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 CHCHCH, although as a ligand it is usually viewed as an allyl anion CH=CH−CH, which is usually described as two equivalent resonance structures. Examples and their syntheses. The allyl ligand is commonly found in organometallic chemistry. Most commonly, allyl ligands bind to metals via all three carbon atoms, the η-binding mode. The η-allyl group is classified as an LX-type ligand in the Green LXZ ligand classification scheme, serving as a  3e donor using neutral electron counting and 4e donor using ionic electron counting. An example of a homoleptic allyl complex is Ir(η-allyl). More common are complexes with allyl and other ligands. Examples include (η-allyl)Mn(CO) and CpPd(allyl). Allyl complexes are often generated by oxidative addition of allylic halides to low-valent metal complexes. This route is used to prepare (allyl)NiCl: A similar oxidative addition involves the reaction of allyl bromide to diiron nonacarbonyl. Other methods of synthesis involve addition of nucleophiles to η-diene complexes and hydride abstraction from alkene complexes. Lastly allyl ligands are produced by salt metathesis  reactions starting with allyl Grignard and allyl lithium reagents. Chelating bis(allyl) complexes. 1,3-Dienes such as butadiene and isoprene dimerize in the coordination spheres of some metals, giving chelating bis(allyl) complexes. Such complexes also arise from ring-opening of divinylcyclobutane. Chelating bis(allyl) complexes are intermediates in the metal-catalyzed dimerization of butadiene to give vinylcyclohexene and cycloocta-1,5-diene. Sigma-allyl. Complexes with η-allyl ligands (classified as X-type ligands) are also known. One example is CpFe(CO)(η-CH), in which only the methylene group is attached to the Fe centre (i.e., it has the connectivity [Fe]–CH–CH=CH). As  is the case for many other η-allyl complexes, the monohapticity of the allyl ligand in this species is enforced by the 18-electron rule, since CpFe(CO)(η-CH) is already an 18-electron complex, while an η-allyl ligand would result in an electron count of 20 and violate the 18-electron rule. Such complexes can convert to the η-allyl derivatives by dissociation of a neutral (two-electron) ligand L. For CpFe(CO)(η-CH), dissociation of L = CO occurs under photochemical conditions: Benzyl complexes. Benzyl and allyl ligands often exhibit similar chemical properties. Benzyl commonly adopt either η or  η bonding modes. The interconversion reactions parallel those of η- or η-allyl ligands: In all bonding modes, the benzylic carbon atom is more strongly attached to the metal as indicated by M-C bond distances, which differ by ca. 0.2 Å in η-bonded complexes. X-ray crystallography demonstrate that the benzyl ligands in tetrabenzylzirconium are highly flexible. One polymorph features four η-benzyl ligands, whereas another polymorph has two η- and two η-benzyl ligands. Applications. 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.