Transition metal alkyne complex

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.

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.

Many alkyne complexes are produced by reduction of metal halides, e.g. titanocene dichloride and bis(triphenylphosphine)platinum dichloride in the presence of the alkyne:

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 elogates and the alkynyl carbon bends away from 180º. For example, in the phenylpropyne complex Pt(PPh₃)₂(C₂)Ph(Me), 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. Because the bending induced by complexation, strained alkynes such as cycloheptyne and cyclooctyne are stabilized by complexation.

In the IR spectra, the C-C vibration of alkynes, which occurs near 2300 cm⁻¹, shifts upon complexation to around 1800 cm⁻¹, indicating a weakening of the C-C bond.

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.

...
Wikipedia