1. In metal carbonyls, the bond between metal atom and the carbonyl ligand consists of two components.
The first component is an electron pair donation from the carbon atom of the carbonyl ligand into a vacant d – orbital of central metal atom. This electron pair donation forms 
and sigma bond.
2. This a bond formation increases the electron density in metal d orbitals and makes the metal electron rich.
3. In order to compensate for this increased electron density, a filled metal d – orbital interacts with the empty n orbital on the carbonyl ligand and transfers the added electron density back to the ligand. This second component is called n back bonding.
4. Thus in metal, carbonyl, electron density moves from ligand to metal through o bonding and from metal to ligand through Pi bonding, this synergic effect accounts for strong M ← CO bond in metal carbonyls.
