Molecular orbital
Molecular orbital calculations have elucidated the relative energies of the orbitals created by these bonding interactions. As shown in the figure below, the lowest energy orbitals are the π bonding orbitals followed by the σ bonding orbital. The next highest are the δ bonding orbitals which represent the HOMO. Because the 10 valence electrons of the metals are used to fill these first 5 orbitals, the next highest orbital becomes the LUMO which is the δ* antibonding orbital. Though the π and δ orbitals are represented as being degenerate, they in fact are not. This is because the model shown here is a simplification and that hybridization of s, p, and d orbitals is believed to take place, causing a change in the orbital energy levels.