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 [n]triangulenes tend to be planar, while hybrids, where just a few
or even alla- orb-type carbon
atoms are silicon-substituted (sila-[n]triangulenes),
tend to be planar/nonplanar when compared with the unsubstituted carbon-[n]triangulenes. Because other spin states of the parent
carbon- and sila-[n]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.