Limitations

The model explained in this section can be justified theoretically/ hypothetically using the quantum mechanical valence bond theory. However, there are several molecules where bonding cannot be explained simply in terms of electron pairs localized among two atoms. Diborane is an instance. The structure (16) as frequently drawn appears to have eight bonds

and so would seem to need 16 valence electrons. Actually, there are only 12 and the molecule is sometimes explained as electron deficient. The electrons' two pairs form three-center bonds each linking two boron atoms and a bridging hydrogen, as demonstrated in the preferable way of drawing the valence structure in 16′. Transition metal chemistry is other area where bonding frequently cannot be explained in terms of localized electron pairs. The resonance concept is one way of conquering some of the restrictions of the localized electron pair model, but such type of cases are treated more naturally by molecular orbital theory, that is not limited to bonds involving two atoms.