CBSE Class 12 · 2002 · Delhi · Coordination CompoundsShort Answer

Ni${}^{2+}$ = [Ar]3d${}^8$.
Tetrahedral complexes (e.g. [NiCl${}_4$]${}^{2-}$): weak-field ligands cause no pairing; sp${}^3$ hybridisation leaves the d${}^8$ set with 2 unpaired electrons — paramagnetic.
Square planar complexes (e.g. [Ni(CN)${}_4$]${}^{2-}$): strong-field ligands force the two unpaired electrons to pair, vacating one 3d orbital for dsp${}^2$ hybridisation — all electrons paired, diamagnetic.