[Ni(Cl)4]2- is paramagnetic while [Ni(Co)4]is diamagnetic though both are tetrahedral. Why?
In [Ni(Cl)4]2- ion, Cl- is a weak field ligand so it will not pair the unpaired electrons of Ni+2 ion.
Electronic configuration of Ni is: [Ar]3d84s2 where [Ar] = 1s22s22p63s23p6
Electronic configuration of Ni+2 = [Ar]3d8
Outer electronic configuration of Ni+2 = 3d8
Overall charge balance:
X + 4(-1) = -2
X = + 2.
Therefore it undergoes sp3 hybridization. So it will have tetrahedral geometry.
Since there are 2 unpaired electrons in the d orbital so it is a paramagnetic compound.
Overall charge is neutral and oxidation state of Ni can be calculated as:
X + 4(0) = 0
X = 0
Ni is in zero oxidation state.
Co is a strong field ligand it causes pairing of the 4 unpaired electrons in d orbital. Also, it causes the 4s electrons to shift to the 3d orbital, thereby undergoes sp3 hybridisation forming tetrahedral geometry. Since it has no unpaired electron, therefore it is diamagnetic compound.