"Excess" electrons in LuGe

The monogermanide LuGe is obtained via high-pressure high-temperature synthesis (5-15 GPa, 1023-1423 K). The crystal structure is solved from single-crystal X-ray diffraction data (structure type FeB, space group Pnma, a=7.660(2) Å, b=3.875(1) Å, and c=5.715(2) Å, R${}_F$=0.036 for 206 symmetry independent reflections). The analysis of chemical bonding applying quantum-chemical techniques in position space was performed. It revealed---beside the expected 2c-Ge-Ge bonds in the germanium polyanion---rather unexpected four-atomic bonds between lutetium atoms indicating the formation of a polycation by the excess electrons in the system Lu${}^{3+}$(2b)Ge${}^{2-}$×1 e${}^{-}$. Despite the reduced VEC of 3.5, lutetium monogermanide is following the extended 8-N rule with the trend to form lutetium-lutetium bonds utilizing the electrons left after satisfying the bonding needs in the anionic Ge-Ge zigzag chain. (© 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH)