Question

If a spherical ligand field is replaced by an octahedral ligand field, what changes
in orbital energies will accompany this?

Answer 1

Spherical ligand field acts uniformly from all directions hence all orbitals are destablized equally.

When spherical field is replaced by octahedral field, the ligands thus approch via the vertices of an octahedron. Hence the dz2 and dx2-y2 orbitals are more destabilized as they are directed along the x,y,z axes and thus experience more repulsion with the incoming ligand field. The dxy, dyz, dxz orbitals on the other hand are directed in between the axes and thus don't experience any direct repulsion from the approaching ligand field and are thus stabilized. The total stabilization and destabilization remain equal.

Refer image below for pictorial representation