CBSE Class 12-science Answered

According to the crystal field theory, the approaching asymmetric field of ligands destablises the five degenerate d-orbitals of transition metal and causes splitting of these orbitals into a triply degenerate, t2g orbital and a doubly  degenerate, eg orbital. The difference of energy between these two sets of d-orbitals is called crystal field splitting energy or crystal field stabilization energy (CFSE).

For example, in octahedral complex, 

 six ligands form an octahedron around the metal ion. In octahedral symmetry the d-orbitals split into two sets with an energy difference, Δ_oct  (which is a crystal field splitting parameter) where the d_xy, d_xz and d_yz orbitals will be lower in energy than the d_z² and d_x²_-y², which will have higher energy, because the former group is farther from the ligands than the latter. Therefore experience less repulsion. The three lower-energy orbitals are collectively referred to as t_2g, and the two higher-energy orbitals as e_g.