The Versatility of the Pentamethylcyclopentadienyl Ligand in Main-Group Chemistry

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Abstract
Examples from different fields have been collected to demonstrate the unique properties of the pentamethylcyclopentadienyl (cp∗) ligand in main-group chemistry. First, compounds with σ-bonded cp∗ ligands are thermally very stable and, therefore, allow detailed investigations of their temperature-dependent dynamic behavior. Drastic differences in the rates of 1,5-sigmatropic rearrangements are observed; the comparison of experimental results with theoretical calculations is very useful in understanding basic principles of fluxionality. Second, the polyhaptobonded cp∗ ligand stabilizes π-complexes with main-group elements as central atoms. Using isolobal and isoelectronic relationships, these π-complexes can be organized in four structurally different classes. Furthermore, the cp∗ ligand is sterically demanding in boron, silicon, phosphorus, and sulfur compounds, leading to kinetically stabilized species and rendering nucleophilic substitution reactions more difficult. Finally, the cp∗ ligand is a good leaving group; cp∗-element bonds can be split by nucleophilic, electrophilic and reducing agents, thus allowing an interesting substitution chemistry.