Donor-Acceptor Complexes of Main-Group Elements

This chapter deals with the nature of the bonding in one-center and two-center donor-acceptor complexes of main group elements. The one-center complexes EL2 have a single atom E which binds to two σ-donor ligands L through donor-acceptor interactions L→E←L. The examples which are discussed in this chapter are tetrylones EL2, where E=C-Pb, which possess two electron lone-pairs at the divalent E(0) atom. Tetrylones exhibit a distinctively different reactivity from tetrylenes ER2 which have E-R electron-sharing bonds and only one electron lone-pair at the divalent E(II) atom. Further examples of the one-center adducts that are discussed are borylene complexes (BH)L2 and the nitrogen cation complexes (N+)L2. We also discuss the bonding situation in two-center complexes E2L2 that contain a diatomic molecule E2 (E=Si-Pb, B, N), which binds two σ-donor ligands L in a linear or anti-periplanar arrangement L→EE←L. It is shown that the donor-acceptor bonding model nicely explains the structures and chemical reactivity of the one- and two-center complexes. The nature of the donor-acceptor interactions is investigated with modern methods of charge and energy decomposition analyses. The results provide a bridge between the physical mechanism of bond formation and molecular orbital models, which are quantified and complemented by considering electrostatic attraction and Pauli repulsion. It is shown that a bonding analysis not only gives insight into the nature of the chemical bonds, but also supplies suggestions for new compounds, which possess unusual bonds, thereby extending the realm of classical Lewis structures.