For a four-electron three-center bonding unit that arises from the interaction between an atomic electron donor ii and a diatomic electron acceptor A:B, it is deduced that some concerted delocalization of the two lone-pair electrons of (Y)double over dot 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 accepters 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)double over dot 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.