The electronic configuration of Ni is Ni (28) = [Ar] 3d8,4s2 Ni2+ = [Ar] 3d8 Both Ni and Ni2+ have two unpaired electrons. CO and CN' are strong field ligands and thus unpaired electrons get paired. Hence, Ni(CO)4 and [Ni(CN)4]2− are diamagnetic. Cl is a weak field ligand hence, no pairing of e will take place, hence NiCl42− is paramagnetic.