ZUSCHRIFTEN 3086  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001 0044-8249/01/11316-3086 $ 17.50+.50/0 Angew. Chem. 2001, 113, Nr. 16 Pentagonal-Dodecahedral La 2 Charge Density in [80-I h ]Fullerene: La 2 @C 80 ** Eiji Nishibori, Masaki Takata,* Makoto Sakata, Atsushi Taninaka, and Hisanori Shinohara New types of endohedral metallofullerenes [1] have been synthesized and structurally characterized in the past few years. [2] In particular, interesting behavior of metal atoms trapped inside carbon cages were revealed by structural studies, e.g. giant thermal motion; [3] formation of metal clusters, [2a] metal nitrides, [2b] and metal carbides; [2e] and violation of the isolated pentagon rule (IPR). [2c,d] These characteristic features were specific to metallofullerenes and closely related to the cage symmetry and the electrostatic interaction between the metal atoms and the carbon cage, which results from charge transfer from metal atoms to the carbon cage. La 2 @C 80 was one of the first endohedral metallofullerenes, and was produced in macroscopic amounts as early as 1991. It was also the first endohedral dimetallofullerene. [4] Although the cage symmetry of C 80 was determined to be I h , similar to that of C 60 , from ab initio theoretical calculations [5] and 13 C NMR spectroscopy, [6] the endohedral structure of La 2 @C 80 remained unknown. Here we report a novel feature of La 2 @C 80 , in which a perfect pentagonal-dodecahedral charge density of La 2 was detected in an icosahedral I h -C 80 cage by a synchrotron radiation (SR) structural study. The character- istic charge density results from a highly selective trajectory of the two La atoms, which hop along the hexagonal rings of the I h -C 80 polyhedral network. The structural analysis was performed by means of SR powder diffraction. The experimental data were analyzed by the MEM/Rietveld method, [7] a self-consistent iterative anal- ysis of a combination of the maximum entropy method (MEM) and Rietveld analysis. The structures of endohedral metallofullerenes such as Y@C 82 , [7a] Sc@C 82 , [8] La@C 82 , [3] Sc 2 @C 84 , [9] Sc 3 @C 82 , [2a] Sc 2 @C 66 , [2e] and Sc 2 C 2 @C 84 [2c] were successfully visualized as charge densities by using synchro- tron radiation powder-diffraction data. Details of this method are described elsewhere. [7, 9] The cell parameters (monoclinic, space group P2 1 ) were determined as a  18.2872(8), b  11.2120(4), c  11.1748(4) , and b  107.91(1)8 by Rietveld analysis. The reliability (R) factor based on the Bragg intensities R I and the weighted profile R factor R wp were 6.3 and 2.6 %, respectively. Figure 1 shows the result of Rietveld fitting. The number of structure factors derived in this analysis was 502. The MEM charge density was derived from these structure factors by dividing the unit cell into 128  72  72 pixels. The R factor of the final MEM charge density was 3.8 %. Figure 1. The result of Rietveld fitting for La 2 @C 80 . A closeup of the region between 14.0 and 35.08 is shown in the insert. The reliability factors are R I  6.3 % and R wp  2.6 %. A three-dimensional representation of the La 2 @C 80 mole- cule is shown in Figure 2 as a surface of equal charge density (1.7 e  3 ). The MEM charge density clearly reveals six- and Figure 2. MEM charge densities of La 2 @C 80 as the equal-density (1.7 e  3 ) contour surface along the a) S 10 and b) C 2 axes. The La 2 dodecahedral charge density is colored brown and is additionally shown beside the fullerene molecule. five-membered rings consistent with the reported I h symmetry of the C 80 cage. Figures 2 a and b represent the top views along S 10 and C 2 axis of I h -C 80 , respectively. A pentagonal-dodeca- hedral cage (brown), which is presumably due to the charge density of the encaged La atoms, is clearly recognizable inside the C 80 cage. The inner dodecahedral cage is also shown separately. To confirm that the charge density of this dodecahedral cage stems from the La atoms encapsulated in the C 80 cage, we counted the total charge of this internal polyhedron. The total charge, calculated on the basis of the MEM results, is 108.8 e, which is very close to the number of electrons for two La 3 ions (108 e). We therefore conclude that the internal dodeca- hedron represents the distribution of two La 3 encapsulated in [*] Prof. Dr. M. Takata, Dr. E. Nishibori, Prof. Dr. M. Sakata Department of Applied Physics, Nagoya University Nagoya 464-8603 (Japan) Fax: ( 81) 52-789-4455 E-mail : a41024a@nucc.cc.nagoya-u.ac.jp A. Taninaka, Prof. Dr. H. Shinohara Department of Chemistry, Nagoya University Nagoya 464-8602 (Japan) [**] This work was supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan, JSPS Future Program on New Carbon Nano-Materials, and the TARA Sakabe Project. This work was also supported by the Toyota-Riken, the Murata Science Foundation, and the Sumitomo Foundation. The synchrotron radiation experiments were performed at SPring-8 BL02B2 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI).