Increased-Valence Structures for Qualitative Valence-Bond Representations of Electronic Structure for Electron-Rich Molecules

Whereas familiar Lewis-type valence-bond structures for singlet-spin systems involve electron-pair bonds and lone-pairs of electrons, increased-valence structures involve one-electron bonds and fractional electron-pair bonds. An increased-valence structure may be easily derived from a familiar Lewis structure via one-electron delocalizations of lone-pair electrons into two-centre bonding molecular orbitals. The increased-valence structure is equivalent to resonance between several Lewis structures, including the familiar Lewis structure from which it was derived, and therefore it is more stable than the latter structure. Some of the properties of increased-valence structures, and their associated Pauling three-electron bonds as diatomic components of these structures, are discussed, with specific reference to N2O and the FeIIO2 linkages of oxyhaemoglobin. Applications to numerous other systems that involve 4-electron 3-centre, 6-electron 4-centre, and 6-electron 5-centre bonding units are presented. Brief consideration is also given to a speculative Bohr approach to electronic structure.