Aromatic Systems:
Aromatic compounds are cyclic strucures containing a continuous ring of electron density above and below the ring, formed from p orbitals located on the nuclei of the ring. Electrons placed in these molecular orbitals are deloaclized, meaning they are not localized to any specific section of the ring. This does not preclude however the existence of nodal surfaces along the orbital, as will be shown here in specific examples. Such simply means that the phase of the wave function for the electron in this orbital changes sign with respect to position.
These are the molecular orbitals of the p system of cyclopentadienyl anion (C5H5- or Cp), a common ligand molecule.
The second and third orbitals are doubly degenerate, meaning there exist two of each and within each pair the phases of the wave function are reversed relative to each other. It readily donates electron density through p and d bonds to metal cations, forming metallocenes.
The most well-known aromatic species is benzene.
It is a six-membered ring of carbon atoms with many interesting properties due to the overlap of p orbitals on the carbons forming a continuous ring of electron density.
The molecular orbitals of the p system of benzene
The orbitals containing one and two nodal surfaces are doubly degenerate.
There are inorganic analogues of benzene. One is borazine:
Borazine has the same colligative properties as benzene, however the two are very different chemically. The electron density along the boron-nitrogen p bond is not distributed evenly, due to the difference in electronegativities between the two types of atoms. Therefore, the molecular orbitals of the p system are lumpy in apperence. This uneven distribution makes borazine prone to additon reactions, making it as a molecule less stable than benzene.
Probability Distributions for borazine (left) and benzene (right)