Synthesis, Crystal Structure and Chemical Bonding of the Zintl Phase Rb 7 NaSi 8

Abstract. The Zintl phase Rb 7 NaSi 8 was synthesized as single‐phase material. Powder and single‐crystal X‐ray diffraction, thermal analysis, chemical analysis, measurement of the magnetic susceptibility, and 23 Na, 29 Si, and 87 Rb nuclear magnetic resonance (NMR) spectroscopy were employed to characterize the material. Rb 7 NaSi 8 crystallizes in the Rb 7 NaGe 8 type of structure forming trigonal pyramidal anions Si 4 4– . Two unique environments of the cations are observed, the linear arrangement [Na(Si 4 ) 2 ] 7– with short Na–Si distances of 2.94 Å and the Rb2 atom coordinated by six Si 4 4– anions with long Rb–Si distances of 4.09 Å. The environment of the Rb1 atom is similar to the coordination of the rubidium atoms in Rb 4 Si 4 . The chemical bonding was investigated by quantum mechanical calculations of the electron localizability indicator (ELI), the electronic density of states (DOS), and NMR coupling parameters. Good agreement of theoretically calculated and experimentally determined NMR coupling parameters was obtained for both chemical shielding and quadrupole coupling. The anisotropy of chemical shielding (CSA) indicates an anisotropic bonding situation of the silicon atoms. This is confirmed by the observation of a lone‐pair‐like feature and three two‐center Si–Si bonds for each silicon atom using the ELI. The electric field gradients (EFG) of 23 Na and 87 Rb indicate the anisotropy of the charge distribution of the cations. In particular, the linear arrangement [Na(Si 4 ) 2 ] 7– and the Rb1 atom feature anisotropic charge distributions of the cations. This is confirmed by calculating the anisotropy ratio defined as Δ n p = n ( p z )/(½[ n ( p x )+ n ( p y )]) with n ( p i ) corresponding to the integrated DOS of the respective state. The long distances of Rb2 to the six coordinating Si 4 4– ions are reflected by the small EFG.