The product formed by the @A00512@ of an @E02021@ or @N04251@ @E02130@ or of a @R05066@ to a ring carbon of an aromatic species so that a new σ-bond is formed and the original @C01267@ is disrupted. (This has generally been called a 'σ-complex', but @A00138@ is more appropriate than complex according to the definitions given.) The term may also be used for analogous adducts to unsaturated (and conjugated) systems in general.
See also:
Meisenheimer adduct
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1170 [Terms] [Paper]