In a metal carbonyl, the metal-carbon bond possesses both $\sigma$ and $\pi$ characters. A $\sigma$ -bond  between metal and carbon atom is formed when a vacant hybrid orbital of the metal atom overlaps with an orbital of 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 overlap with a vacant antibonding ${\pi }^{*}$ orbital of C atom of CO. This overlap is aslo called back donation of electrons by metal atom to carbon.



(a) The formation of the metal$\leftarrow$carbon $\sigma$ - bond using an unshared pair of the C atom . (b) The formation of the  metal $\to$carbon $\pi$ - bond.
The $\mathrm{\pi -}$ $\sigma$-bond. overlap is perpendicular to the nodal plane of