Ca10Pt7Tt3 (Tt = Si, Ge): New Platinide Phases Featuring Electron-Rich 4c–6e Bonded [Pt7Tt3]20– Intermetalloid Clusters

Two new phases Ca10Pt7Tt3 (with Tt = Si, Ge) were obtained by reacting stoichiometric mixtures of the elements at high temperature. Their structures were refined from single crystal X-ray diffraction data. They are isostructural and crystallize in the Ba10Al3Ge7 type structure, space group P63/mcm (No. 193) with a = b = 8.7735(3) Å, c = 13.8260(5) Å, V = 921.66(6) Å3, Z = 2 for Tt = Si, and a = b = 8.7995(6) Å, c = 13.9217(14) Å, V = 933.56(16) Å3 for Tt = Ge phase. The most interesting structural features in these phases are the propeller shape {Pt7Tt3} (Tt = Si, Ge) intermetalloid clusters in a D3h local symmetry. LMTO electronic structure calculations and COHP analyses reveal that both Ca10Pt7Tt3 (Tt = Si, Ge) phases are charge optimized, which is not predicted by the classical Zintl concept and the octet or Wade–Mingo’s rules, but rather by a more complex bonding model based on the unprecedented electron-rich 4c–6e multicenter bonding. The clusters are best described as three-condensed trigonal planar [TtPt3]8– units, resulting in a central Pt atom also with a trigonal planar coordination of three symmetrical equivalent Si/Ge atoms that are further connected to two terminal Pt atoms each. The “trefoil” electron-rich multicenter bonding is proposed here for the first time, and may be viewed as a unique bonding feature with potential relevance for the catalytic properties of the noble metal platinum.