Semihydrogenation

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.

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 may be formed by reaction of the alkyne with dicobalt octacarbonyl.

: Co₂(CO)₈ + R₂C₂ → Co₂(C₂R₂)(CO)₆ + 2 CO

Many alkyne complexes are produced by reduction of metal halides:
: Cp₂TiCl₂ + Mg + Me₃SiC≡CSiMe₃ → Cp₂Ti CSiMe₃)₂+ MgCl₂

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.Crabtree, R. H. ''Comprehensive Organometallic Chemistry V'', 2009, John Wiley & Sons

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. Variants of this reaction are exploited for some syntheses of substituted pyridines.

The Pauson–Khand reaction provides a route to cyclopentenones via the intermediacy of cobalt-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.
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References

{{Coordination complexes
Organometallic chemistry
Transition metals
Coordination chemistry