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.

Examples
The allyl ligand is commonly found in organometallic chemistry. Most commonly, allyl ligands bind to metals via all three carbon atoms, the η3-binding mode. The η3-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. More common are complexes with allyl and other ligands. Examples include (η3-allyl)Mn(CO)4 and CpPd(allyl).

Homoleptic complexes

 * bis(allyl)nickel
 * bis(allyl)palladium
 * bis(allyl)platinum
 * tris(allyl)chromium
 * tris(allyl)rhodium
 * tris(allyl)iridium

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.

Allyl σ ligands
Complexes with η1-allyl ligands (classified as X-type ligands) are also known. One example is CpFe(CO)2(η1-C3H5), in which only the methylene group is attached to the Fe centre (i.e., it has the connectivity [Fe]–CH2–CH=CH2). As is the case for many other η1-allyl complexes, the monohapticity of the allyl ligand in this species is enforced by the 18-electron rule, since CpFe(CO)2(η1-C3H5) is already an 18-electron complex, while an η3-allyl ligand would result in an electron count of 20 and violate the 18-electron rule. Such complexes can convert to the η3-allyl derivatives by dissociation of a neutral (two-electron) ligand L. For CpFe(CO)2(η1-C3H5), dissociation of L = CO occurs under photochemical conditions:
 * CpFe(CO)2(η1-C3H5) → CpFe(CO)(η3-C3H5) + CO

Synthetic methods
Allyl complexes are often generated by oxidative addition of allylic halides to low-valent metal complexes. This route is used to prepare (allyl)2Ni2Cl2:
 * 2 Ni(CO)4 + 2 ClCH2CH=CH2 → Ni2(μ-Cl)2(η3-C3H5)2 + 8 CO

A similar oxidative addition involves the reaction of allyl bromide to diiron nonacarbonyl. Oxidative addition route has been used for Mo(II) allyl complexes as well:
 * Mo(CO)3(pyridine)3 + BrCH2CH\dCH2 -> Mo(CO)2(Cl)(C3H5)(pyridine)2

Other methods of synthesis involve addition of nucleophiles to η4-diene complexes and hydride abstraction from alkene complexes. For example, palladium(II) chloride attacks alkenes to give first an alkene complex, but then abstracts hydrogen to give a dichlorohydridopalladium alkene complex, and then eliminates hydrogen chloride:
 * PdCl2 + >C=CHCH< → Cl2Pd–(η2-(>CCHCH<)) → Cl2Pd(H)⚟(>CCHC<) → ClPd⚟(>CCHC<) + HCl

One allyl complex can transfer an allyl ligand to another complex. An anionic metal complex can displace a halide, to give an allyl complex. However, if the metal center is coordinated to 6 or more other ligands, the allyl may end up "trapped" as a σ (η1-) ligand. In such circumstances, heating or irradiation can dislocate another ligand to free up space for the alkene-metal bond.

In principle, salt metathesis reactions can adjoin an allyl ligand from an allylmagnesium bromide or related allyl lithium reagent. However, the carbanion salt precursors require careful synthesis, as allyl halides readily undergo Wurtz coupling. Mercury and tin allyl halides appear to avoid this side-reaction.

Benzyl complexes
Benzyl and allyl ligands often exhibit similar chemical properties. Benzyl ligands commonly adopt either η1 or η3 bonding modes. The interconversion reactions parallel those of η1- or η3-allyl ligands:
 * CpFe(CO)2(η1-CH2Ph) → CpFe(CO)(η3-CH2Ph) + CO

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 η3-bonded complexes. X-ray crystallography demonstrate that the benzyl ligands in tetrabenzylzirconium are highly flexible. One polymorph features four η2-benzyl ligands, whereas another polymorph has two η1- and two η2-benzyl ligands.

Applications
In terms of applications, a popular allyl complex is allyl palladium chloride.

The reactivity of allyl ligands depends on the overall complex, although the influence of the metal center can be roughly summarized as
 * (more reactive) Fe ≫ Pd > Mo > W (less reactive)

Such complexes are usually electrophilic (i.e., react with nucleophiles), but nickel allyl complexes are usually nucleophilic (resp. with electrophiles). In the former case, the addition may occur at unusual locations, and can be useful in organic synthesis.