The renaissance of homoatomic nine-atom polyhedra of the heavier carbon-group elements Si-Pb

Homoatomic nine-atom clusters of the elements Si, Ge, Sn, and Pb represent a fascinating class of molecules. The Zintl anions [E₉]^x- occur with different charges x and various shapes. Mixed valent compounds and polymeric chains via cluster linkage promise outstanding physical and chemical properties. A summary of recent progress in synthesis, crystal structure, EXAFS investigations, chemical bonding, Raman spectroscopy, mass spectrometry, and chemical reactivity is given. The shapes of about 30 Zintl ions [E₉] are analysed. Finally, it is shown that a close relationship exists between the compounds with anionic nine-atom clusters of elements of the carbon group and the superconducting fulleride salts, which contain negatively charged polyhedra of the lightest element in this group.