ИСТИНА

The idea on metal-containing nonbenzenoid aromatic systems was suggested in INEOS RAS by Mark Vol’pin. Namely, the three-membered metallacycle in complex Cp2Ti(PhC2Ph) [1] was considered as aromatic, its -electron delocalizes system involving two -electrons of carbon atoms and the vacant orbital of the Cp2Ti species. Thus any metal atoms with vacant p-orbital can theoretically participate in aromatic system. In this study, vibrational spectra and quantum-chemical results for aromatic compounds of four types (I-IV) will be presented and discussed. These are two-electron metallacyclopropenes (I) as well as six-electron metallacyclocumelenes (II) [2], Arduengo type N-heterocyclic unsaturated carbene analogs (III) [3], and their benzannulated derivatives (IV) [4]. Aromaticity of compounds of these types has been estimated using modern experimental and theoretical criteria, (vibrational and UV−vis spectroscopy, nucleus-independent chemical shifts, and isomerization stabilization energies). In spite of revealed complications due to specific properties of a metal atom, it can be concluded that the degree of aromaticity generally increases descending the group.