Transition-metal allyl complexes are

coordination complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

es with

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

and its derivatives as

ligand In coordination chemistry, a ligand is an ion or molecule (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 electr ...

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 found 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 ...

. 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 Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and bonding. Many rules in chemistry rely on electron-counting: *Octet rule is used with Lewis structures for main group eleme ...

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

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 Diiron nonacarbonyl is an organometallic compound with the formula Fe2(CO)9. This metal carbonyl is an important reagent in organometallic chemistry and of occasional use in organic synthesis. It is a more reactive source of Fe(0) than Fe(CO)5. ...

. Oxidative addition route has been used for Mo(II) allyl complexes as well: : Other methods of synthesis involve addition of nucleophiles to η⁴-diene complexes and hydride abstraction from alkene complexes. Lastly allyl ligands are produced by

salt metathesis reaction A salt metathesis reaction, sometimes called a double displacement reaction, is a chemical process involving the exchange of bonds between two reacting chemical species which results in the creation of products with similar or identical bonding a ...

s starting with

allylmagnesium bromide Allylmagnesium bromide is a Grignard reagent used for introducing the allyl group. It is commonly available as a solution in diethyl ether. It may be synthesized by treatment of magnesium with allyl bromide Allyl bromide (3-bromopropene) is an or ...

or related allyl lithium reagents.

Chelating bis(allyl) complexes

1,3-

Diene In organic chemistry a diene ( ) (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 nomenclature. ...

s such as

butadiene 1,3-Butadiene () is the organic compound with the formula (CH2=CH)2. 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 viny ...

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. Isoprene is an unsaturated hydrocarbon. It is produced by many plants and animals ...

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 Cycloocta-1,5-diene is a cyclic hydrocarbon with the chemical formula , specifically . There are three configurational isomers with this structure, that differ by the arrangement of the four C–C single bonds adjacent to the double bonds. Each ...

Sigma-allyl 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

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 determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

demonstrate that the benzyl ligands in

tetrabenzylzirconium Tetrabenzylzirconium is an organozirconium compound with the formula Zr(CH2C6H5)4. The molecule features diamagnetic Zr(IV) bonded to four benzyl ligands. It is an orange air- and photo-sensitive solid, which is soluble in hydrocarbon solvents. ...

are highly flexible. One polymorph features four η²-benzyl ligands, whereas another polymorph has two η¹- and two η²-benzyl

Applications

In terms of 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.addition Addition (usually signified by the Plus and minus signs#Plus sign, plus symbol ) is one of the four basic Operation (mathematics), operations of arithmetic, the other three being subtraction, multiplication and Division (mathematics), division. ...

, which can be useful in

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

.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