The theory that can completely/properly explain the nature of bonding in Ni CO 4 is:

Step-by-step solution

There is synergic bonding present in Ni CO 4 . As CO is neutral ligand, so oxidation state of nickel is zero i.e. Ni 0 . So, electronic configuration of Ni 0 = Ar 3 d 8   4 s 2 As CO is strong field ligand, it will back paired the 4s electrons to 3 d orbitals. So in complex electronic configuration of Ni 0 = Ar   3 d 10   4 s 0 here inner d orbital is filled. In ligand CO (carbonyl) the anti-bonding orbital is vacant. CO :   σ 1 s 2 ,   σ * 1 s 2 ,   σ 2 s 2 ,   &#9