Metal π‐Complexes with Di‐ and Oligo‐olefinic Ligands

To the somewhat casual observer, the concept “metal π‐complex” is primarily associated with the names “ferrocene” and “dibenzenechromium”. The preparation of these two compounds was in fact the starting point of a development which has had fruitful effects on theoretical as well as preparative chemistry and the limits of which are not yet in sight. In recent years, many straight‐chain and cyclic olefins with varying numbers of double bonds have been added to the list of ligands in metal π‐complexes, besides such “aromatic” systems as the cyclopentadienyl anion, benzene, and the tropylium cation. The literature in this area is already so copious that it seems advisable to adopt a division into compounds with mono‐olefins and those with di‐ and oligo‐olefins. The present review is concerned exclusively with the latter group.