Quantum-chemical calculations of the geometries and electronic structures of a series of dicoordinated silicon compounds SiL(2), in which L is a five-membered cyclic species suggest that the molecules are divalent silicon(0) compounds that possess two L-->Si donor-acceptor bonds and two lone-pair MOs with pi and sigma symmetry at silicon. The classification as a dicoordinate silicon compound with L-->Si<--L donor-acceptor bonds applies not only to molecules in which L is an N-heterocyclic carbene but also when L is a cyclic silylene. The recently synthesized "trisilaallene" (S. Ishida, T. Iwamoto, C. Kabuto, M. Kira, Nature 2003, 421, 725), which has a bending angle of 136.5(o) for the central moiety, and which was written as Si=Si=Si, is probably better considered as a divalent silicon(0) compound. We suggest the name silylones for the latter species in analogy to silylenes which identify divalent Si(II) compounds. This bonding interpretation explains the theoretically predicted large values for the first and second proton affinities and for the large bond dissociation energies for one and two BH(3) ligands. The calculations predict that the first protonation of the divalent silicon(0) compounds takes place at the pi lone-pair orbital, which yields protonated silylones that have a pyramidal arrangement of the ligands at the central tricoordinate silicon atom. Silylones SiL(2) could be interesting ligands for transition-metal compounds. The calculated structures and bonding situation of the analogous carbon compounds are also reported.

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.