Coordination compounds have magnetic and optical properties. They also show colour. To explain these properties shown by coordination compounds H Bethe presented his Crystal Field Theory in 1929.
The main points of this theory are as follows:
1. The nature of bonding in the complexes is electrostatic i.e. the metal ligand bond is supposed to be ionic.
2.Anionic ligands are supposed negative points or point charges & neutral ligands are taken as point dipoles.
3.CFT is based on the concept of splitting of d-orbitals.
Partition of d-orbitals: The five d-orbitals are divided into two sets: t2g (dxy, dyz and dxz) and eg(dx2-y2 and dz2)
I.The crystal field splitting in octahedral coordination entities: In an octahedral coordination entity, there is repulsion between the electrons of d-orbitals of the central atom or ion and the electrons of the ligands.
The repulsion is more when d-orbitals are directed towards ligands, than when they are away from ligands. Since, eg orbitals which are along axes i.e. in the direction of ligands face more repulsion which finally leads the raising of energy of orbitals and the energy of t2g orbitals is lowered. Due to this, d-orbitals are splitted as follows:
The splitting of degenerate orbitals due to the presence of ligands is called crystal field splitting.
Crystal field splitting energy or crystal field stablising energy(CFSE): The energy required for the separation of d-orbitals is called CFSE. It is denoted by ∆o.