An arenium ion is a cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution."Stable carbocations. CXVIII. General concept and structure of carbocations based on differentiation of trivalent (classical) carbenium ions from three-center bound penta- of tetracoordinated (nonclassical) carbonium ions. Role of carbocations in electrophilic reactions" George A. Olah J. Am. Chem. Soc.; 1972; 94(3) 808–820; For historic reasons this complex is also called a Wheland intermediate,"A Quantum Mechanical Investigation of the Orientation of Substituents in Aromatic Molecules" G. W. Wheland J. Am. Chem. Soc.; 1942; 64(4) 900–908; or a σ-complex.
