How can carbon monoxide-type ligands help transition metals reach low oxidation states?

Carbon monoxide (CO) ligands stabilize transition metals in low or zero oxidation states primarily through a process known as synergic bonding. This mechanism involves two complementary electron interactions between the metal and the ligand:1

• $\sigma$ (Sigma) Donation: The carbon atom of the CO molecule possesses a lone pair of electrons in an orbital with $\sigma$ symmetry. It acts as a Lewis base, donating this electron pair into an empty orbital on the metal atom to form a direct metal-carbon $\sigma$ bond.213
• $\pi$ (Pi) Back-bonding: Simultaneously, the metal atom donates electron density from its filled $d$ orbitals into the empty, low-energy antibonding orbitals (${\pi }^{*}$) of the CO ligand.21

This back-donation of electron density from the metal to the ligand is crucial for stabilizing low oxidation states. By effectively "offloading" excess electron density onto the CO ligands, the metal can maintain a stable electronic configuration—often adhering to the 18-electron rule—even when its formal oxidation state is zero or low. This unique ability to act as both a $\sigma$-donor and a $\pi$-acceptor makes carbon monoxide an exceptionally effective ligand for stabilizing metals in these reduced states compared to ligands that are only moderate $\sigma$-donors.4513