Divalent E(0) compounds (E = Si-Sn).

Quantum-chemical calculations at the BP86/TZVPP level of theory have been carried out for compounds EL(2) for E = Si, Ge, Sn, where L is a five-membered cyclic ylidene or N-heterocyclic ylidene. The theoretical results provide evidence for the classification of the complexes as divalent E(0) compounds, where the bonding situation is best described in terms of donor-acceptor interactions between a bare atom E, which retains its valence electrons as two lone pairs, and two donor ligands L --> E <-- L. The molecules are very strong donors, which may bind one or two Lewis acids. Divalent E(0) compounds have unusually high second proton affinities and they are strong sigma donor ligands. The calculations predict that complexes of EL(2) with one or two BH(3) ligands are stable enough to become isolated in a condensed phase. It is also shown that the bond dissociation energies (BDEs) of transition-metal complexes [(CO)(5)WD] and [(CO)(3)NiD], where D = EL(2) are rather high. The BDE of some ligands D are higher than those of CO in the metal carbonyls.