Why is the metallic atom in low oxidation state in metallic carbonyls?

The metal atom in metal carbonyls is in low oxidation state. This is because of the

fact that there is a higher electron density (electronic charge) on the metal atom in its lower

oxidation state than in its higher oxidation state. Thus, since the metal is electropositive in

nature, it would feel more comfortable, if some of the excess electronic charge present on

the metal atom gets transferred to the vacant π- orbitals of CO (ligand) by forming M->CO

π- bond.