Why is [Ni (NH3)6]2+paramagnetic while [Ni (CN) 6]4-diamagnetic?

In the both cases, the co-ordination number of nickel is six with octahedral geometry. The oxidation number of nickel is +2 with 3d8 system.

Crystal field theory requires the splitting of the degenerate d-orbitals with the approaching of the ligands.

Therefore, under the influence of octahedral crystalfield, the d-orbitals are split up into set of energetically different orbitals.

One is energetically higher eg level and the other is energetically lower t2g level.

Now, in case of [Ni (NH3)6]2+complex ion, the ligand NH3 is neither a strong field ligand nor a weak field ligand. Actually, it is mild strong field ligand.

But in case of [Ni (NH3)6]2+ ion, ligand NH3 act as a weak field ligand as crystal field stabilization energy is less than pairing energy. That is, 10Dq < P.

Therefore, under the influence of octahedral crystal field, the electronic configuration is t2g6 eg2.

From the above electronic configuration, it has been found that the complex has two unpaired electrons. Hence the complex [Ni (NH3)6]2+ is paramagnetic.

On the other hand, in case of [Ni (CN) 6]4- complex ion, the ligand CN–is sufficiently strong field ligand. Hence, in this case the interactions are much more.

So, the single electron in dx2-y2 orbital is repelled by the four ligands whereas the electron in the dz2 orbital is repelled by only two ligands.

Thus, the energy of dx2-y2 orbital becomes higher than that of dz2 orbital.

Hence, in presence of a sufficiently strong field ligand the energy difference between these two orbitals exceeds the pairing energy of the electrons. That is, 10Dq > P.

Under this condition, in order to get a more stable configuration both eg electrons are paired up and occupy the low lying dz2 orbital leaving the dx2-y2 orbital empty.

Therefore, the preferred electronic configuration is dxz2, dyz2, dz22, dxy2, dx2-y20.

From the above electronic configuration, it has been found that the complex has no unpaired electrons. Hence the complex [Ni (CN) 6]4- is diamagnetic.

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