Explain synergic bonding.

(N/A) The metal-carbon bond in metal carbonyls possesses both $\sigma$ and $\pi$ character.
The $M-C$ $\sigma$-bond is formed by the donation of a lone pair of electrons on the carbonyl carbon into a vacant orbital of the metal.
The $M-C$ $\pi$-bond is formed by the donation of a pair of electrons from a filled $d$-orbital of the metal into the vacant antibonding $\pi^{*}$ orbital of carbon monoxide $(CO)$.
The metal-to-ligand bonding creates a synergic effect which strengthens the bond between $CO$ and the metal.