Valence bond structures for three-electron three-center and four-electron three-center bonding units: some further examples.

A formula is derived for the R-atom valence in the valence bond structure X.R:Y, with one overlapping atomic orbital per atomic center. Using this formula, the results of STO-6G valence bond calculations for linear H(3), HFH, FHF, and F(3) transition states show that for each molecule, the R-atom exhibits increased-valence or electronic hypervalence, that is, its valence exceeds that for the Lewis structure X R:Y, with no one-electron bond. Consideration is also given to (a) valence bond formulations for the conversion of the excited states of the reactants into the excited states of the products for the X() + R:Y --> X:R + Y() and X(:(-)) + R:Y --> X:R + Y(:(-)) reactions and the state correlation diagram for X() + R:Y --> X:R + Y(), (b) Kekule and Dewar increased-valence structures for D(6h) symmetry systems, and (c) the three-center molecular orbital interaction of the X(*) electron with the antibonding molecular orbital of R:Y, to form the one-electron X-R bond of X.R:Y.