8.4 Molecular orbital theory  (Page 7/26)

As we saw in valence bond theory, σ bonds are generally more stable than π bonds formed from degenerate atomic orbitals. Similarly, in molecular orbital theory, σ orbitals are usually more stable than π orbitals. However, this is not always the case. The MOs for the valence orbitals of the second period are shown in [link] . Looking at Ne ₂ molecular orbitals, we see that the order is consistent with the generic diagram shown in the previous section. However, for atoms with three or fewer electrons in the p orbitals (Li through N) we observe a different pattern, in which the σ _p orbital is higher in energy than the π _p set. Obtain the molecular orbital diagram for a homonuclear diatomic ion by adding or subtracting electrons from the diagram for the neutral molecule.

This switch in orbital ordering occurs because of a phenomenon called s-p mixing    . s-p mixing does not create new orbitals; it merely influences the energies of the existing molecular orbitals. The σ _s wavefunction mathematically combines with the σ _p wavefunction, with the result that the σ _s orbital becomes more stable, and the σ _p orbital becomes less stable ( [link] ). Similarly, the antibonding orbitals also undergo s-p mixing, with the σ _s* becoming more stable and the σ _p* becoming less stable.

s-p mixing occurs when the s and p orbitals have similar energies. When a single p orbital contains a pair of electrons, the act of pairing the electrons raises the energy of the orbital. Thus the 2 p orbitals for O, F, and Ne are higher in energy than the 2 p orbitals for Li, Be, B, C, and N. Because of this, O ₂ , F ₂ , and N ₂ only have negligible s-p mixing (not sufficient to change the energy ordering), and their MO diagrams follow the normal pattern, as shown in [link] . All of the other period 2 diatomic molecules do have s-p mixing, which leads to the pattern where the σ _p orbital is raised above the π _p set.

Using the MO diagrams shown in [link] , we can add in the electrons and determine the molecular electron configuration and bond order for each of the diatomic molecules. As shown in [link] , Be ₂ and Ne ₂ molecules would have a bond order of 0, and these molecules do not exist.