Syntheses and structures of the germanides CaNiGe and MgCoGe as well as chemical bonding in CaNiGe and CaNi₂Ge₂

The intermetallic germanides CaNiGe and MgCoGe have been synthesized from the elements in sealed tantalum tubes in a high-frequency furnace. The compounds were investigated by X-ray diffractions both on powders and single crystals: CeFeSi structure type, P4/nmm, a = 4.19341(3), c = 6.6264(1)Å, wR2 = 0.030, 124 F² values, 10 variable parameters for CaNiGe and a = 3.8960(4), b = 6.1929(11)Å, wR2 = 0.048, 104 F² values, 10 variable parameters for MgCoGe. In CaNiGe and MgCoGe the transition metal and germanium atoms build [TGe] layers (T = Ni, Co), which are separated by the calcium and magnesium atoms, respectively. The crystal structures of CaNiGe and MgCoGe as well as chemical bonding in CaNiGe and CaNi₂Ge₂ are discussed in terms of LMTO bond structure calculation and analysis using the Electron Localization Function (ELF). The Ge-Ge bond formation in polyanionic _∞³[NiGe]^- network of CaNi ₂Ge₂ can formally be regarded as oxidative coupling product of _∞²[NiGe]^2- layers of CaNiGe.