Met-Cars: a unique class of molecular clusters

Currently there is extensive interest in systems of finite size as they often give rise to unique properties that differ from those of an extended solid or the individual molecular constituents of which they are comprised. Particularly interesting are systems whose composition can be selectively chosen, and ones whose individual characteristics may be retained, thus allowing them to serve as the building blocks for nanostructured/cluster-assembled materials. In 1992 we discovered a new class of molecular clusters termed metallocarbohedrenes, or Met-Cars for short, which involve bonding between early transition metals and carbon with a stoichiometry of M8C12. Calculations, as well as recent experimental findings, suggest that these species exhibit considerable free electron behavior which becomes manifested through observations of changing electronic energy levels with the nature of the metal. Indications that it is possible to produce Met-Cars with various endohedral atoms, as well as the finding that other metals and non-metal atoms may also be substituted in the cage lattice, suggest that these cluster materials are valuable in the context of unraveling the properties of condensed matter of finite size. This also opens an avenue for exploring the prospect that they may provide building blocks for new materials. Their discovery, formation, and ionization dynamics are reviewed herein. (C) 2002 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.