The geometry and magnetic behavior of the complex [Ni(CO)₄] are

In Ni(CO)₄ Ni is in the zero oxidation state, i.e., it has a configuration of 3d⁸4s².
Since CO is a strong field ligand, it causes the pairing of unpaired 3d electrons.
Also, it causes the 4s electrons to shift to the 3d orbital, thereby giving rise to sp³ hybridization and,
hence, tetrahedral shape. Since no unpaired electrons are present in this case, [Ni(CO)₄] is
diamagnetic