In metal carbonyl having and genral formula $M{\left(CO\right)}_x$, where, M = metal, x = 4 and the metal is bonded to

In metal carbonyl $M{\left(CO\right)}_4$ , metal is bonded to the ligand CO through carbon atoms. These compounds contam both $\sigma \text{ and }\pi$-bonded complexes.
$\sigma$-bond between metal and carbon atom is formed when a vacant hybrid orbital of the metal atom overlaps with an orbital on C-atom of carbon rnonooxide containing a low pair of electron Formation of $\pi$-bond is occurs when a filled orbital of the metal atom overlaps with a vacant antibonding $\pi$-orbital of C-atom of Co.