Hypothetical three-dimensional metallic compounds of C-60 with transition metal clusters

Solid C60 crystallizes at room temperature in a face-centered cubic (fcc) lattice, a = 14.11 A, with a shortest C-C contact of 3.10 A. The fcc lattice has tetrahedral and octahedral interstices, into which alkali and alkaline earth metal atoms enter, in the remarkable superconducting M3C60 species, M = Na, K, Rb, Cs. The parent C60 fcc lattice expands just a little (a = 14.24 A for K3C60) in the process. The holes in the fcc C60 lattice are in fact quite large. And π-bonded substructure of the fullerene surround the holes-so 8 of the 20 C6 rings of C60 face the 8 tetrahedral holes around. The octahedral holes all face a C-C bond connecting two 5-membered rings, or a 6,6-ring junction. We suggest these interstices might be filled with transition metal clusters, not just donating electrons to the C60, but bonded to the fullerenes. What we see is that the bonding in these hypothetical solids is quite different from that of the well-studied alkali-metal fullerene systems. While the latter can be regarded as nearly pure ionic systems-the valence electrons of the metal are practically fully transferred to the empty C60 orbitals - the hypothetical transition-metal cluster fullerenes discussed by us display significant degrees of covalent metal-carbon bonding. Our calculations suggest that the two (C60(M4)2 phases will be conductors, but we cannot say if they will be superconducting.