Sigma and Pi-bonded Organometallic compounds:

Metal carbonyls, compounds formed between metal and carbon monoxide belong to this class. These compounds possess both Sigma and Pi-bonding. The oxidation state of metal atoms in these compounds is zero. Carbonyls may be monomeric, bridged or polynuclear. Carbonyls are mainly formed by the transition metals of VIth, VIIth and VIIIth groups.

Some well known examples are:

In a metal carbonyl, the metal-carbon bond possesses both the Sigma and Pi- character. A sigma bond between metal and carbon atom is formed when a vacant hybrid bond of the metal atom overlaps with an orbital on C atom of carbon monoxide containing a lone pair of electrons.

Formation of Pi-bond is caused when a filled orbital of the metal atom overlaps with a vacant antibonding (Pi-star) orbital of C atom of carbon monoxide. This overlap is also called back donation of electrons by metal atom to carbon. It has been shown below:

The Pi-overlap is perpendicular to the nodal plane of sigma-bond.

Pi-bonded Organometallic compounds :

These are the compounds of metals with alkenes, alkynes, benzene and other ring compounds. In these complexes, the metal and ligand form a bond that involves the Pi electrons of the ligand. Three common examples are Zeise’s salt, ferocene and dibenzene chromium. These are shown here:

Sigma Bonded Complexes:

In these complexes, the metal atom and carbon atom of the ligand are joined together with a sigma bond, i.e., the ligand contributes one electron and is, therefore, called one electron donor. Examples are :

(1)  Grignard reagent, R-Mg-X where R is an alkyl or aryl group and  X is a halogen.

(2)  Zinc compounds of the formula R₂Zn such as (C₂H₅)₂Zn. This was first isolated by Frankland in 1849. Other similar compounds are Sn(CH₃)_4, Pb(C₂H₅)₄, Al₂(C₂H₅)₆  Al₂(CH₃)₆  , Pb(CH₃)₄ etc.

      Al₂(CH₃)₆ is a dimeric compound and has a structure similar to diborane, B₂H₆ It is an electron deficient compound and two methyl groups act as bridges between two aluminium atoms.