Explain about crystal field theory.

Explanation

`(1)` Crystal field theory assumes that the bond between the ligand and the central metal atom is purely ionic. i.e., the bond is framed due to the electronstatic attraction between the electron rich ligand and electron deficient metal.
`(2)` In the coordination compounds, the central atom / ion and the ligands are considered as point charges (or) electric dipoles.
`(3)` According to crystal field theory, the complex formation is considered as the following series of hypothetical steps.
Step 1 : In an isolated gaseous state, all the five d orbitals of the central metal ion are degenerate. Initially , the ligands from a spherical field of negative charge around the metal. In this field, the energies of all the five d orbitals will increase due to the repulsion between the electrons of the metal and the ligand.
Step 2 : The ligands are approaching the metal atom in actual bond directions. Consider an octahedral field, in which the central metal ion is located at the origin and six ligands are coming from the `+x`, `-x`, `+y`, `-y`,`+z` and `-z` directions. The orbitals lying along the axes `dx^(2)-y^(2)` and `dz^(2)` orbitals will experience strong repulsion and raise in energy to a greater extent than the orbitals with lobes directed between the axes `(dxy,dyz "and" dxz)`. Thus the degenerate d orbitals now split into two sets and the process is called crystal field splitting.
Step 3 : Upto this point the complex formation would not be favoured. However when the ligands approach further, there will be an attraction between the negatively charged electron and the positively charged metal ion that resulta in a net decrease in energy. This decrease in energy is the driving force for the complex formation.