Explain why the Crystal Field Theory is not applied to complexes of main group metals ?
Crystal field theory describes the breaking of orbital degeneracy in transition metal complexes due to the presence of ligands. Crystal field theory qualitatively describes the strength of the metal-ligand bonds.
When applied to alkali metal ions containing a symmetric sphere of charge, calculations of bond energies are generally quite successful.
This approach leads to the correct prediction that large cations of low charge, such as K+ and Na+, should form few coordination compounds. For transition metal cations that contain varying numbers of d electrons in orbitals that are not spherically symmetric, however, the situation is quite different. The shape and occupation of these d-orbitals then becomes important in an accurate description of the bond energy and properties of the transition metal compound.