Of the following complex ions, which is diamagnetic in nature?

* ${\text{Ni}}^{2+}$ is $d^8$.In $[{\text{Ni(CN)}}_4{]}^{2-}$, ${\text{CN}}^{-}$ is a strong field ligand. It causes pairing of electrons, leading to $ds{p}^2$ hybridization and no unpaired electrons. Thus, it is diamagnetic.In $[{\text{NiCl}}_4{]}^{2-}$, ${\text{Cl}}^{-}$ is a weak field ligand ($s{p}^3$), so it remains paramagnetic.