[ Prev | Up | Next ]

Organometallic Chemistry:

Carbon monoxide (CO) acts as a common ligand in organometallic compounds. Although CO itself is not an organic compound, it is prominent in metallic complexes containing organic ligands. CO demonstrates some of the interesting non-covalent chemical bonding which is typical for organometallic compounds.

CO may attach to a metal center and thereby act as an acceptor of electron density. This type of bonding requires a metal which has electrons to spare, meaning one in a low oxidation state. Electron density from the d orbitals on the metal flows into the antibonding p orbital of CO. Due to the difference in electronegativities, carbon in free CO is always electropositive in comparison to oxygen, and is therefore the atom which bonds to the metal.

Carbon Monoxide Ligand Accepts Electron Density

This influx of electron density into the antibonding orbital results in a weakening of the overall CO bond, but not nearly enough to cause dissociation. In this case, CO is a terminal ligand, meaning it is not bonded to anything else but one metal center. However, CO can act as a bridging ligand, meaning it can interact with two or more metal centers. An example of bonding with two metal ions is shown here:

Notice in the first diagram, CO acts as a donor of electron density, giving from the back-lobes of its sigma bond to the metals. In the second , CO is an acceptor of electron density, taking from the d orbitals of the metals. Both of these cases are common in binary metal complexes.

Here're some metal complexes containing pentahapto cyclopentadienyl ligands. They're called metallocenes.

Ferrocene (h5-Cp)2Fe

(h5-Cp)Co(CO)2

A type of organometallic compound which is crucial in performing a variety of biological functions is the metalloporphyrins. These consist of a metal center and a planar ligand constructed from four pyrrole derivatives which chelates the metal. The behavior of each is determined mostly by the type of metal being chelated and the types of substituents on the edge of the porphyrin. An example of such is the heme complex.

Heme Complex

This metalloporphrin is a constituent in cytochromes, the proteins responsible for transporting and storing blood oxygen. In myoglobin and hemoglobin, the iron center of heme becomes the center of an octahedral ligand field. It has four coordination sites filled by the porphyrin ring and one by another part of the cytochrome. This leaves one possible position available at which a dioxygen molecule can coordinate.

Oxygen Coordination to Hemoglobin

In the case of cytochromes, the sixth position is coordinated reversibly in the presence of molecular oxygen. This reversibility, which is necessary for continued oxygen transfer, does not occur with just ordinary heme. Oxygen coordinated to one heme complex will eventually coordinate to another, forming hematin. In the case of cytochromes however, steric hinderance caused by the bulky protein part prevents another heme-protein from coming close enough to the bound oxygen, allowing for reversibility.(19)

Another example of a metalloporphyrin is chlorophyll a: