Transition-metal allyl complexes are

coordination complex A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

es with

allyl In organic chemistry, an allyl group is a substituent with the structural formula . It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, . In 1844, Theodor Wertheim isolated a ...

and its derivatives as

ligand In coordination chemistry, a ligand is an ion or molecule with a 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 el ...

s. 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

is commonly in

organometallic chemistry Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...

. Usually, 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 In chemistry, electron counting is a formalism for assigning a number of valence electrons to individual atoms in a molecule. It is used for classifying compounds and for explaining or predicting their electronic structure and chemical bond, bondi ...

and 4e^– donor using ionic electron counting.

Scope

Commonly, allyl ligands occur in mixed ligand complexes. Examples include (η³-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-

Diene In organic chemistry, a diene ( ); also diolefin, ) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alk''ene'' units, with the standard prefix ''di'' of systematic nome ...

s such as

butadiene 1,3-Butadiene () is the organic compound with the formula CH2=CH-CH=CH2. It is a colorless gas that is easily condensed to a liquid. It is important industrially as a precursor to synthetic rubber. The molecule can be viewed as the union of two ...

and

isoprene Isoprene, or 2-methyl-1,3-butadiene, is a common volatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. It is produced by many plants and animals (including humans) and its polymers ar ...

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 e��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 Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre. Oxidat ...

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. The oxidative addition route has also been used to prepared Mo(II) allyl complexes: : Other methods of synthesis involve addition of nucleophiles to η⁴-diene complexes and hydride abstraction from alkene complexes. For example,

palladium(II) chloride Palladium(II) chloride, also known as palladium dichloride and palladous chloride, are the chemical compounds with the formula PdCl2. PdCl2 is a common starting material in palladium chemistry – palladium-based catalysts are of particular value ...

attacks alkenes to give first an alkene complex, but then abstracts hydrogen to give a dichlorohydridopalladium alkene complex, and then eliminates

hydrogen chloride The Chemical compound, compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hyd ...

: :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 reaction A salt metathesis reaction (also called a double displacement reaction, double replacement reaction, or double decomposition) is a type of chemical reaction in which two ionic compounds in aqueous solution exchange their component ions to form two ...

s 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 X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

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

Applications

Allyl complexes are often discussed in academic research, but few have commercial applications. A popular allyl complex is allyl palladium chloride.Tatsuno, Y.; Yoshida, T.; Otsuka, S. "(η³-allyl)palladium(II) Complexes"

Inorganic Syntheses ''Inorganic Syntheses'' is a book series which aims to publish "detailed and foolproof" procedures for the synthesis of inorganic compounds.nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...

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 Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...

.Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis; University Science Books: New York, 2010.

References

{{Coordination complexes Organometallic chemistry Transition metals Allyl complexes Coordination chemistry