Question

why do [Ni(CN)4]2- and [NiCl4]2- differ in magnetic property?

Answer 1

CN is strong field ligand

Answer 2

The valence shell electronic configuration of ground state Ni atom is 3d8 4s2.

both the compounds have Ni in +2 oxidation state.

CN being strong ligand pairs electron whereas Cl cannot

here you see the hybridisation is sp3 and geometry is tetrahedral.

Now , for CN-

The hybridisation is dsp2, and geometry is square planar.

For the concept for strong and weak ligand.