Magnesium and barium in two substructures: BaTMg₂ (T = Pd, Ag, Pt, Au) and the isotypic cadmium compound BaAuCd₂ with MgCuAl₂ type structure

The intermetallic barium compounds BaTMg2 (T = Pd, Ag, Pt, Au) and BaAuCd2 were synthesized by reactions of the elements in sealed tantalum ampoules in muffle furnaces. The five compounds crystallize with the orthorhombic MgCuAl2 type structure, space group Cmcm, with small differences in chemical bonding between the magnesium and cadmium series. All samples were characterized through their Guinier powder diffraction patterns. The structures of BaPdMg2 (a = 444.57(4), b = 1174.67(10), c = 827.58(7) pm, wR2 = 0.0460, 475 F-2 values, 16 variables), BaAuMg2 (a = 450.27(6), b = 1183.94(16), c = 838.76(11) pm, wR2 = 0.0355, 473 F-2 values, 16 variables) and BaAuCd2 (a = 463.31(5), b = 1112.79(12), c = 826.63(8) pm, wR2 = 0.0453, 469 F-2 values, 16 variables) were refined from single crystal X-ray diffraction data. The large barium atoms push the [TMg2] respectively [AuCd2] substructures apart. This allows fast moisture attack and leads to fast hydrolyzes of the samples when they get in contact with water. The influence of the difference in electronegativity between magnesium and cadmium is reflected for the pair of compounds BaAuMg2 and BaAuCd2. The magnesium compound shows the higher auridic character, while the cadmium compound shows a tendency towards a three-dimensional cadmium substructure.