Carbon-[]Triangulenes and Sila-[]Triangulenes: Which Are Planar?

Using our recently suggested concept of a quasi-molecule ("tile") and, in the case of the planarity here at stake, its generalization to larger than tetratomics, we explain why carbon []triangulenes tend to be planar, while hybrids, where just a few or even all - or -type carbon atoms are silicon-substituted (sila-[]triangulenes), tend to be planar/nonplanar when compared with the unsubstituted carbon-[]triangulenes. Because other spin states of the parent carbon- and sila-[]triangulenes tend to correlate with the same tiles, it is conjectured that no structural changes are expected to depend on their spin state. Other polycyclic and sila-compounds are also discussed.