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 <i>P</i>6₃<i>/mcm</i> (No. 193) with <i>a</i> = <i>b</i> = 8.7735(3) Å, <i>c</i> = 13.8260(5) Å, <i>V</i> = 921.66(6) ų, <i>Z</i> = 2 for Tt = Si, and <i>a</i> = <i>b</i> = 8.7995(6) Å, <i>c</i> = 13.9217(14) Å, <i>V</i> = 933.56(16) ų for Tt = Ge phase. The most interesting structural features in these phases are the propeller shape {Pt₇Tt₃} (Tt = Si, Ge) <i>intermetalloid</i> clusters in a <i>D</i>_3<i>h</i> 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