Ring contraction of metallacyclobutadiene to metallacyclopropene driven by π- and σ-aromaticity relay

π-Aromaticity is an important driving force in directing the synthesis of aromatic compounds; contrast, reactions induced by σ-aromaticity are uncommon. Here we report a strategy based on π- and relays to realize structurally defined ring contraction metallacyclobutadiene metallacyclopropene. This reaction involves release π-antiaromaticity afford π-aromatic intermediate, followed reclosure generate σ-aromatic The opening–reclosing mechanism versatile switching aromaticity metallacyclic species supported experimental results theoretical calculations. work demonstrates importance relay with successive decrease energy will stimulate efforts exploiting vital role synthetic chemistry. Reactions uncommon compared those π-aromaticity. Now, developed form