Hey ,

Ni(CO)4 = Ni + 4CO

* The valence shell electronic configuration of ground state Ni atom is 3d8 4s2. 

* All of these 10 electrons are pushed into 3d orbitals and get paired up when strong field CO ligands approach Ni atom. The empty 4s and three 4p orbitals undergo sp3 hybridization and form bonds with CO ligands to give Ni(CO)4. Thus Ni(CO)4 is diamagnetic.

Thank You !