Synthesis, Crystal Structure, and Bonding Analysis of the Hypoelectronic Cubic Phase Ca5Pd6Ge6

The title compound, Ca5Pd6Ge6, was obtained during a systematic investigation of the Ca-Pd-Ge ternary phase diagram. The crystal structure was determined and refined from single-crystal X-ray diffraction data. It crystallizes in a new structure variant of the Y4PdGa12-type structure (Im (3) over barm, a = 8.7764(4) angstrom) that features an arrangement of vertex-sharing body-centered cubes of calcium, Ca@Ca-8, with a hierarchical bcc network, interpenetrating a second (Pd6Ge6) network consisting of Ge-2 dunibbells surrounded by Pd in a strongly flatteUed octahedron with Pd(mu(2)-eta(2),eta(4)-Ge-2)-like motifs. These octahedra are condensed through the Pd to form a 3D open fcc network Theoretical band structure calculations suggested that the compound is hypoelectronic with predominantly multicenter-type interatomic interactions involving all three elements and essentially a Hume-Rothery-like regime of electronic stabilization. The similar electronegativity between germanium and palladium atoms has a decisive impact on the bonding picture of the system.