CO
The MO diagram for CO demonstrate a more difficult instance, helpful for understanding the bonding in carbonyl compounds like Ni(CO)4. Formation of the π MOs is simple to understand as there is only one pair of equivalent AOs on every atom. They can combine to create the bonding 1π (concentrated more on oxygen) and the antibonding 2π MOs. The σ MOs are more difficult as there are four AOs involved, 2s and 2pσ on each atom. They give 4 MOs labeled 1σ-4σ in Fig. 3. Their energies and forms can be understood by imagining the preliminary formation of two sp hybrids on every atom. The hybrids that point in the direction of each other overlap strongly and
combine to create the strongly bonding 1σ and strongly antibonding 4σ MOs. The other two hybrids each one point away from the other atom and so do not overlap strongly; they stay nearly nonbonding and create 'lone-pair' type orbitals, with 2σ localized on oxygen and the 3σ on carbon. The 10 valence electrons take the MOs as displayed. The highest occupied MO (HOMO) is the 'carbon lone-pair' 3σ, and the lowest unoccupied MO (LUMO) the antibonding 2π. The HOMO and the LUMO are called the frontier orbitals and can be employed to understand the interaction of a molecule with other species. Their approximate form in CO is displayed in Fig. 3. The bonding in CO comes mainly from two electrons in 1σ and four in the 1π, giving a bond order of three as predicted by the valence structure.