Polar‐covalent bonding beyond the Zintl picture in intermetallic rare‐earth germanides

A comparative chemical bonding analysis for the germanides La${}_2$MGe${}_6$ (M=Li, Mg, Al, Zn, Cu, Ag, Pd) and Y${}_2$PdGe${}_6$ is presented, together with the crystal structure determination for M=Li, Mg, Cu, Ag. The studied compounds adopt the two closely related structure types oS72‐Ce${}_2$(Ga${}_{0.1}$Ge${}_{0.9}$)${}_7$ and mS36‐La${}_2$AlGe${}_6$, containing zigzag chains and corrugated layers of Ge atoms bridged by M species, with La/Y atoms located in the biggest cavities. Chemical bonding was studied by means of the quantum chemical position‐space techniques QTAIM (quantum theory of atoms in molecules), ELI‐D (electron localizability indicator), and their basin intersections. The new penultimate shell correction (PSC0) method was introduced to adapt the ELI‐D valence electron count to that expected from the periodic table of the elements. It plays a decisive role to balance the Ge−La polar‐covalent interactions against the Ge−M ones. In spite of covalently bonded Ge partial structures formally obeying the Zintl electron count for M=Mg${}^{2+}$, Zn${}^{2+}$, all the compounds reveal noticeable deviations from the conceptual 8−N picture due to significant polar‐covalent interactions of Ge with La and M ≠ Li, Mg atoms. For M=Li, Mg a formulation as a germanolanthanate M[La${}_2$Ge${}_6$] is appropriate. Moreover, the relative Laplacian of ELI‐D was discovered to reveal a chemically useful fine structure of the ELI‐D distribution being related to polyatomic bonding features. With the aid of this new tool, a consistent picture of La/Y−M interactions for the title compounds was extracted. (© 2019 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)