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 system s involve electron-pair bonds and lone-pairs of electrons, increased-valenc e 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 equi valent to resonance between several Lewis structures, including the familia r Lewis structure from which it was derived, and therefore it is more stabl e than the latter structure. Some of the properties of increased-valence st ructures, and their associated Pauling three-electron bonds as diatomic com ponents of these structures, are discussed, with specific reference to N2O and the (FeO2)-O-II linkages of oxyhaemoglobin. Applications to numerous ot her systems that involve 4-electron 3-centre, 6-electron 4-centre, and 6-el ectron 5-centre bonding units are presented. Brief consideration is also gi ven to a speculative Bohr approach to electronic structure.