An ''ansa''-metallocene is a type of organometallic compound containing two

cyclopentadienyl Cyclopentadienyl can refer to *Cyclopentadienyl anion, or cyclopentadienide, **Cyclopentadienyl ligand *Cyclopentadienyl radical, • *Cyclopentadienyl cation, See also *Pentadienyl In organic chemistry, pentadienyl refers to the organic radic ...

ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...

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. The term ''ansa''-metallocene (ansa being Greek for "handle") was coined by Lüttringhaus and Kullick to describe alkylidene-bridged

ferrocene Ferrocene is an organometallic compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, a ...

s, which were developed in the 1950s. Often ''ansa''-metallocenes are described in terms of the angle defined by the two Cp rings. In

titanocene dichloride Titanocene dichloride is the organotitanium compound with the formula ( ''η''5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowl ...

, this angle is 58.5° whereas in the ''ansa''-titanocene Me₂Si(C₅H₄)₂TiCl₂ the angle is 51.2°. The prototypical linker groups are of the type (CH₂)_n where n = 1, 2, and 3. More easily installed are linker groups consisting of heteroatoms, e.g. (CH₃)₂Si. 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 Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene. Polypropylene belongs to the group of polyolefins and ...

in the presence of

methylaluminoxane Methylaluminoxane, commonly called MAO, is a mixture of organoaluminium compounds with the approximate formula (Al(CH3)O)''n''. It is usually encountered as a solution in (aromatic) solvents, commonly toluene but also xylene, cumene, or mesitylene ...

(MAO) can be used to direct whether the product is syndiotactic, isotactic, or atactic.{{cite book, title = Landmarks in Organo-Transition Metal Chemistry: A Personal View, author = Werner, H., year = 2008, publisher =

Springer Science Springer Science+Business Media, commonly known as Springer, is a German multinational publishing company of books, e-books and peer-reviewed journals in science, humanities, technical and medical (STM) publishing. Originally founded in 1842 i ...

, page = 161, isbn = 978-0-387-09847-0, url = https://books.google.com/books?id=dP4LTfaPzAMC&q=ansa-zirconocene&pg=PA161 File:HAWROMskew.png, File:HAWROMtop.png, File:HAWROMside.png,

References

Metallocenes