# 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 CH$_2$CHCH$_2$, although as a ligand it is usually viewed as an allyl anion CH$_2$=CH−CH$_2$$^−$, which is usually described as two equivalent resonance structures.

## Examples

The allyl ligand is commonly in organometallic chemistry. Usually, allyl ligands bind to metals via all three carbon atoms, the η$^3$-binding mode. The η$^3$-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.

### Scope

Commonly, allyl ligands occur in mixed ligand complexes. Examples include (η$^3$-allyl)Mn(CO)$_4$ and CpPd(allyl).
Substituents on the allyl group are also common, e.g. 2-methallyl.

### 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 η$^1$-allyl ligands (classified as X-type ligands) are also known. One example is CpFe(CO)$_2$(η$^1$-C$_3$H$_5$), in which only the methylene group is attached to the Fe centre (i.e., it has the connectivity [Fe]–CH$_2$–CH=CH$_2$). As is the case for many other η$^1$-allyl complexes, the monohapticity of the allyl ligand in this species is enforced by the 18-electron rule, since CpFe(CO)$_2$(η$^1$-C$_3$H$_5$) is already an 18-electron complex, while an η$^3$-allyl ligand would result in an electron count of 20 and violate the 18-electron rule. Such complexes can convert to the η$^3$-allyl derivatives by dissociation of a neutral (two-electron) ligand L. For CpFe(CO)$_2$(η$^1$-C$_3$H$_5$), dissociation of L = CO occurs under photochemical conditions:
    CpFe(CO)$_2$(η$^1$-C$_3$H$_5$) → CpFe(CO)(η$^3$-C$_3$H$_5$) + 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)$_2$Ni$_2$Cl$_2$:
    2 Ni(CO)$_4$ + 2 ClCH$_2$CH=CH$_2$ → Ni$_2$(μ-Cl)$_2$(η$^3$-C$_3$H$_5$)$_2$ + 8 CO
A similar oxidative addition involves the reaction of allyl bromide to diiron nonacarbonyl. The oxidative addition route has also been used to prepared Mo(II) allyl complexes:
    Mo(CO)$_3$(pyridine)$_3$ + BrCH$_2$CH=CH$_2$ → Mo(CO)$_2$(Br)(C$_3$H$_5$)(pyridine)$_2$ + pyridine + CO
Other methods of synthesis involve addition of nucleophiles to η$^4$-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:
    PdCl$_2$ + >C=CHCH< → Cl$_2$Pd–(η$^2$-(>CCHCH<)) → Cl$_2$Pd(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 (η$^1$-) 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 η$^1$ or η$^3$ bonding modes. The interconversion reactions parallel those of η$^1$- or η$^3$-allyl ligands:
    CpFe(CO)$_2$(η$^1$-CH$_2$Ph) → CpFe(CO)(η$^3$-CH$_2$Ph) + 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 η$^3$-bonded complexes. X-ray crystallography demonstrate that the benzyl ligands in tetrabenzylzirconium are highly flexible. One polymorph features four η$^2$-benzyl ligands, whereas another polymorph has two η$^1$- and two η$^2$-benzyl ligands.

## Applications

Allyl complexes are often discussed in academic research, but few have commercial 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.