“Simple” MOs for 2p overlap Because σp overlap has increased electron density between nuclei, it would be expected to be lower in energy than pi overlap. There is some p-s mixing that occurs which complicates matters. The s orbital can mix with the p orbital with σ symmetry.

MO Diagrams for B2 Expected Actual

MO Diagram for N2 with p-s mixing

Summary of p-s mixing There is electron density in the p orbital along a given axis which can overlap with the s orbital of the other atom. The overlap of electron densities results in electron-electron repulsions which raise the energy of the σp bonding orbital. With increased nuclear charge there is a greater energy difference between the s and p orbitals and this overlap decreases.

How do we know? B2 is paramagnetic.

MO Diagrams for B2 through F2

Example 14.6 in the text Bond order: 2 2.5 1.5 Bond energy: 495 643 395 (kJ/mol)

What about NO? Odd number of electrons (like He2+, O2+ and O2–). Heteronuclear diatomic. Paramagnetic.

MO for HF Overlap with H1s and F2p. Unequal sharing.

Combining Models Simplicity of localized electron model (VBT) with delocalization characteristics of the MO model.

Combining Models Simplicity of localized electron model (VBT) with delocalization characteristics of the MO model.

“Electrons in a Box” E = n2h2/8mL2 ΔE = (nf2 – ni2)[h2/8mL2] ≈ 4.6 x 10–19 J nf = 6; ni = 5 L ≈ 12 Å = 1.2 x 10-9 m m = 9.10939 x 10–31 kg h = 6.62608 x 10–34 Js ΔE = hc/λ λ ≈ 4.3 x 10–7 m or about 420-440 nm

Hybridization vs. MO for Methane