Charge transfer and bonding in endohedral fullerenes from high-energy spectroscopy

This contribution deals with the investigation of charge transfer and bonding in endohedral fullerenes in the solid state using high-energy spectroscopies such as photo-emission and x-ray absorption. An overview is given of the detailed and direct information that can be won as regards the valence state of the encapsulated species, the degree of hybridisation between the electronic states of the endohedral entity and the carbon cage and finally the charge transfer between the host and guest. Examples are presented of rare-earth monometallofullerenes, rare-earth and transition-metal dimetallofullerenes and endohedrals containing transition-metal-nitride clusters, both in the pristine state and intercalated with alkali metals.