The Extended-Huckel Perturbation-Theory and the Concept of Orbital Balance

The Huckel perturbation molecular orbital theory is briefly discussed with particular reference to the importance of pi-complexes and orbital following. A consideration of Extended-Huckel perturbation theory leads to a general concept termed orbital balance, which is based on the relative magnitude of the two-electron and four-electron terms. This concept is applied in a qualitative (or in special cases in a semi-quantitative) manner to molecular conformation, gas-phase acidities and nucleophilic substitution, and also to the E1 CB elimination process. The limitations of this approach are also discussed.