Limitations of valence bond theory

Valence bond theory, for all its successes, has three specific, well-documented limitations. First, it explains only the ORDINARY covalent bond, where each of the two bonded atoms contributes exactly one electron to the shared pair; it offers no account at all of a CO-ORDINATE (dative) covalent bond, in which BOTH electrons of the shared pair are contributed by just ONE of the two bonded atoms. Second, VB theory predicts that the O2 molecule should be DIAMAGNETIC — since a simple Lewis/VB picture fills every valence orbital on both oxygen atoms, leaving no unpaired electrons — yet O2 is experimentally found to be PARAMAGNETIC, carrying two unpaired electrons; VB theory has no way to explain this mismatch, and molecular orbital theory (section 5.5, specifically the worked O2 configuration in 5.5.6) is needed to correctly account for it. Third, VB theory cannot explain the bonding found in ELECTRON-DEFICIENT mo …