Lowest unoccupied molecular orbital . square-planar

Figure 15 depicts the splittings of the central metal d-orbitals for (a) an octahedral and a distorted octahedral compound and for (b) a square planar and a distorted square planar compound. For this model discussion, one 7i -orbital is introduced to represent the lowest unoccupied molecular orbital (LUMO). In these simplified schemes, other unoccupied 7i -orbitals and occupied 7i-orbitals are neglected for clarity. Thus, all considered electronic states are MLCT states with a central metal d-orbital as highest occupied molecular orbital (HOMO). 

Square-planar bis(dithiolene) complexes have also been the subject of theoretical investigations (43-46). For example, density functional theory (DFT) calculations indicated that the highest occupied molecular orbital (HOMO) for Ni(S2C2H2)2 is primarily a ligand-based orbital comprising of four 3pz orbitals of sulfur, perpendicular to the molecular xy plane, and four 2p-orbitals of carbon with opposite phases. The lowest unoccupied molecular orbital (LUMO) is a mix of ligand-metal orbitals, but still mostly of the ligand... 

One always begins with the monomer. What are its frontier levels The classical crystal field or molecular orbital picture of a square planar complex (Fig. 2) leads to a 4 below 1 splitting of the d block.11 For 16 electrons we have z2, xz, yz, and xy occupied and x2-y2 empty. Competing with the ligand field-destabilized x2-y2 orbital for being the lowest unoccupied molecular orbital (LUMO) of the molecule is the metal z. These two orbitals can be manipulated in understandable ways x acceptors push z down, x donors push it up. Better a donors push x2-y2 up. 

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