Divalent silicon(0) compounds.

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.