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

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 e_g level and the other is energetically lower t_2g 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 t_2g6 e_g2.

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 d_x2-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 d_x2-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 e_g electrons are paired up and occupy the low lying dz² orbital leaving the d_x2-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.

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

Why is [Ni (NH₃)₆]²⁺ ion paramagnetic?

Why is [Ni (CN) ₆]^4- ion diamagnetic?

What is the hybridization of [Ni (NH₃)₆]²⁺ and [Ni (CN) ₆]^4- complex ions?

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