Ternary palladium – Group 12 metal compounds of the Pd5TlAs-type: A case study

Three metal-rich palladium-zinc and platinum-zinc selenium- and phosphorus-containing compounds, Pd5ZnSe, Pd5ZnP, and Pt5ZnP, were synthesized using a high-temperature ampoule technique. Their crystal structures were determined from single-crystal synchrotron data (Pd5ZnSe) or Rietveld analysis of powder diffraction data (Pd5ZnSe, Pd5ZnP, and Pt5ZnP). All the compounds crystallize in tetragonal system with P4/mmm space group and belong to the Pd5TlAs structure type, with their main structural units being zinc-centered [TM12Zn] cuboctahedra (TM = Pd, Pt) of the AuCu3-type, single-stacked along the c axis, alternating with [TM8P] rectangular prisms of the PtHg2 type. DFT electronic structure calculations predict all compounds to be 3D metallic conductors and to show diamagnetic behavior. Charge density analysis shows that all compounds are intermetallic in nature. According to the bonding analysis based on the electron localizability indicator topology, all compounds in the TM5MQ series (TM ​= ​Pd, Pt; M ​= ​Zn, Cd, Hg; Q ​= ​Se, P) feature four-centered interactions of the 3TM ​+ ​M type between the transition metal and group 12 atoms in their heterometallic fragments. Additionally, essentially pairwise interactions between platinum atoms are also observed, indicating a somewhat more localized bonding in the case of platinum-based compounds. The use of iodine-assisted synthesis in the Pd–Zn–Se system reveals a new compound, Pd8Zn2Se, that exists as an admixture to Pd5ZnSe and, according to the Rietveld refinement data, can be regarded a second homologue to Pd5ZnSe.