In $\mathrm{iron} \mathrm{carbonyl} \mathrm{complexes}, \mathrm{Fe}-\mathrm{C}$ bond possess
 

Synergic bonding is the type of bonding in which the transfer of electrons of ligands to metals along with the transfer of electrons from metals to the antibonding orbitals of ligands occurs. 

It is known as "synergic bonding." The ligand (CO) creates the sigma bond by donating its single pair of electrons to the iron atom. Since the iron atom has some electrons in its d-orbitals as well, it transfers those electrons back to the ligand's molecular orbitals to create a pi-bond.