sigma complex
{{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).{{Citation |last1=Weller |first1=A. S. |title=Chapter Five - Transition Metal Alkane-Sigma Complexes: Synthesis, Characterization, and Reactivity |date=2016-01-01 |work=Advances in Organometallic Chemistry |volume=66 |pages=223–276 |editor-last=Pérez |editor-first=Pedro J. |url=https://www.sciencedirect.com/science/article/pii/S0065305516300211 |access-date=2024-08-11 |publisher=Academic Press |last2=Chadwick |first2=F. M. |last3=McKay |first3=A. I.|doi=10.1016/bs.adomc.2016.09.001 |url-access=subscription }} In some cases, even C–C bonds function as sigma ligands.{{Cite journal |last1=Brayshaw |first1=Simon K. |last2=Sceats |first2=Emma L. |last3=Green |first3=Jennifer C. |last4=Weller |first4=Andrew S. |date=2007-04-24 |title=C–C σ complexes of rhodium |journal=Proceedings of the National Academy of Sciences |language=en |volume=104 |issue=17 |pages=6921–6926 |doi=10.1073/pnas.0609824104 |doi-access=free |issn=0027-8424 |pmc=1855424 |pmid=17435164}}
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.{{Cite book
| publisher = Kluwer
| isbn = 0-306-46465-9
| last = Kubas
| first = Gregory J.
| title = Metal Dihydrogen and σ-Bond Complexes: Structure, Theory, and Reactivity
| date = 2001-08-31
}}
File:MnSiH2.png.{{cite journal |doi=10.1016/S0022-328X(00)98993-9|title=Hydrido-silyl-Komplexe|year=1986|last1=Schubert|first1=U.|last2=Scholz|first2=G.|last3=Müller|first3=J.|last4=Ackermann|first4=K.|last5=Wörle|first5=B.|last6=Stansfield|first6=R.F.D.|journal=Journal of Organometallic Chemistry|volume=306|issue=3|pages=303–326}}]]
Types of sigma complexes
=== Wheland complex ===
{{Main|Arenium ion}}
The Wheland complex is an intermediate in the electrophilic substitution reaction on an aromatic compound.{{cite web|access-date=2024-10-18 |title=Sigma-Komplex |url=http://www.u-helmich.de/che/lexikon/S/Sigma-Komplex.html}}
==== 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|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).