Scientists have synthesized a stable pentasilacyclopentadienide (Si5R5−, where R is aryl), which is a negatively charged five-membered silicon ring similar to the known carbon cyclopentadienide.[1] This element is essentially planar and exhibits Hückel aromaticity according to the 4n+2 π electron rule.[1] Since 1981, when it was possible to stabilize Si–Si π-bonds, the goal of research has been exclusively silicon aromatics, but so far only a three-membered analogue of cyclo-propene (Si3R3+) has been experimentally achieved.[1] They characterized the composition both in the solid state and in solution, where it showed signs of aromaticity in the electronic structure.[1] Experimental and computational data indicate an ultrafast equilibrium between planar and non-planar isomers.[1] Upon reaction with trimethylchlorostannate (Me3SnCl), the Si5 core rearranges to a three-membered isomer, which confirms the key role of the lithium cation for stability.[1] This substance promises rich chemistry similar to carbon cyclopentadienides, and its structural flexibility challenges the clear distinction between resonance and equilibrium.[1]