Sigma complex

Sigma complexes are of great mechanistic significance, despite their frequent fragility. They represent an initial interaction between the metal center and a hydrocarbon substrate. As such, sigma complexes are generally assumed to be intermediates prior to full oxidative addition. . == Types of sigma complexes ==

Wheland complex The Wheland complex is an intermediate in the electrophilic substitution reaction on an aromatic compound. Example - Halogenation of benzene In the halogenation of benzene, the sigma complex comprises the six carbon atoms of the benzene ring, each bonded to a hydrogen atom. An additional halogen atom is bonded to one of the carbon atoms, which is sp3-hybridized, while the other carbons remain sp2-hybridized. In this state, the ring loses its aromaticity and acquires a positive charge, with the charge delocalized across the ring. Dihydrogen complexes Sigma complexes with agostic interactions Sigma complexes with agostic interactions represent a particularly common subgroup of sigma complexes. In these, a C-H-σ bond from one of the ligands interacts with the coordinatively unsaturated metal center, forming a chelate complex. Transition metal-alkane complexes Transition metal-alkane complexes bind exclusively through the C-H bond. Structurally characterized examples are rare, as C-H σ-bonds generally act as weak electron donors. In many cases, the weakened C-H bond undergoes complete cleavage (oxidative C-H addition). ==References==