Tetrahedral Stars as Flexible Basis Clusters in sp-Bonded Intermetallic Frameworks and the Compound BaLi 7 Al 6 with the NaZn 13 Structure Ulrich Ha 1 ussermann,* Christer Svensson, and Sven Lidin Contribution from the Department of Inorganic Chemistry 2, Lund UniVersity, P.O. Box 124, S-22100 Lund, Sweden ReceiVed September 23, 1997. ReVised Manuscript ReceiVed December 29, 1997 Abstract: The cluster units obtained by the capping of all faces of a tetrahedron (tetrahedral star, TS) and a trigonal bipyramid (double tetrahedral star, DTS) are used as building units to describe and rationalize framework structures found in the intermetallic structure types NaBa, NaZn 13 , Th 6 Mn 23 , Ba 2 Li 4.21 Al 4.79 , BaHg 11 , BaCd 11 , Cr 23 C 6 , -Mn, Ba 3 Li 3 Ga 4.1 , BaLi 4 , CaZn 3 , EuMg 5.2 , ErZn 5 , Sr 3 Mg 13 , and Sr 9 Li 17.5 Al 25.5 . The electronic requirements for optimum structural stability of these frameworks in the case of sp-bonding have been investigated with the simple tight-binding Hu ¨ckel model. As a result the networks exhibit a pronounced maximum of stability in the range of 2.1-2.6 electrons per atom with the particular optimum values slightly depending on the kind of basis cluster and its connectivity. Considering the sp-bonded representatives of the above mentioned structure types, the frameworks described by the basis clusters TS and DTS are usually formed by the divalent metals Be, Mg, Zn, Cd, and Hg or a combination of a mono- and a trivalent metal, e.g., (Li,Al), (Cu,Al), or (Ag,Al). More electropositive atoms, like the heavier alkaline earth metals Ca, Sr, and Ba, are embedded in such frameworks. When applying a formal electron transfer from these atoms to the slightly more electronegative framework forming atoms, the obtained values for the framework valence electron concentration are very close to the calculated optimum ones. Therefore it is argued that this family of intermetallic compounds can still be interpreted as electron compounds following the Zintl-Klemm concept. The new representative BaLi 7 Al 6 , which has been synthesized by fusion of the elements and characterized by single-crystal X-ray diffraction methods, supports this idea. BaLi 7 Al 6 is isotypic to NaZn 13 (Fm3 hc, a ) 12.9377- (9) Å, Z ) 8) with the Zn sites mixed occupied by Li and Al atoms. 1. Introduction In the past few years research on polar intermetallic com- pounds has developed into one of the most exciting fields in inorganic chemistry. A wealth of novel and peculiar structures has been discovered and unforeseen bonding patterns based on multicenter bonding has been analysed. 1-4 This has led to an increased understanding of chemical bonding in systems at the metal nonmetal border reflecting the transition from covalent to metallic bonding. 5,6 Polar intermetallics are compounds formed between two or more metallic elements with distinctly different electronega- tivities. In particular the electronegative component is one of the group 13 elements which are called triels (Tr), and the more electropositive counterpart is represented by an alkali metal (A) or/and an alkaline earth metal (Ae) or a trivalent rare-earth element (R). In these systems triel atoms are reduced by the electropositive component and thus obtain a valence electron concentration (VEC, number of valence electrons of a formula unit per Tr atom) which enables them to fit the electronic requirement for the formation of a remarkable variety of sp- bonded networks, clusters, or cluster frameworks. The formal electron transfer is in the spirit of the Zintl-Klemm concept, and indeed many of the polar intermetallics can be counted as Zintl phases, especially when applying the criterion diamagne- tism for defining (closed-shell) Zintl phases. 3 However, a rather large number of polar intermetallics exhibit small deviations from the electron count which would follow from the Zintl- Klemm concept. This indicates that other contributions to structural stability, like packing and interrelated size effects, gain importance compared to the stability-determining VEC in Zintl phases. But still the Zintl-Klemm concept provides an excellent starting point for the interpretation and prediction of bonding motifs in these compounds. One may roughly distinguish between three different kinds of Tr substructures occurring in the polar intermetallics: open networks found in the systems Ae/Tr in a range of VEC between 3.5 and 5.0; isolated clusters in the systems A/Tr and A/A′/Tr for values of VEC between 3.6 to 5.0; and in the same systems frameworks of clusters in a range of VEC between 3.14 and 3.6. Characteristically these clusters represent deltahedral units or are related to them and thus correspond to entities with a (1) Belin, C.; Tillard-Charbonell, M. Prog. Solid State Chem. 1993, 22, 59 and references therein. (2) Eisenmann, B.; Cordier, G. In Chemistry, Structure and Bonding of Zintl Phases and Ions; Kauzlarich, S. M., Ed.; VCH Publishers: New York, 1996; Chapter 2 and references therein. (3) Corbett, J. D. In Chemistry, Structure and Bonding of Zintl Phases and Ions; Kauzlarich, S. M., Ed.; VCH Publishers: New York, 1996; Chapter 3 and references therein. (4) Nesper, R. Habilitationsschrift; University of Stuttgart: Stuttgart, Germany, 1988; (in German). (5) Nesper, R. Angew. Chem., Int. Ed. Engl. 1991, 30, 789 and references therein. (6) Miller, G. J. In Chemistry, Structure and Bonding of Zintl Phases and Ions; Kauzlarich, S. M., Ed.; VCH Publishers: New York, 1996; Chapter 1 and references therein. 3867 J. Am. Chem. Soc. 1998, 120, 3867-3880 S0002-7863(97)03335-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/14/1998