Question

Explain the following observation:

Tetrahedral Ni(II) complexes are usually paramagnetic but square planar Ni(II) complexes are diamagnetic.

Answer 1

1. The observation that tetrahedral Ni(II) complexes are usually paramagnetic, while square planar Ni(II) complexes are diamagnetic, can be explained based on valence bond theory and the crystal field splitting pattern in different geometries.
2. Nickel in the +2 oxidation state (Ni²⁺) has the electron configuration 3d⁸.
3. In a tetrahedral field: 
   • The crystal field splitting is relatively small, and the energy difference between the split d-orbitals (t₂ and e) is not sufficient to cause pairing of electrons.
   • As a result, the 3d electrons remain unpaired, leading to paramagnetic behaviour (i.e., the complex has unpaired electrons and is attracted to a magnetic field).
4. In a square planar field:
   • The crystal field splitting is much larger and more asymmetric.
   • The splitting pattern is such that the highest energy orbital (dx²–y²) lies well above the others.
   • For a d⁸ configuration in a strong field square planar environment, the electrons pair up in the lower energy orbitals to avoid occupying the high-energy dx²–y² orbital.
   • This results in all electrons being paired, and the complex becomes diamagnetic (i.e., no unpaired electrons and not attracted to a magnetic field).