Increased valence (qualitative valence bond) descriptions of the electronic structures of electron-rich fluorine-containing molecules

Valence bond (VB) descriptions of the electronic structures of molecules provide a valid domain of research which is receiving increasing attention. Many bench chemists are interested in qualitative VB representations of electronic structure. In this review, we are concerned primarily with how to represent the electronic structures of many electron-rich fluorine-containing molecules via VB methods. Without valence shell expansion for atoms of second-row and higher row elements, our review provides VB structures for the following molecules: FOOF, FSSF, F2SS, F2SO, FNO, FSN, FNO2, F3NO, F3CO“, PF5, SF4, ClF3, XeF2, SF6, ClF5, XeF4, BrF6”, IF7, (HF)2, HF2“ and F3”, via the use of increased-valence representations for the four-electron three-centre bonding units which are present in each of them. Increased-valence structures are equivalent to resonance between familiar standard or Kekule’-type Lewis structures and “long-bond” or Dewar-type Lewis structures. Consideration is also given to several hypercoordinate molecules whose bond lengths suggest that valence shell expansion does occur in the primary VB structures. The results of density functional estimates of the energies and geometries for several pairs of fluorine-containing isomers are also reported.