[Cr(NH₃)₆]³⁺ is paramagnetic while [Ni(CN)₄]^2– is diamagnetic. Explain why?

Overall charge balance in [Cr(NH₃)₆]³⁺ complex:

X + 6(0) = + 3

X = + 3

Cr is in + 3 oxidation state.

Electronic configuration of Cr in + 2 state: 3d³ . Now ammonia is a weak field ligand so it not causes pairing of the unpaired electron and undergoes hybridisation to form 6 sp³d² hybrid orbitals filled by the six ammonia ligands. It's geometry is octahedral with unpaired electrons and hence is paramagnetic complex.

In case of [Ni(CN)₄]^2– ion :

Overall charge balance in [Ni(CN)₄]^2–complex:

X + 4(-1) = -2

X = + 2

Ni is in + 2 oxidation state.

Electronic configuration of Ni in + 2 state: 3d⁸. Now cyanide ion is a strong field ligand so it causes pairing of the unpaired electron and undergoes hybridisation to form 4 dsp² hybrid orbitals filled by the four cyanide ligands. It's geometry is square planar with no unpaired electrons and hence is diamagnetic complex.