Synthesis, Structure and Bonding of SrCa2In2Ge: A New Zintl Phase with an Unusual Inorganic π-System

The Zintl concept provides an effective and useful way to describe the chemical bonding in a wide variety of main group intermetallic structures. The success of the concept in rationalizing the syntheses and discovery of complex Zintl phases has enhanced its validity as an effective approach in the search for new polar intermetallics and for rationalization of their chemical bonding.1-5 In evaluating the limits of the Zintl concept, an empirical boundary between elements of groups 13 (trelides) and 14 (tetrelides) represents the violations for many group 13 polar intermetallics. This is illustrated by the unlikely situation that the elements of group 13 have enough effective core potential to accumulate a high number of electrons, up to 5 electrons, without significant mixing with the electronic states of the metal component. However, existence of normal trelide Zintl phases and the non-Zintl behavior of several tetrelides and pnictides certainly imply that the Zintl boundary is not exact. The novelty in the chemical bonding and crystal structure of polar main group intermetallics that lie near the Zintl border allows for the possibility of discovering unique electronic phenomena. In recent years, Corbett and co-workers, as well as Belin and co-workers, have reported on many novel Ga, In, and Tl cluster compounds that follow Wade’s rules as in the boranes.7-9 Studies on trelides have been spurred by the novelty of their cluster chemistry. Other reports on polar intermetallics of trelides and mixed In-Ge anions have resulted in intriguing questions concerning the ability of In to accommodate high negative charges.1,3,6-10 Recently, it was shown that La3In4Ge contains layers of indium with a delocalized open-shell band structure.10 Also, recent work by von Schnering and co-workers on new silicides and germanides reveals the existence of Si and Ge arene-like systems in describing complex electron-deficient silicides and germanides.11 Our present studies on polar intermetallics involving trelide and tetrelide post-transition-metals focus on the structural, chemical, and physical characteristics of “electron-deficient” Zintl phases.16 These phases represent a class of Zintl phases where the normal picture of covalent networks of singly bonded metalloids may not be sufficient to satisfy the valence requirements of the anionic substructures. Our exploratory syntheses along the Zintl border have led to the discovery of one such compound, SrCa2In2Ge. Its crystal structure and chemical bonding feature a novel [In2Ge] chain analogous and isoelectronic with an allyl anion chain. The air-sensitive title compound was synthesized through high-temperature reactions of nearly stoichiometric amounts of the pure elements (distilled Ca and Sr metals from APL Engineering Labs: in shots, 99.9999%; Ge pieces, 99.9999%) in welded Ta tubing within an evacuated quartz jacket. A slight excess (∼10%) of indium and germanium were necessary to obtain high yields (>80%). The reactions were performed at 1050 °C over 5 days with prior heating under dynamic vacuum at 350-450 °C for 5-8 h. The reaction was terminated by rapid quenching to room temperature. All sample manipulations were done within a purified Argon atmosphere glovebox. The title compound, SrCa2In2Ge, forms as dark metallic rods and decomposes peritectically, which makes it difficult to obtain as pure phase product. Single crystals were obtained from reactions with large excess (>100%) of In and Ge. Dimensional analyses do not show any significant changes in the lattice parameters with variations in the loaded reaction stoichiometries. This is confirmed by chemical analysis on several single crystals which indicates a nearly constant stoichiometry corresponding to SrCa2In2Ge. Crystal structure investigations were performed on powder and single crystals by X-ray diffraction. SrCa2In2Ge crystallizes in the orthorhombic space group Pnna with four formula units in the unit cell.17 The crystal structure of SrCa2In2Ge, an ordered derivative of the CrB-type, is shown in Figure 1. The crystal structure can be described as an ordered-derivative of CrB structure type.18,19 This structure type is characterized by trigonal prisms of the metal atoms (Cr) centered by nonmetal atoms (B). The nonmetal atom arrangement results in parallel zigzag chains along the c-axis. Most alkaline-earthand rareearth-metal monosilicides and monogermanides crystallize in