Highest occupied molecular orbital energy average

In 1934, Mulliken proposed a different electronegativity scale based upon the average of the ionization potential in volts (IP X X + e ) and the electron affinity (EA X + e" X ) of the gas-phase atoms. This approach is more in character with Mulliken s MO approach since IP reflects the energy of the highest occupied molecular orbital (HOMO) while the EA has a relationship to the energy of the lowest unoccupied molecular orbital (LUMO). Not surprisingly, there has been continued debate ever since over the superiority of these two and numerous later electronegativity scales. 

Fig. 7.17. Molecular-orbital energy-level diagrams for the (a) FeO, " and (b) FeO, clusters calculated using the MS-SCF-Ya method by Tossell (1976a). The variation in MO energies is shown as Fe-O distance (7 ) is decreased to simulate the effects of increasing pressure. The horizontal line corresponds to a constant l/,g6t, .l 2u average energy. Crosses represent O 2p nonbonding orbitals open circles represent other orbitals in FeO ". The highest occupied orbital is 3Cg with two electrons in FeO, it is 2t2 with one electron (after Tossell, 1976a reproduced with the publisher s permission).

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