chemistry

Transition metal allyl complex

Transition metal allyl complex is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Transition metal allyl complex rather than just read about it. In short: Transition-metal allyl complexes are coordination complexes with allyl and its derivatives as ligands. The allyl group, the connectivity CH2=CHCH2−, binds to metal predominantly to form π-allyl complexes. π-Allyl complexes In π-allyl complexes, all three carbon atoms bind to the metal.

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Reference excerpt

Transition-metal allyl complexes are coordination complexes with allyl and its derivatives as ligands. The allyl group, the connectivity CH2=CHCH2−, binds to metal predominantly to form π-allyl complexes.

π-Allyl complexes In π-allyl complexes, all three carbon atoms bind to the metal. This bonding mode is referred to as η3-allyl, meaning three contiguous atoms form bonds to the metal. π-Allyl is classified as an LX-type ligand in the LXZ ligand classification scheme, serving as a 3e– donor using neutral electron counting and a 4e– donor using ionic electron counting. Characteristically, the central carbon binds more tightly to the metal, as verified by X-ray crystallography. In bis(allyl)nickel, the Ni-C(central) distance is 198 picometers, and the two flanking Ni-C distances are 202 pm. Substituents on the allyl group are also common:

2-methallyl. 1-methylallyl (crotyl)

Homoleptic complexes bis(allyl)nickel bis(allyl)palladium bis(allyl)platinum tris(allyl)chromium tris(allyl)rhodium tris(allyl)iridium

Mixed ligand complexes

Many mixed ligand complexes are known: (η3-allyl)Mn(CO)4 and CpPd(allyl).

Synthetic methods A variety of routes, some serendipitous, have been reported.

Oxidative addition of allyl halides 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. The oxidative addition route has also been used to prepare Mo(II) allyl complexes:

Mo(CO)3(pyridine)3 + BrCH2CH=CH2 → Mo(CO)2(Br)(C3H5)(pyridine)2 + pyridine + CO The oxidative addition route proceeds via complexes with η1-allyl ligands. Also called σ-allyl, it is classified as an X-type ligand. 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). Decarbonylation of CpFe(CO)2(η1-C3H5) gives CpFe(CO)(η3-C3H5).

From dienes and related compounds η4-Diene ligands are viable precursors to π-allyl complexes. For example, cationic butadiene complexes are susceptible to hydride reduction to give complexes of crotyl (η3-C3H4CH3). Complementarily, anionic diene complexes are known to protonate at carbon to also give crotyl derivatives. The addition of butadiene to hydrido cobalt tetracarbonyl to give the crotyl complex:

C4H6 + HCo(CO)4 → (C3H4Me)Co(CO)3 + CO This complex exists as a mixture of syn and anti isomers, depending on the location of the methyl (Me) substituent. 1,3-Dienes such as butadiene and isoprene dimerize in the presence of some metals, giving chelating bis(allyl) complexes. Chelating bis(allyl) complexes are intermediates in the metal-catalyzed dimerization of butadiene to give vinylcyclohexene and cycloocta-1,5-diene. Such complexes also arise from ring-opening of divinylcyclobutane. Allene is another precursor of allyl complexes. Reduction of cobalt(II) chloride with sodium in the presence of 1,5-cyclooctadiene gives Co(cyclooctadiene)(cyclooctenyl). In this case one nonconjugated diene serves as a source of a cyclic allyl ligand.

From alkenes Hydride abstraction from alkene complexes represents yet another route to π-allyl complexes.

Salt metathesis reactions Salt metathesis reactions are important routes to allyl complexes:

2 C3H5MgBr + NiBr2 → Ni(C3H5)2 + 2 MgBr2 3 C3H5MgBr + Co(C5H5O2)3 → Co(C3H5)3 + 3 MgBr(C5H5O2) (where C5H5O2 is acetylacetonate)

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 (Ph = phenyl) 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 demonstrates that the benzyl ligands in tetrabenzylzirconium are flexible. One polymorph features four η2-benzyl ligands, whereas another polymorph has two η1- and two η2-benzyl ligands.

Reactions The behavior of π-allyl complexes has attracted considerable attention. The reactivity of the allyl ligand is affected by the ancillary ligands. Protonolysis can afford free propene:

2 L2Rh(C3H5) + 2 HCl → [L2RhCl]2 + 2 CH3CH=CH2 (L = phosphine ligand)

Applications π-Allyl complexes are often discussed in mechanistic organometallic chemistry. For example, allyl-metal-hydride intermediates are often invoked as intermediates in the isomerization of alkene complexes. Many homogeneous catalysts have been developed from allyl complexes, but few have commercial applications. In the area of organic synthesis, a popular allyl complex is allyl palladium chloride.

Further reading Powell, P. (1982). "Synthesis of η3-allyl complexes". In Hartley, Frank R.; Patai, Saul (eds.). The Chemistry of the Metal–Carbon Bond. Vol. 1: The structure, preparation, thermochemistry and characterization of organometallic compounds. Chichester, UK: Interscience (published April 1987). pp. 326–8. ISBN 0471100587.

References

Illustrations

Worked examples

Example 1 — a first encounter with Transition metal allyl complex

Start with the simplest possible case. Write down what Transition metal allyl complex claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Transition metal allyl complex before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Transition metal allyl complex ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Transition metal allyl complex

In research: Transition metal allyl complex appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Transition metal allyl complex in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Transition metal allyl complex is common in secondary-school and first-year university syllabi. It links to neighbouring topics Allyl complexes, Coordination complexes, Organometallic chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Transition metal allyl complex outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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