Ternary Early-Transition-Metal Palladium Pnictides Zr₃Pd₄P₃, Hf₃Pd₄P₃, HfPdSb, and Nb₅Pd₄P₄

Several ternary palladium pnictides of the early transition metals have been prepared by arc-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 structure type (Pearson symbol oP40), crystallizing in the orthorhombic space group Pnma with Z = 4 and unit cell parameters of a = 16.387(2), b = 3.8258(5), and c = 9.979(1) Å for Zr3Pd4P3 and a = 16.340(2), b = 3.7867(3), and c = 9.954(1) Å for Hf3Pd4P3. The antimonide HfPdSb was identified by powder X-ray diffraction (orthorhombic, Pnma, Z = 4, a = 6.754(1) Å, b = 4.204(1) Å, and c = 7.701(2) Å) and confirmed to be isostructural to ZrPdSb, which adopts the TiNiSi-type structure. The phosphide Nb5Pd4P4 adopts the Nb5Cu4Si4-type structure, crystallizing in the tetragonal space group I4/m with Z = 2, a = 10.306(1) Å, and c = 3.6372(5) Å. 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 structures; tetracapped tetragonal prisms for Nb are also found in Nb5Pd4P4. In common with many metal-rich compounds whose metal-to-nonmetal ratio is equal or close to 2:1, the variety of structures formed by these ternary palladium pnictides arises from the differing connectivity of pnicogen-filled trigonal prisms. Pnicogen−pnicogen bonds are absent in these structures, but metal−metal bonds (in addition to metal−pnicogen bonds) are important interactions, as verified by extended Hückel band structure calculations on Zr3Pd4P3.