Qualitative valence-bond descriptions of bonding for S-N, S-O, S-F and S-S compounds via increased-valence structures

Abstract

Valence-bond (VB) structures are developed for a variety of molecular species with S-S, S-O, S-Y (Y = H, CH 3, or halogen) and S-N bonds, and one or more sets of three-electron two-centre, four-electron three-centre and six-electron four-centre bonding units. The valence bond structures that are mostly considered are of the increased-valence type, with one-electron bonds, fractional electron-pair bonds and normal electron-pair bonds. If the sulphur atoms do not utilize their 3 d atomic orbitals as valence orbitals in the increased-valence structures, then for many species these structures may be obtained by bonding together the ground states of their diatomic and monatomic components. The participation of sulphur 3 d orbitals in bonding as valence orbitals in the primary VB structures is assumed to occur when a sulphur atom carries a formal charge of + 2 in the Kekulé-type octet Lewis structures, i.e. when either ▪ occurs in these structures. On the basis of bond-length considerations, it is suggested that expansion of the sulphur valence shell occurs in R 2SO type compounds, with R = alkyl or aryl. In the primary increased-valence structures with no sulphur valence shell expansion, the S-O and S-S double bonds are represented as S÷O and S÷S, respectively, instead of SO and SS. The increased-valence structures are equivalent to resonance between Kekulé- and Dewar-type Lewis structures; therefore they must always provide VB representations which are lower in energy than those obtained when (as is usually done) only Kekulé-type structures are considered. Inclusion of an “increased-valence” component in a valence bond structure reduces the extent to which the sulphur 3 d orbitals are required as valence orbitals in order that the formal charge on a sulphur atom be small in magnitude.