Sigma complex
In the following article, we will explore in detail Sigma complex, a relevant topic that has captured the attention of experts and the general public. Over the years, Sigma complex has been the subject of debate, study and analysis, generating endless research and conflicting opinions. Its importance and impact on modern society make it a topic worthy of exploration and reflection. Through this article, we will seek to further understand what Sigma complex is, what its implications are and how it can influence various aspects of our daily lives.
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).[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 substrate. As such, sigma complexes are generally assumed to be intermediates prior to full oxidative addition.[3]
Types of sigma complexes
Wheland complex
The Wheland complex is an intermediate in the electrophilic substitution reaction on an aromatic compound.[5]
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.[5]
Dihydrogen complexes
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).[1]
References
- ^ a b 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", Advances in Organometallic Chemistry, vol. 66, Academic Press, pp. 223–276, doi:10.1016/bs.adomc.2016.09.001, retrieved 2024-08-11
- ^ Brayshaw, Simon K.; Sceats, Emma L.; Green, Jennifer C.; Weller, Andrew S. (2007-04-24). "C–C σ complexes of rhodium". Proceedings of the National Academy of Sciences. 104 (17): 6921–6926. doi:10.1073/pnas.0609824104. ISSN 0027-8424. PMC 1855424. PMID 17435164.
- ^ Kubas, Gregory J. (2001-08-31). Metal Dihydrogen and σ-Bond Complexes: Structure, Theory, and Reactivity. Kluwer. ISBN 0-306-46465-9.
- ^ 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:10.1016/S0022-328X(00)98993-9.
- ^ a b "Sigma-Komplex". Retrieved 2024-10-18.