Divalent silicon(0) compounds

Quantum-chemical calculations of the geometries and electronic structures of a series of dicoordinated silicon compounds SiL₂, 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 π and σ 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° 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 BH₃ ligands. The calculations predict that the first protonation of the divalent silicon(0) compounds takes place at the π lone-pair orbital, which yields protonated silylones that have a pyramidal arrangement of the ligands at the central tricoordinate silicon atom. Silylones SiL₂ could be interesting ligands for transition-metal compounds. The calculated structures and bonding situation of the analogous carbon compounds are also reported.