Sir,please explain crystal field theory and why is the energy of eg set greater than t2g set?

The Crystal field theory considers ionic bond arising purely from electrostatic interaction between metal atom (positively charged) and ligands ( negatively charged). The transition metal ion is considered to be positive charge equal to its oxidation state. The central metal atom is surrounded by negatively charged ligands. The ligands were considered as point charges.

The energy of d orbitals remains degenerate (same energy) in the free transition metal ion . When ligand approach the metal ion, there arises attraction between the nucleus of the metal ion and the ligands. In the same time, there will be repulsion between the electrons present in the metal atom and the negative ends of the ligands. Due to this repulsion, the energy of the metal ion will no longer be degenerate. Consequently, the degeneracy is split.

The energy of e_gset greater than t₂g set

The e_g set consists of the d_x2-y2 and d_z2 orbitals. The lobes of these orbitals point along the axes x, y and z.The lobes of t₂g orbitals (d_xy, d_yz, d_yz) point in between the axes. The energy of the e_g orbital is high because the orbitals d_x2-y2 and d_z2 falls directly along the path of the incoming ligand . Therefore they experience great force of repulsion and have higher energy than t_2g.