Write a note on crystal field theory.

(N/A) The crystal field theory $(CFT)$ is an electrostatic model which considers the metal-ligand bond to be ionic,arising purely from electrostatic interactions between the metal ion and the ligand.
If the ligands are anions,they are treated as point charges,and if they are neutral molecules,they are considered point dipoles.
The five $d$-orbitals in an isolated gaseous metal atom/ion have the same energy,i.e.,they are degenerate. This degeneracy is maintained if a spherically symmetrical field of negative charges surrounds the metal atom/ion.
However,when this negative field is due to ligands (either anions or the negative ends of dipolar molecules like $NH_3$ and $H_2O$) in a complex,it becomes asymmetrical,and the degeneracy of the $d$-orbitals is lifted,which results in the splitting of the $d$-orbitals. The pattern of splitting depends upon the nature of the crystal field.