{{Short description|Organometallic compound}} {{DISPLAYTITLE:''Ansa''-metallocene}} [[File:Ansa-Metallocene.svg|thumb|200px|Structure of a generic ''ansa''-metallocene. X is the linker group, often (CH<sub>2</sub>)<sub>n</sub> or R<sub>2</sub>Si (R = alkyl). L<sub>n</sub> refers to additional ligands that may reside on the metal M.]] An '''''ansa''-metallocene''' is a type of organometallic compound containing two [[cyclopentadienyl]] [[ligand]]s that are linked by a bridging group such that both cyclopentadienyl groups are bound to the same metal. The link prevents rotation of the cyclopentadienyl ligand and often modifies the structure and reactivity of the metal center. Some ansa-metallocenes are active in [[Ziegler-Natta catalysis]], although none are used commercially.<ref>{{cite journal | doi = 10.1016/j.ccr.2005.05.012 | last1 = Wang | first1 = B | title = ''Ansa''-metallocene polymerization catalysts: Effects of the bridges on the catalytic activities | journal = Coordination Chemistry Reviews | volume = 250 | pages = 242–258 | year = 2006| issue = 1–2 }}</ref> The term ''ansa''-metallocene (ansa being Greek for "handle") was coined by Lüttringhaus and Kullick to describe alkylidene-bridged [[ferrocene]]s, which were developed in the 1950s.<ref>{{cite journal | doi = 10.1002/ange.19580701407 | last1 = Lüttringhaus | first1 = A. | last2 = Kullick | first2 = W. | title = ''Ansa''-Ferrocene | journal = [[Angewandte Chemie]] | volume = 70 | pages = 438 | year = 1958 | isbn = 958-07-0140-7 | issue = 14| bibcode = 1958AngCh..70R.438L }}</ref>
Often ''ansa''-metallocenes are described in terms of the angle defined by the two Cp rings. In [[titanocene dichloride]], this angle is 58.5° whereas in the ''ansa''-titanocene Me<sub>2</sub>Si(C<sub>5</sub>H<sub>4</sub>)<sub>2</sub>TiCl<sub>2</sub> the angle is 51.2°.<ref>{{cite journal |last1 = Bajgur |first1 = Chandrasekhar S. |last2 = Tikkanen |first2 = Wayne |last3 = Petersen |first3 = Jeffrey L. |title = Synthesis, structural characterization, and electrochemistry of [1]metallocenophane complexes, [Si(alkyl)<sub>2</sub>(C<sub>5</sub>H<sub>4</sub>)<sub>2</sub>]MCl<sub>2</sub>, M = Ti, Zr |journal = [[Inorganic Chemistry (journal)|Inorganic Chemistry]] |volume = 24 |pages = 2539 |year = 1985 |doi = 10.1021/ic00210a015 |issue = 16}}</ref>
The prototypical linker groups are of the type (CH<sub>2</sub>)<sub>n</sub> where n = 1, 2, and 3. More easily installed are linker groups consisting of heteroatoms, e.g. (CH<sub>3</sub>)<sub>2</sub>Si.{{cn|date=May 2026}} Small linker groups require non-coplanar Cp rings, and thus introduce strain into the resulting complex. Some undergo [[ring-opening polymerization]].<ref>{{Cite journal|journal=Chemical Reviews|year=1999|volume=99|title=Organometallic polymers with transition metals in the main chain|first=Paul|last=Nguyen|first2=Paloma|last2=Gómez-Elipe|first3=Ian|last3=Manners|orig-date=7 Dec 1998|doi=10.1021/cr960113u|publisher=American Chemical Society|p=1519}}</ref>
One potential application of ''ansa''-metallocenes is in polymerisation reactions. Specifically, the ansa bridge allows greater control of the stereochemistry at the metal centre. For example, three distinct ''ansa''-zirconocene dichloride catalysts used in the synthesis of [[polypropylene]] in the presence of [[methylaluminoxane]] (MAO) can be used to direct whether the product is [[Tacticity#Polymers|syndiotactic, isotactic, or atactic]].<ref>{{cite book|title = Landmarks in Organo-Transition Metal Chemistry: A Personal View|author = Werner, H.|year = 2008|publisher = [[Springer Science]]|page = 161|isbn = 978-0-387-09847-0|url = https://books.google.com/books?id=dP4LTfaPzAMC&q=ansa-zirconocene&pg=PA161}}</ref> <gallery class="center" caption="Three views of ansa-dimethylsilylhafnocene dichloride (from X-ray coordinates)."> File:HAWROMskew.png| File:HAWROMtop.png| File:HAWROMside.png| </gallery>
==References== <references/>
[[Category:Metallocenes]]