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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.

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