Aspects of hypercoordination and one-electron delocalizations via increased-valence structures

Abstract

Consideration is given to increased-valence descriptions of bonding for four-electron three-centre and three-electron three-centre bonding units, with special attention given to hypercoordinated AH 5 − and AH 5 systems with D 3h symmetry. Increased-valence structures of types I–IV ▪ accommodate the nearest-neighbour bonding electrons in localized molecular orbitals (MOs). When the A-atom orbital in I and II includes some s as well as pa character in the formation of the one-electron bonds, it is shown that each of these structures is equivalent to resonance between one of the Lewis structures V or VI and four structures in which an electron is transferred ▪ from the H − of the Lewis structure into one of the four A-H antibonding σ MOs that are vacant in this structure. Using increased-valence structures which are similar to I–IV, master equations are provided for S N2 and radical substitution reactions via one-electron delocalizations or shifts, as in ▪ for the latter type of reaction. The associated state correlation diagrams and valence bond (VB) representations for these reactions are compared with those provided by Shaik and Press. Valence bond representations are also provided for (a) electron conduction in an n-type semi-conductor and the YBa 2Cu 3O 7 superconductor, and (b) hydrogen bonding in the N 20 + HF complex.