Let us explain this splitting in different crystal fields ( a ) Crystal field splitting in octahedral coordination entities
In an octahedral coordination entity with six ligands surrounding
the metal atom/ion, there will be repulsion between the electrons in
metal d orbitals and the electrons (or negative charges) of the ligands Such a repulsion is more when the metal d orbital is directed towards
the ligand than when it is away from the ligand Thus, the 
2
2
x
y
d
and
dz2
 orbitals which point towards the axes along the direction of
the ligand will experience more repulsion and will be raised in
energy; and the dxy, dyz and dxz orbitals which are directed between
the axes will be lowered in energy relative to the average energy in
the spherical crystal field