According to Crystal Field Theory (CFT), when ligands approach the central metal in an octahedral geometry (along the x, y, z axes):

• The $d_{x^2-y^2}$ and $d_{z^2}$ orbitals point directly at the ligands and experience greater repulsion. Their energy increases; they are called $e_g$ orbitals.
• The $d_{xy},d_{yz},$ and $d_{zx}$ orbitals point between the axes, experiencing less repulsion. Their energy is lower; they are called $t_{2g}$ orbitals.
• This separation of energy levels is known as Crystal Field Splitting (${\Delta }_o$).