Nonbonding orbitals square planar complexes

The main difference between the d blocks of oaahedral and square-planar complexes concerns the number of nonbonding or weakly antibonding orbitals there are three in the former but four in the latter. 

The comparison on four-coordinate complexes provides us a very interesting illustration on the effect of d-p mixing on expected valence electron count. In a square planar complex, the nonbonding orbitals have minimal d-p mixings on the other hand, for a tetrahedral complex, we would expect stabilization of the bonding orbitals and destabilization of three nonbonding orbitals (mainly contributed by d y. 

Divalent late transition metals like cobalt (d ), nickel (d ), and copper (d ) in the first row of the d-block can use five 3d orbitals, one 4s orbital, and three 4p orbitals to form 4-, 5-, or 6-coordinate complexes. As a general rule, if there are N ligands in the first-shell coordination sphere of a transition metal complex, then there should be N bonding molecular orbitals, N anti-bonding molecular orbitals, and 9-N nonbonding molecular orbitals. Exceptions to this rule occur in some square-planar complexes in which three orbitals with the same symmetry properties overlap and form chemical bonds. Usually, some coordination sites in the first-shell of the... 

Exceptions to the E.A.N. rule do occur, particularly with d8 metal ions, where many examples of square-planar, 16-electron complexes are known. In these complexes, the high-lying pz orbital is nonbonding and remains empty. This deviation from the rule is often said to be due to the large s-to-p promotion energies found for the free atoms. As the atomic number increases across a given transition-metal series, the... 

Figure 6 Schematic MO diagram for a square planar low-spin CumL4 complex only sections with the occupied, weakly a-antibonding or nonbonding a -MO, the empty strongly a-antibonding (with respect to Cum) big (dx2-yi)-MO and occupied nonbonding, ligand-centered orbitals (w) are shown—cases of high...

The maintenance of an octahedral geometry within the RhFe3 (PPh)2(C5 Me5)(CO)8/R.hFe3(PPh)2(C5 Me5)(CO)9 interconversion, formally caused by a two-electron addition/removal, can be understood if one considers that, in these complexes, the electrons enter or leave nonbonding frontier orbitals centred on the square-planar tetrametal fragment stabilized by the interaction with the apical phosphorus atoms [89]. 

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