[AdiDex]

How we came to know ligand should approach from t_2g orbitals in d⁵ configuration for tetrahedral complexes? What will be the difference in energy if ligand approach from e_g ?
As far as I can think of, The major driving force for formation complex is the Nucleus-Ligand interaction and the opposing minor force is ligand's e^--Metal's e^- but important for determining the coordination no. and orientation.  The field of nucleus should be spherically symmetrical so It can't be the reason.
So we've left with only one reason which is Ligand's e^--Metal's e^- interaction .Does CFT have any answer to it? 

A similar question can be asked for Octahedral, but we know it is associated with CFSE, so it plays an important role for orientation.

But what difference will it make in octahedral complexes if we assume that ligand will approach from x', y' and z' (particularly in case of d⁴ configuration)
Let's assume it is a weak ligand
in this case, the CFSE should be -Δ'2/5 , as e_g orbitals, will be in lower energy state , but if ligand approaches from x,y and z then CFSE will be -Δ3/5 .
Is this the reason , that ligand will always approach from x,y and z .