Insertion, into metal-hydrogen bonds acetylenes

However, because of the difficulties involved in handling this reagent, other methods for preparing these compounds are generally preferred. Olefins and acetylenes insert into metal-hydrogen bonds to give alkyl and alkenyl complexes, respectively. Examples are... 

Insertion of olefins and acetylenes into metal-hydrogen bonds... 

The addition of hydrogen to olefinic or acetylenic bonds is symmetry-forbidden [87, 88]. However, the participation of a catalyst subdivides the addition of H2 to an unsaturated system into a series of successive steps which do not suffer from these symmetry restrictions. These successive steps are oxidative addition of hydrogen, insertion of the coordinated unsaturated system into a metal-hydrogen bond, and reductive elimination of the hydrogenation product. Irrespective of the individual mechanism there is overwhelming evidence from D2 addition experiments that the catalytic addition of H2 to carbon-carbon double and triple bonds is a cA-addition [20]. 

A quite different process, called Reppe carbonylation, has been used to convert acetylene to acrylic acid esters. The catalysts are carbonyls of iron, cobalt or nickel and the hydrogen source is a hydrogen halide, HX. The process is thought to involve oxidative addition of HX to the metal carbonyl, followed by coordination and insertion of alkyne into the M—H bond and insertion of CO into the M—C bond. The resulting acyl complex is cleaved by alcohol to produce the ester and the metal hydride catalyst. De Angelis et al. have reported a theoretical analysis of the Ni(CO)4 system. 

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