| Abstract: | In this review article complexes of the type XnM—R are dealt with in which a σ-bonded organic ligand R is additionally linked by a π-bond with the metal M. In other words, the complexes contain metallacycles with three and a half, four and a half, five and a half members, etc. Well-Known five and a half membered ring systems are those with 4-pentenyl, 1,4-pentadienyl, norbornyl norbornenyl, cyclooctenyl, and endo-dicyclopentadienyl ligands. Metallacycles of this size are strain-free and stable. Donor ligands such as amines and phosphanes can displace the coordinated double bond of the π-ligand from the metal. This C?C double bond is lengthened from ca. 133 pm to 136–146 pm by the coordination. |
