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. Dihydrogen complexes are examples. 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. 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.