Pentasilacyclopentadienide: A Hückel aromatic species at the border of resonance and equilibrium

Aromatic rings such as the ubiquitous π-ligand cyclopentadienide (C₅H₅^-) adhere to the famous Hückel rule with their 4n+2 π cyclically delocalized electrons. Since the advent of stable Si-Si π-bonds in 1981, all-silicon aromatics have been targeted, yet only a three-membered cyclopropenium analog (Si₃R₃⁺; R, silyl) has been experimentally realized. We report a stable persilacyclopentadienide (Si₅R₅^-; R, aryl), which is essentially planar and decidedly aromatic, although experimental and computational data suggest an ultrarapid equilibrium between nonplanar isomers. With trimethylchlorostannane, Si₅R₅^- rearranges the Si₅ core to a tricyclic isomer confirming the pivotal role of the lithium cation for stability. The persilacyclopentadienide promises a rich chemistry akin to carbon-based cyclopentadienides. Its structural flexibility questions the paradigm of a clear-cut distinction between resonance and equilibrium.