Hypothetical Three-Dimensional Metallic Compounds of C60 with Transition Metal Clusters

Abstract

Solid C{sub 60} 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 M{sub 3}C{sub 60} species, M = Na, K, Rb, Cs. The parent C{sub 60} fcc lattice expands just a little (a = 14.24 A for K{sub 3}C{sub 60}) in the process. The holes in the fcc C{sub 60} lattice are in fact quite large. And {pi}-bonded substructure of the fullerene surround the holes - so 8 of the 20 C{sub 6} rings of C{sub 60} 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 C{sub 60}, 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 more » transferred to the empty C{sub 60} orbitals - the hypothetical transition-metal cluster fullerenes discussed by us display significant degrees of covalent metal-carbon bonding. Our calculations suggest that the two C{sub 60}(M{sub 4}){sub 2} phases will be conductors, but we cannot say if they will be superconducting. 17 refs., 1 fig. « less