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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 $\mathrm{A_{1g}}$.

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  • 1
    $\begingroup$ Why can't all three orbitals mix? $\endgroup$ – Ben Norris Sep 30 '15 at 16:29
  • $\begingroup$ Could you refresh my memory in the meaning of SALC? $\endgroup$ – Jan Sep 30 '15 at 16:33
  • $\begingroup$ Symmetry adapted linear combinations. $\endgroup$ – user21417 Sep 30 '15 at 16:36
  • $\begingroup$ How should I draw the MO diagram if all three mix together? $\endgroup$ – user21417 Sep 30 '15 at 16:36
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All orbitals of the same symmetry will interact (mix) to some degree.

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 $\mathrm{4s}$.

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