Why do that ligands approach along axes, raise energies of orbitals pointing away from those axes?

I'm studying crystal field theory.

In the case of an octahedral complex, why do ligands that approach along the axes raise the energies of orbitals other than $\mathrm{d}_{x^2-y^2}$ and $\mathrm{d}_{z^2}$?

• It's not like ligands approach along the axes, it's the axes are chosen so as to point at the ligands. – Ivan Neretin Sep 25 '15 at 12:00
• Yes,that answers it,but all the d orbitals are not identical in shape,aren't they?The d-xy and the d-z^2 orbitals,for example,are,as I happen to understand,significantly different in shape. – Aditya Anand Sep 25 '15 at 12:04
• Shape of d-orbitals is somewhat of an abstraction. Anyway, the "true" orbitals are expressed in complex numbers, and these commonly used nice pictures are but linear combinations. They could have been combined differently. – Ivan Neretin Sep 25 '15 at 12:55