Sigma complex

This dihydrogen complex ([HFe(H₂)(dppe)₂]⁺) is an example of a sigma complex.

See also: arenium ion

In chemistry, a sigma complex or σ-complex usually refers to a family of coordination complexes where one or more ligands interact with the metal using the bonding electrons in a sigma bond. Transition metal silane complexes are often especially stable sigma complexes. A particularly common subset of sigma complexes are those featuring an agostic interaction where a C–H σ-bond on one of its ligands 'leans' towards and interacts with the coordinatively unsaturated metal center to form a chelate. Transition metal alkane complexes (e.g., a methane complex) that bind solely through the C–H bond are also known but structurally characterized examples are rare, as C–H σ-bonds are generally poor electron donors, and, in many cases, the weakened C–H bond cleaves completely (C–H oxidative addition) to form a complex of type M(R)(H). In some cases, even C–C bonds function as sigma ligands.

Significance

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.

Structure of (MeC₅H₄)Mn(CO)(PMe₃)(η²-H₂SiPh₂, a sigma complex of diphenylsilane. Selected distances: Si-Mn = 325, H-Fe = 149, Si-H(Mn) = 177, Si-H_terminal = 135 picometer.

Types of sigma complexes

Wheland complex

Main article: Arenium ion

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

Main article: Dihydrogen complex

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).