The oligomerization of acetylenes induced by metals of the nickel triad

Acetylenes react with many transition metal complexes to give a wide variety of products, including benzenes and cyclopentadienes, formed by cyclotrimerization of the acetylene. Among the factors which determine the mechanism of cyclotrimerization, an important and hitherto unrecognized one is the formal oxidation state of the metal. This is well illustrated by palladium where two distinct modes of reaction, depending on whether the reactivity entity is Pd or Pd°, are observed. The nature of the acetylene and of the other ligands on the metal are also important. Such reactions offer synthetically useful routes to organic compounds as well as metal complexes otherwise difficult to prepare. Acetylenes are highly reactive compounds which in addition to their normal organic reactions also interact with a wide variety of metal complexes. Their reactions with transition metal compounds in particular have yielded a rich harvest of new types of complexes and novel transformations. This field was first explored by Reppe and his collaborators who found that the then elusive and rare cyclooctatetraene could be synthesized in one step