# Sigma complex > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Sigma_complex > Markdown URL: https://mediated.wiki/source/Sigma_complex.md > Source: https://en.wikipedia.org/wiki/Sigma_complex > Source revision: 1344548528 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) This dihydrogen complex ([HFe(H2)(dppe)2]+) is an example of a sigma complex. See also: [arenium ion](/source/Arenium_ion) In chemistry, a **sigma complex** or **σ-complex** usually refers to a family of [coordination complexes](/source/Coordination_complex) where one or more [ligands](/source/Ligand) interact with the metal using the bonding electrons in a [sigma bond](/source/Sigma_bond). [Transition metal silane complexes](/source/Transition_metal_silane_complexes) are often especially stable sigma complexes. A particularly common subset of sigma complexes are those featuring an [agostic interaction](/source/Agostic_complex) 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](/source/Chelation). 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](/source/Electron_donor), and, in many cases, the weakened C–H bond cleaves completely (C–H [oxidative addition](/source/Oxidative_addition)) to form a complex of type M(R)(H).[1] In some cases, even C–C bonds function as sigma ligands.[2] ## Significance Sigma complexes are of great mechanistic significance, despite their frequent fragility. They represent an initial interaction between the metal center and a [hydrocarbon](/source/Hydrocarbon) substrate. As such, sigma complexes are generally assumed to be intermediates prior to full [oxidative addition](/source/Oxidative_addition).[3] 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-Hterminal = 135 [picometer](/source/Picometer).[4] ## Types of sigma complexes ### Wheland complex Main article: [Arenium ion](/source/Arenium_ion) The Wheland complex is an intermediate in [the electrophilic substitution reaction](/source/Electrophilic_aromatic_substitution) on an [aromatic compound](/source/Aromatic_compound).[5] #### Example - Halogenation of benzene In the [halogenation](/source/Halogenation) of [benzene](/source/Benzene), the sigma complex comprises the six [carbon](/source/Carbon) atoms of the [benzene ring](/source/Benzene_ring), each bonded to a [hydrogen](/source/Hydrogen) atom. An additional halogen atom is bonded to one of the carbon atoms, which is sp3-[hybridized](/source/Carbon#atomic_model_of_carbon), while the other carbons remain sp2-hybridized. In this state, the ring loses its aromaticity and acquires a positive charge, with the charge [delocalized](/source/Delocalization) across the ring.[5] ### Dihydrogen complexes Main article: [Dihydrogen complex](/source/Dihydrogen_complex) ### Sigma complexes with agostic interactions Sigma complexes with [agostic interactions](/source/Agostic_interaction) 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](/source/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).[1] ## References 1. ^ [***a***](#cite_ref-:1_1-0) [***b***](#cite_ref-:1_1-1) Weller, A. S.; Chadwick, F. M.; McKay, A. I. (2016-01-01), Pérez, Pedro J. (ed.), ["Chapter Five - Transition Metal Alkane-Sigma Complexes: Synthesis, Characterization, and Reactivity"](https://www.sciencedirect.com/science/article/pii/S0065305516300211), *Advances in Organometallic Chemistry*, vol. 66, Academic Press, pp. 223–276, [doi](/source/Doi_(identifier)):[10.1016/bs.adomc.2016.09.001](https://doi.org/10.1016%2Fbs.adomc.2016.09.001), retrieved 2024-08-11 1. **[^](#cite_ref-2)** Brayshaw, Simon K.; Sceats, Emma L.; Green, Jennifer C.; Weller, Andrew S. (2007-04-24). ["C–C σ complexes of rhodium"](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1855424). *Proceedings of the National Academy of Sciences*. **104** (17): 6921–6926. [doi](/source/Doi_(identifier)):[10.1073/pnas.0609824104](https://doi.org/10.1073%2Fpnas.0609824104). [ISSN](/source/ISSN_(identifier)) [0027-8424](https://search.worldcat.org/issn/0027-8424). [PMC](/source/PMC_(identifier)) [1855424](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1855424). [PMID](/source/PMID_(identifier)) [17435164](https://pubmed.ncbi.nlm.nih.gov/17435164). 1. **[^](#cite_ref-3)** Kubas, Gregory J. (2001-08-31). *Metal Dihydrogen and σ-Bond Complexes: Structure, Theory, and Reactivity*. Kluwer. [ISBN](/source/ISBN_(identifier)) [0-306-46465-9](https://en.wikipedia.org/wiki/Special:BookSources/0-306-46465-9). 1. **[^](#cite_ref-4)** Schubert, U.; Scholz, G.; Müller, J.; Ackermann, K.; Wörle, B.; Stansfield, R.F.D. (1986). "Hydrido-silyl-Komplexe". *Journal of Organometallic Chemistry*. **306** (3): 303–326. [doi](/source/Doi_(identifier)):[10.1016/S0022-328X(00)98993-9](https://doi.org/10.1016%2FS0022-328X%2800%2998993-9). 1. ^ [***a***](#cite_ref-:0_5-0) [***b***](#cite_ref-:0_5-1) ["Sigma-Komplex"](http://www.u-helmich.de/che/lexikon/S/Sigma-Komplex.html). Retrieved 2024-10-18. --- Adapted from the Wikipedia article [Sigma complex](https://en.wikipedia.org/wiki/Sigma_complex) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Sigma_complex?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.