Question

[NiCl₄]²⁻ is paramagnetic while [Ni(CO)₄] is diamagnetic though both are tetrahedral.
Why?

Answer 1

But CO is a strong field ligand. Therefore, 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. Since no unpaired electrons are present in this case, [Ni(CO)₄] is diamagnetic.