In ${\text{Fe(CO)}}_5$, the $\text{Fe}-\text{C}$ bond possesses:

In metal carbonyls, bonding occurs through synergic bonding:A $\sigma$ -bond is formed by donation of a lone pair of electrons from the carbonyl carbon into a vacant orbital of the metal.A $\pi$ -bond is formed by back-donation of electrons from a filled $d$ -orbital of the metal into the vacant antibonding ${\pi }^{\ast }$ orbital of $\text{CO}$.