# Why do t2g set of orbitals have lower energy in octahedral coordination compunds, but its the opposite in tetrahedral compounds?

In Octahedral compounds ,Ligands are directed towards the axis . Since eorbitals i.e (dx2-y2 and dz2) are directed towards axis thus they face strong repulsion from ligands approaching towards them and hence their energy becomes higher than that of t2g orbitals.Figure below shows how the ligands are approaching in case of octahedral.
In first image ligands approaching towards eg orbitals is shown.
In second image ligands approaching towards t2g orbital(dxy,dyz,dzx) is shown.

Now let us take a case of d orbital splitting in tetrahedral complex. In tetrahedral geometry when ligands approach towards d orbitals of central metal atom,they are directed more towards the t2g set of orbitals that is dxy,dyz,dzx as compared to that of eg orbitals.Figure below shows the position of ligands with respect to that of d orbitals of central metal atom.
first image shows the ligands approaching towards t2g orbitals
second image shows the direction of ligands approaching with respect to eg orbital.

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