CFT assumes that the ligands are point charges and it follows that anionic ligands should exert the greatest splitting effect. Further it does not take into account the covalent character of bonding which exists between the metal and the carbon atom of the carbonyl. The $\mathrm{M}-\mathrm{C} \sigma$ bond is formed by the donation of lone pair of electrons on the carbonyl carbon into a vacant orbital of the metal and the M-C $\pi$ bond is formed by the donation of a pair of electrons from a filled d-orbital of metal into vacant antibonding $\pi^*$ orbital of carbon monoxide. So, MOT explains the bonding in $\mathrm{Ni}(\mathrm{CO})_4$.