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[[File:ConstrainedGeomCmpx.png|thumb|A constrained geometry organotitanium complex in the (inactive) chloride form.]]

In [[organometallic chemistry]], a '''"constrained geometry complex"''' (CGC) is a kind of catalyst used for the production of [[polyolefin]]s such as [[polyethylene]] and [[polypropylene]].<ref>Klosin, J.; Fontaine, P. P.; Figueroa, R., "Development of Group Iv Molecular Catalysts for High Temperature Ethylene-Α-Olefin Copolymerization Reactions", Accounts of Chemical Research 2015, 48, 2004-2016. {{doi|10.1021/acs.accounts.5b00065}}</ref> The catalyst was one of the first major departures from [[metallocene]]-based catalysts and ushered in much innovation in the development of new plastics.

==Structure==
CGC complexes feature a [[pi-bond]]ed moiety (e.g. [[cyclopentadienyl complex|cyclopentadienyl]]) linked to one of the other [[ligand]]s on the same [[metal]] centre in such a way that the angle at this metal between the centroid of the pi-system and the additional ligand is smaller than in comparable unbridged complexes.<ref>{{cite journal | title = Constrained geometry complexes—Synthesis and applications | author = Holger Braunschweig and Frank M. Breitling | journal = [[Coordination Chemistry Reviews]] | volume = 250 | issue = 21–22 | year = 2006 | pages = 2691–2720 | doi = 10.1016/j.ccr.2005.10.022}}</ref> More specifically, the term CGC was used for [[Bridge (chemical)|ansa-bridged]] cyclopentadienyl amido complexes, although the definition goes far beyond this class of compounds. The term CGC is frequently used in connection with other more or less related ligand systems that may or may not be [[isolobal]] and/or [[isoelectronic]] with the ansa-bridged cyclopentadienyl amido ligand system.<ref>{{Cite journal |last=Braunschweig |first=Holger |last2=Breitling |first2=Frank M. |date=2006-11-01 |title=Constrained geometry complexes—Synthesis and applications |url=https://www.sciencedirect.com/science/article/pii/S001085450600083X |journal=Coordination Chemistry Reviews |series=18th Main Group Chemistry |volume=250 |issue=21 |pages=2691–2720 |doi=10.1016/j.ccr.2005.10.022 |issn=0010-8545|url-access=subscription }}</ref> Furthermore, the term is frequently used for related complexes with long ansa-bridges that induce no [[steric strain|strain]]. Ansa-bridged cyclopentadienyl amido complexes are known for the [[Group 3 element|Group 3]], [[Group 4 element|4]], [[Group 5 element|5]], [[Group 6 element|6]] and some [[Group 8 element|Group 8]] metals, with the Group 4 [[Congener (chemistry)|congener]]s being the most studied ones.{{cn|date=March 2024}}

==Applications==
Like Group 4 [[metallocene]]s, suitable Group 4 CGCs may be activated for the [[polymerisation]] of [[ethylene]] and [[alpha-olefin]]s by reaction with [[co-catalyst]]s, e.g. [[methylaluminoxane]] (MAO), [[tris(pentafluorophenyl)borane]]s, and [[trityl]] borates. The catalytic systems based on CGCs, however, display incorporation of alpha-olefin [[comonomer]]s to a greater extend than comparable metallocene based systems. This superiority of CGCs in copolymerisation reactions is ascribed to; (i) a high accessibility of the reactive centre, (ii) a low tendency of the bulk polymer chain to undergo [[chain transfer reaction]]s. CGC derived polymers are currently marketed by The Dow Chemical Company as part of their INSITE technology.<ref>Chum, P. S., W. J. Kruper, and M. J. Guest "Materials properties derived from INSITE metallocene catalysts." Advanced Materials 12.23 (2000): 1759-1767.</ref>

Beyond the use of CGCs for polymerisation reactions, a number of other transformations catalysed by CGCs (both of Group 3 and 4 metals) have been reported from academic laboratories. These include the application of CGCs as catalysts for [[hydrogenation]] of [[imine]]s, [[hydroboration]] of alkenes, carboalumination of alkenes, [[hydrosilylation]] of alkenes, hydroamination/cyclisation of alpha, omega-aminoalkenes and dimerisation of terminal [[alkyne]]s.{{cn|date=March 2024}}

==History==
The first CGC was reported by Shapiro and Bercaw for a [[scandium]] complex.<ref>Shapiro, P. J.; Bunel, E.; Schaefer, W. P.; Bercaw, J. E. "Scandium Complex [{(η5-C5Me4)Me2Si(η1-NCMe3)}(PMe3)ScH]2: A Unique Example of a Single-Component α-Olefin Polymerization Catalyst" Organometallics 1990, volume 9, pp. 867−869.</ref> The following year patents were issued to [[The Dow Chemical Company]] and [[Exxon]] for applications in alkene polymerization.<ref>"Success is Insite for Dow's unique patented catalyst technology", ICIS Chemical Business, 4 January 1997.</ref> and today are made at the billion pound scale.<ref>{{cite book |last=Lloyd |first=Lawrie |title=Handbook of industrial catalysts |year=2011 |publisher=Springer |pages=334}}</ref><ref>Dow Chemical. Solutionism at Work: 2011 Databook, www.dow.com/financial, Form No. 161-00770, April 2012, page 62.</ref>

== References ==
{{Reflist}}

[[Category:Organometallic chemistry]]
[[Category:Metal amides]]

Constrained geometry complex - Revision history

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