Let us assume that the six ligands are positioned symmetricallyalong the Cartesian axes, with metal atom at the origin.As the ligands approach, first there is an increase in energy of d-orbitalsrelative to that of the free ion just as would be the case in a sphericalfield.
The orbitals lying along the axes (dz2 and dx2−y2 ) get repelled more stronglythan dxy,dyz and dzx orbitals which have lobes directed between the axes.The dz2 and dx2−y2 orbitals get raised in energy and dxy,dyz,dzx orbitals arelowered in energy relative to the average energy in the spherical crystalfield. Thus, the degenerate set of d-orbitals get split into two sets : thelower energy orbitals set, t2g and the higher energy orbitals set, eg. Theenergy is separated by Δo.