Question

Why do ligand electrons make occupancy of the e_g orbitals by metal electrons unfavourable?

Answer 1

In octahedral symmetry the d-orbitals split into two sets with an energy difference, Δ_oct (the 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 and 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.

_{Since the eg orbitals are directly pointing towards the ligands , as a result the electron density on ligand will repel with eg electrons . Thus it makes the electrons in eg orbitals unfavourable due to high repulsion.}