chemistry

Transition metal alkyne complex

Transition metal alkyne 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 alkyne complex rather than just read about it. In short: In organometallic chemistry, a transition metal alkyne complex is a coordination compound containing one or more alkyne ligands. Such compounds are intermediates in many catalytic reactions that convert alkynes to other organic products, e.g. hydrogenation and trimerization.

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Reference excerpt

In organometallic chemistry, a transition metal alkyne complex is a coordination compound containing one or more alkyne ligands. Such compounds are intermediates in many catalytic reactions that convert alkynes to other organic products, e.g. hydrogenation and trimerization. They are distinct from acetylide complexes.

Synthesis Transition metal alkyne complexes are often formed by the displacement of labile ligands by the alkyne. For example, a variety of cobalt-alkyne complexes arise by the reaction of alkynes with dicobalt octacarbonyl.

Co2(CO)8 + R2C2 → (R2C2)Co2(CO)6 + 2 CO Many alkyne complexes are produced by reduction of metal halides:

Cp2TiCl2 + Mg + Me3SiC≡CSiMe3 → Cp2Ti[(CSiMe3)2] + MgCl2 Complexes of dichloroacetylene are produced by in situ reduction of the metal by the alkyne:

4 C2Cl2 + 2 WCl6 → [WCl4(C2Cl2)]2 + 2 C2Cl4.

Reactions Transition metal alkyne complexes participate in many reactions. Protonation of alkyne ligands gives alkenyl complexes:

LnM(RC≡CR) + H+ → [LnM−CR=CRH]+ Complexes of terminal alkenes are prone to rearrange to vinylidene derivatives:

LnM(HC≡CR) → LnM=C=CRH The Pauson–Khand reaction provides a route to cyclopentenones via the intermediacy of cobalt-alkyne complexes.

Structure and bonding

The coordination of alkynes to transition metals is similar to that of alkenes. The bonding is described by the Dewar–Chatt–Duncanson model. Upon complexation the C-C bond elongates and the alkynyl carbon bends away from 180º. For example, in the phenylpropyne complex Pt(PPh3)2(MeC2Ph), the C-C distance is 1.277(25) vs 1.20 Å for a typical alkyne. The C-C-C angle distorts 40° from linearity upon complexation. Because the bending induced by complexation, strained alkynes such as cycloheptyne and cyclooctyne are stabilized by complexation. The C≡C vibration of alkynes occurs near 2300 cm−1 in the IR spectrum. This mode shifts upon complexation to around 1800 cm−1, indicating a weakening of the C-C bond.

η2-coordination to a single metal center When bonded side-on to a single metal atom, an alkyne serves as a dihapto usually two-electron donor. For early metal complexes, e.g., Cp2Ti(C2R2), strong π-backbonding into one of the π* antibonding orbitals of the alkyne is indicated. This complex is described as a metallacyclopropene derivative of Ti(IV). For late transition metal complexes, e.g., Pt(PPh3)2(MeC2Ph), the π-backbonding is less prominent, and the complex is assigned oxidation state 0. In some complexes, the alkyne is classified as a four-electron donor. In these cases, both pairs of pi-electrons donate to the metal. This kind of bonding was first implicated in complexes of the type W(CO)(R2C2)3.

η2, η2-coordination bridging two metal centers Because alkynes have two π bonds, alkynes can form stable complexes in which they bridge two metal centers. The alkyne donates a total of four electrons, with two electrons donated to each of the metals. And example of a complex with this bonding scheme is η2-diphenylacetylene-(hexacarbonyl)dicobalt(0).

Benzyne complexes Transition metal benzyne complexes represent a special case of alkyne complexes since the free benzynes are not stable in the absence of the metal.

Applications Metal alkyne complexes are intermediates in the semihydrogenation of alkynes to alkenes:

C2R2 + H2 → cis-C2R2H2 This transformation is conducted on a large scale in refineries, which unintentionally produce acetylene during the production of ethylene. It is also useful in the preparation of fine chemicals. Semihydrogenation affords cis alkenes. Metal-alkyne complexes are also intermediates in the metal-catalyzed trimerization and tetramerizations. Cyclooctatetraene is produced from acetylene via the intermediacy of metal alkyne complexes. Acrylic acid was once prepared by the hydrocarboxylation of acetylene:

C2H2 + H2O + CO → H2C=CHCO2H With the shift away from coal-based (acetylene) to petroleum-based feedstocks (olefins), catalytic reactions with alkynes are not widely practiced industrially. Polyacetylene has been produced using metal catalysis involving alkyne complexes.

Cuprous chloride also catalyzes the dimerization of acetylene to vinylacetylene, once used as a precursor to various polymers such a neoprene. Mechanistic studies suggest that this reaction proceeds by insertion of acetylene into a copper(I) acetylide complex.

References

Illustrations

Worked examples

Example 1 — a first encounter with Transition metal alkyne complex

Start with the simplest possible case. Write down what Transition metal alkyne 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 alkyne 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 alkyne 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 alkyne complex

In research: Transition metal alkyne 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 alkyne 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 alkyne complex is common in secondary-school and first-year university syllabi. It links to neighbouring topics Coordination complexes, Organometallic chemistry, Transition metal compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Transition metal alkyne 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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Frequently asked questions

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