Limitations of VBT
This theory could not explain the quantisation of the magnetic data, existence of inner orbital and outer orbital complex, change of magnetic moment with temperature and colour of complexes.
Crystal Field Theory( eFT)
This theory was proposed by H. Bethe and van Vleck. Orgel. in 1952, applied this theory to coordination compounds. In this theory, ligands are treated as point charges in case of anions and dipoles in case of neutral molecules.
The five d-orbitals are classified as
( i) Three d-orbitals i. e., dxy, dyz and dzx are oriented in between the coordinate axes and are called t2g – orbitals.
( ii) The other two d-orbitals, i. e., d x 2- y 2 and d z 2 oriented along the x – y % axes are called eg – orbitals.
Due to approach of ligands, the five degenerate d-orbitals split. Splitting of d-orbitals depends on the nature of the crystal field.
[ The energy difference between t2g and eg level is designated by and is called crystal field splitting energy.]
By using spectroscopic data for a number of coordination compounds, having the same metal ions but different ligand, the crystal field splitting for each ligand has been calculated. A series in which ligand are arranged in order of increasing magnitude of crystal field splitting, is called spectrochemical series.