Four-Electron Three-Center Bonding:  One-Electron and Concerted Two-Electron Delocalizations into Bonding and Antibonding Molecular Orbitals

For a four-electron three-center bonding unit that arises from the interaction between an atomic electron donor Y and a diatomic electron acceptor A:B, it is deduced that some concerted delocalization of the two lone-pair electrons of Y into the antibonding molecular orbital of A:B is equivalent to the concerted delocalization of these electrons into a Y−A bonding molecular orbital. For neutral intermolecular electron donors and acceptors that involve a four-electron three-center bonding unit, second-order perturbation theory is used to deduce a condition for which concerted delocalization of the two Y electrons into the Y−A bonding molecular orbital may generate a higher energy than does a one-electron delocalization into this molecular orbital. The theory is illustrated via the results of STO-6G valence-bond studies for the hydrogen-bonding interactions that arise for an idealized HF dimer.