Transition metal allyl complex
Transition-metal allyl complexes are coordination complexes with allyl and its derivatives as ligands. Allyl is the radical with the connectivity CH2CHCH2, although as a ligand it is usually viewed as an allyl anion CH2=CH−CH2−, which is usually described as two equivalent resonance structures.
The allyl ligand is commonly in organometallic chemistry. Most commonly, allyl ligands bind to metals via all three carbon centers, the η3-binding mode. An example of a homoleptic allyl complex is Ir(η3-allyl)3. Mixed ligand allyl complexes are more common. Examples include (η3-allyl)Mn(CO)4 and CpPd(allyl).
Allyl complexes are usually generated by oxidative addition of allylic halides to low-valent metal complexes. This route is used to prepare (allyl)2Ni2Cl2:
Some complexes with η1-allyl ligands are also known, e.g., CpFe(CO)2(η1-allyl) where only the methylene group is attached to the Fe centre. Such compounds often convert to the η3-allyl derivatives by dissociation of a ligand:
Benzyl and allyl ligands often exhibit similar chemical properties. Benzyl favors the η1 bonding mode.
In terms of applications, a popular allyl complex is allyl palladium chloride. Allyl ligands are susceptible to nucleophilic addition, which can be useful in organic synthesis.
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