Cp* Chemistry of main-group elements

In the chemistry of main-group elements the pentamethylcyclopentadienyl (Cp*) group represents a very important substituent. Owing to its quite variable bonding modes, ranging from mainly ionic to mainly covalent and from eta(5) via eta(2/3) to eta(1), an effective adjustment to the electronic situation at the respective element centre is feasible. Furthermore, the steric requirements of the sigma- or pi-bound Cp* group enable the kinetic stabilization of otherwise highly reactive species. Covalent Cp*-element bonds are comparatively weak, consequently allowing fast sigmatropic and haptotropic rearrangement processes. A further consequence is that the Cp* radical as well as the Cp*(-) anion are potential leaving groups. This phenomenon forms the basis of a rich faceted substitution and elimination chemistry. As proof of this statement, several examples from phosphorus, silicon, and gallium chemistry are presented.