Covalent, ionic and resonating single bonds

Three bond types of electron-pair bonding emerge from multi-structure valence bond (VB) computations of 10 different single bonds. The first bond type is observed in HH, LiLi, CH and SiH. These are all covalent bond types whose major bonding comes from the covalent Heitler-London (HL) configuration, with a minor perturbation from the resonance interaction between the covalent and zwitterionic (Z) configurations. The second bond type is observed for NaF. This is an ionic bond type in which the major bonding is provided by the electrostatic stabilization of the ionic configuration, Na⁺F^-, with a slight perturbation from the HLZ resonance interaction. The third bond type is observed for FF, HF, CF and SiF. These are the resonating bond types in which the major bonding event is the resonance energy stabilization due to the HLZ mixing. No special status should be attached to either the covalency or ionicity of these last bond types, even if they may appear purely "covalent", such as FF, or "highly ionic", as CF, by charge distribution criterion. The phenomenon of resonating bonding is shown to emerge from weakly bound or unbound covalent HL configurations which originate when the "preparation" for bonding of the fragments becomes energy demanding, as for fluorine. The mechanism of HL bond weakening is through costly promotion energy and overlap repulsion of a lone pair with a bond pair of the same symmetry. The essential requirements for a fragment A to qualify as a resonating binder are therefore: (a) to possess two AOs which maintain a very large energy gap between them, and which by virtue of overlap capability can both enter into bonding; and (b) to have three electrons in these two AOs which thereby mutually antagonize each other's bonding. The propensity for resonating bonding is discussed, in the light of these qualifications, for the main elements across the Periodic Table. It is concluded that the elements with the highest propensity for resonating bonding are F, O and N. Any combination AB where either A or B or both are resonating binders is likely to lead to a resonating bond (e.g. OO, NF, CF, CO, and so on). The resonating bonds are shown to coincide with the group of "weakened" bonds in the classification of Sanderson, and with those bonds which exhibit negative or marginally positive deformation densities in electron density determinations. Negative or marginally positive deformation densities may serve as the experimental signature of the theoretical concept of resonating bonding. The LiH bond appears to possess a special status. While the computations tend to classify this bond among the covalent types, the results also show that the HL and ionic Li⁺H^- configurations are nearly degenerate and maintain a very weak coupling. Therefore the LiH bond will have a metastable character, as far as ionicity-covalency, in the presence of medium perturbations which are at least of the magnitude of the coupling between the ionic and covalent structures.