Ternary early-transition-metal palladium pnictides Zr3Pd4P3, Hf3Pd4P3, HfPdSb, and Nb5Pd4P4

Several ternary palladium pnictides of the early transition metals have bee n prepared by are-melting of the elemental metals and the binary pnictides ZrP, HfP, HfSb2, or NbP, and their structures have been determined by X-ray diffraction methods. The phosphides M3Pd4P3 (M = Zr, Hf) adopt a new struc ture type (Pearson symbol oP40), crystallizing in the orthorhombic space gr oup Pnma with Z = 4 and unit cell parameters of a = 16.387(2), b = 3.8258(5 ), and c = 9.979(1) Angstrom for Zr3Pd4P3 and a = 16.340(2), b = 3.7867(3), and c = 9.954(1) Angstrom for H3Pd4P3. The antimonide HfPdSb was identifie d by powder X-ray diffraction (orthorhombic, Pnma, Z = 4, a = 6.754(1) Angs trom, b = 4.204(1) Angstrom, and c = 7.701(2) Angstrom) and confirmed to be isostructural to ZrPdSb, which adopts the TiNiSi-type structure. The phosp hide Nb5Pd4P4 adopts. the Nb5Cu4Si4-type structure, crystallizing in the te tragonal space group I4/m with Z = 2, a = 10.306(1) Angstrom, and c = 3.637 2(5) Angstrom. Coordination geometries of pentacapped pentagonal prisms for the early transition metal, tetracapped distorted tetragonal prisms for Pd , and tricapped trigonal prisms for the pnicogen are found in the three str uctures; tetracapped tetragonal prisms for Nb are also found in Nb5Pd4P4 In common with many metal-rich compounds whose metal-to-nonmetal ratio is equ al or close to 2:1, the variety of structures formed by these ternary palla dium pnictides arises from the differing connectivity of pnicogen-filled tr igonal prisms. Pnicogen-pnicogen bonds are absent in these structures, but metal-metal bonds (in addition to metal-pnicogen bonds) are important inter actions, as verified by extended Huckel band structure calculations on Zr3P d4P3.