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 .