IIT JEE Advanced 2011 Paper 1

In NiCl42− :
Ni (28) = [Ar] 3d84s2
NI2+ = [Ar]3d8
[NiCl4]2− : 3d :

↑↓

↑↓

↑↓

↑

↑

4s :

x

4p :

x

x

x

Cl− being a weak ligand will not cause pairing of electrons, so Ni2+ will show sp3 hybridisation and the geometry of the complex will be tetrahedral.
In [Ni(CN)4]2− :
Ni2+ = [Ar]3d8 :

↑↓

↑↓

↑↓

↑

↑

3d :

↑↓

↑↓

↑↓

↑↓

x

4s :

x

4p :

x

x

Here pairing of electrons takes place as CN− is a strong field ligand. Therefore Ni2+ will show dsp2 hybridisation and the geometry of the complex is square planar.
In Ni(H2O)
[Ni(H2O)6]2+ = Ni2+ = [Ar]3d8
4s :

↑↓

↑↓

↑↓

↑↓

x

4p :

x

x

x

4d :

x

x

As H2O is a weak field ligand, no pairing of electrons will take place and Ni2+ will show sp3d2 hybridisation and the geometry of the complex is octahedral.