In chemistry, a sigma complex or σ-complex usually refers to a family of

coordination complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

es where one or more ligand interacts with the metal using the bonding electrons in a sigma bond.

Dihydrogen complex Dihydrogen complexes are coordination complexes containing intact H2 as a ligand. They are a subset of sigma complexes. The prototypical complex is W(CO)3( PCy3)2(H2). This class of compounds represent intermediates in metal-catalyzed reactions i ...

es are examples.

Transition metal silane complexes Transition metal silane complexes are coordination compounds containing hydrosilane ligands. An early example is (MeC5H4)Mn(CO)2(η2-HSiPh3) (Ph = C6H5). The bonding in silane sigma complexes is similar to that invoked in agostic interactions. T ...

are often especially stable sigma complexes. A subset of sigma complexes are

agostic complex In organometallic chemistry, agostic interaction refers to the interaction of a coordinatively-unsaturated transition metal with a C−H bond, when the two electrons involved in the C−H bond enter the empty d-orbital of the transition metal, r ...

es, where a C-H sigma bond functions as the donor ligand. 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 saturated substrates. Sigma complexes are generally assumed to be intermediates prior to full

oxidative addition Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre. Oxidat ...

References

Transition metals Metal hydrides