TOPOLOGICAL ASPECTS OF METALS IN CARBON CAGES - ANALOGIES WITH ORGANOMETALLIC CHEMISTRY

Metals can interact with carbon cages in the following ways: (1) stabl e carbon cages (i.e., fullerenes) function as electronegative olefins in their exohedral eta(2) bonding to transition metals; (2) endohedral metallofullerenes with a highly electropositive lanthanide (Ln) insid e the carbon cage can be considered to be ionic with lanthanide cation s, Ln(3+), and fullerene anions; (3) fullerenes too small for independ ent existence can be stabilized by internal covalent bonding to an end ohedral metal atom using the central carbon atoms of pentagon triplets , i.e. triquinacene units, in complexes such as M@C-28 (M = Ti, Zr, Hf , and U), derived from the tetrahedral fullerene C-28; (4) metal atoms can occur as vertices of binary mixed metal-carbon cages in both earl y transition metal complexes of the types M14C13, M8C12, and M13C22 (e .g., M = Ti) and copper-carbon cages of the types Cu2n+1C2n+ (n less t han or equal to 10), Cu7C8+, Cu9C10+, and Cu12C12+. The presence of me tal atoms as vertices of carbon cages changes radically their stoichio metries and thus their structures. Thus, early transition metals form cages such as Ti14C13 assumed to have titanium atoms at the vertices a nd face midpoints of a 3 x 3 x 3 cube and carbon atoms at the edge mid points and center of the cube and Ti13C22 assumed to have titanium ato ms at the edge midpoints and center of a 3 x 3 x 3 cube as well as C-2 units and carbon atoms at the vertices and face midpoints, respective ly, of the cube. Elimination of the face metal atoms from the Ti14C13 structure as well as the center carbon atom, which has been achieved e xperimentally by photofragmentation, leads to the Ti8C12 cluster. The structure of this cluster is based on a tetracapped tetrahedron with T -d symmetry with two distinct quartets of titanium atoms, six distinct C-2 pairs, and 36 direct Ti-C interactions. The copper-carbon cages o f various stoichiometries are suggested to have prismatic, antiprismat ic, or cuboctahedral structures in which the electronic configurations of the copper atoms approach the favored 18-electron rare gas configu ration.