Ring currents in silicon tetramer (Si4, Si42+) and planar tetracoordinate carbon doped cluster Si4C2+: σ versus π aromaticity

We revisited the aromaticity of silicon tetramer Si4, Si42+ and isoelectronic four-membered rings without and with a planar tetracoordinate carbon dopant (Si4C2+). Electron localizability indicators and magnetic ring currents were used to probe bonding patterns. Comparison with Al42−, Si2Al2, Al4C2− and Si2Al2C was performed. The 14 (Si42+), 16 (Si4) and 18 (Si4C2+) valence electrons systems exhibit diatropic ring current, but this is determined by σ electrons. Electrons in π orbitals do not significantly take part in the diatropic magnetic response which determines aromaticity. These clusters can thus be regarded as σ-aromatic species and do not follow the classical Hückel rule.