ABSTRACT The symbolisms used by Shaik and Hiberty (2003, Helvet. Chim Acta, 86, 1063) to provide valence-bond (VB) structures for the Pauling three-electron bond (or three-electron two atomic orbital bond) and the ground-state of O2, are compared with those provided by Green and Linnett (1960, J. Chem. Soc. 4949). The incorporation of the Green-Linnett VB structure for O2 into the VB structures for numerous diamagnetic as well as paramagnetic tri- and polyatomic molecules, to generate increased-valence structures, is re-described. Illustrative examples include molecular systems of the types R-O2 and R-O2-R, with for either or both species, R = F, NO, CoII(S = 1/2), O(S = 1), FeII(S = 1), O2(S = 1), and CuI(S = 1). The use of increased-valence structures to provide VB formulations of SN2 reactions is also redescribed. The origin of the existence of a triplet-spin (S = 1) ground-state for O2 is re-examined, with attention given to two-electron delocalisation (exchange) resonance which is associated with reduced electron-electron repulsion.
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