Transition metals show low oxidation states with carbon monoxide
Synergic bonds enable the formation of complexes by transition elements in zero oxidation states. In metal carbonyl, the metal-carbon bond has both s and p characters. The M-C sigma bond is created when a lone pair of electrons from the carbonyl carbon are donated to a metal's empty orbital. The M-C bond is created when a pair of electrons from a metal's full d orbital is donated to carbon monoxide's unoccupied antibonding pi* orbital. The synergic action of metal to ligand bonding enhances the connection between carbon monoxide and metal.
