Acetylenic bond, formation from allene

The hydrogenation of cumulative double bonds (allenes) takes place at a rate comparable to that of the corresponding acetylene but no isomerization to the acetylene is observed during the reaction. "5 Selective hydrogenation to the alkene occurs since the allene readily displaces the product olefin from the catalyst before the hydrogenation can take place. Selectivities for monoalkene formation of greater than 95% were commonly obtained in the hydrogenation of various substituted allenes." This reaction selectivity is probably facilitated by the fact that the n electron clouds of the two double bonds are perpendicular to... 

Structures of these compounds are analogous to those of olefin and alkyne complexes. In square-planar compounds, the allene molecule is approximately perpendicular to the ML3 plane, while in the remaining types of compounds, the allene molecule lies almost in the ML2 plane. The angle between this plane and the plane containing the allene carbon atoms equals about 10°. The character of the metal-allene bond may be described by means of the Dewar-Chatt-Duncanson model as in the case of alkene and acetylene complexes. The formation of the a(d -n) and n d -n ) bonds causes an increase in the carbon-carbon distance and tilting of the allene substituents away from... 

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