Reports Conventional binary solid-state compounds, A x B y , are infinite, crystalline arrays of atoms A and B. Here we describe analogous binary solids in which the "atomic" building blocks are pseudo-spherical molecular clusters rather than simply atoms [for reviews on molecular clusters, see (1-3)]. We prepare these new solids by simply combining independently synthesized molecular clusters (4-6). The internal structures of the constituent clusters remain unchanged, but charge is transferred between them, forming ionic solids analogous to NaCl. We report three new solids: [ [C 60 ]. The former two assemble into a superatomic relative of the CdI 2 structure type, and the latter forms a simple rock-salt crystal. Despite their ready availability, molecular clusters have been used infrequently as electronic materials. Noteworthy examples of success in this area are the organic-inorganic hybrid materials reported by Batail and Mitzi (7-11). Nanocrystals have been assembled into striking superlattices (12-14), but they do not have discrete structural, electronic and magnetic properties and cannot be regarded as genuine artificial atoms. Here, we combine independently prepared electronically and structurally complementary molecular cluster building blocks to form atomically precise binary solid-state compounds. When the building blocks are atoms (ions), binary solids assemble into simple crystalline arrays such as the rock-salt and CdI 2 lattices [for an authoritative text on solid-state inorganic chemistry, see (15)]. We show that when similarlysized clusters combine the same lattice results, albeit at the dramatically increased length scale of nanometers rather than Angstroms. The constituent clusters interact to produce collective properties such as electrically conducting networks and magnetic ordering. Our strategy was to use constituent molecular clusters that have the same, roughly spherical, shape but very different electronic properties in order to encourage reaction and subsequent structural association. By analogy to "atomic" solid-state chemistry, we reasoned that the in situ transfer of charge would produce ions (or the equivalent) that could then form an ordered solid. Thus, we sought cluster pairs in which one cluster is relatively electron-poor and the other is relatively electron-rich. C 60 carbon clusters are good electron acceptors (16). The electrically neutral metal chalcogenide clusters Co 6 Se 8 (PEt 3 ) We combined 1 and two equivalents of C 60 in toluene and obtained black crystals after ~12 hours. Single-crystal x-ray diffraction (SCXRD) revealed that this solid is a 1:2 stoichiometric combination of 1 and C 60 (1•2C 60 ) Nanoscale Atoms in Solid-State Chemistry We measured how much charge was transferred between the components in the solid-state material using Raman spectroscopy. The A 2 g pentagonal pinch mode of C 60 (1468 cm -1 for pristine C 60 ) shifts to lower energy by 6 cm -1 per electron transferred to C 60 independent of the dopant or the crystal structure [see, for example, (19); for a review on discrete fulleride anions, see •− (20). Cluster 1 has four weak transitions between 350 and 700 nm that were observed in 1•2C 60 but not in 2•2C 60 . We can compare these solids to traditional simple M 2+ X 1-2 solids. The CdI 2 structure type (21) is formed by a hexagonally close-packed array of monoanions with half of the octahedral interstitial sites occupied by dications. The cations are ordered such that along the crystallographic c-direction the cation layers are alternatively empty and fully occupied, and the layers are held together by van der Waals bonding between anions of neighboring layers. The structures of compounds 1•2C 60 and 2•2C 60 can be appreciated in these same terms. Wireframe representation of 1•2C 60 are shown i
