl complexes serve as catalysts in acetic acid manufactured through the Monsanto and Cativa processes. Iron and manganese carbonyls are studied for their role in carbon-hydrogen bond activation and for their utility as organometallic precursors. In laboratory and industrial organic synthesis, metal carbonyls are used in stoichiometric reactions to build complex organic molecules.
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