# Orbital mixing in square planar D4h complexes

Consider a square planar $\ce{ML4}$ complex with $D_\mathrm{4h}$ symmetry. Which orbital should the $\mathrm{a_{1g}}$ SALC of the ligands mix with, $\mathrm{3d}_{z^2}$ or $\mathrm{4s}$? Both of those AOs transform as $\mathrm{A_{1g}}$.

• Why can't all three orbitals mix? – Ben Norris Sep 30 '15 at 16:29
• Could you refresh my memory in the meaning of SALC? – Jan Sep 30 '15 at 16:33
• Symmetry adapted linear combinations. – user21417 Sep 30 '15 at 16:36
• How should I draw the MO diagram if all three mix together? – user21417 Sep 30 '15 at 16:36

So the ligand SALC will interact with both $\mathrm{a_{1g}}$ orbitals on the metal. The degree of interaction will depend on how close in energy the ligand $\mathrm{a_{1g}}$ is to the $\mathrm{4s}$ or $\mathrm{3d}_{z^2}$ orbitals on the metal.
Generally, the ligand orbitals are closer in energy to the $\mathrm{3d}_{z^2}$ orbital than to the $\mathrm{4s}$.