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.