Extracting [Pd@Sn₉]^4– and [Rh@Pb₉]^4– Clusters from their Binary Alloys Using “Metal Scissors”

The search for novel ternary intermetallic compounds with specific structures is still a challenge. We found that the two-step synthesis giving a typical alloy in the first step followed by the reaction of this alloy with alkali metals is a promising route. The intermetallic compounds K₁₂Pd_0.47Sn₁₇ and K₄RhPb₉ were synthesized by high-temperature reactions of preformed Pd-Sn and Rh-Pb alloys with K acting as “metal scissors” and were characterized by means of single crystal and powder X-ray diffractometry. The salt-like ternary phases K₁₂Pd_0.47Sn₁₇ and K₄RhPb₉ contain novel endohedrally filled intermetalloid clusters [Pd@Sn₉]^4– and [Rh@Pb₉]^4–, respectively. The crystal packing of the products corresponds to filled variants of the binary Zintl phases K₁₂Sn₁₇ and K₄Pb₉, respectively. The crystal structure of K₁₂Pd_0.47Sn₁₇ can be regarded as a hierarchical replacement variant of the hexagonal Laves phase MgZn₂, with [Pd@Sn₉]^4–/[Sn₉]^4– and [Sn₄]^4– on Mg and Zn positions, respectively, whereas the packing of [Rh@Pb₉]^4– clusters in K₄RhPb₉ is hcp. K₁₂Pd_0.47Sn₁₇ was characterized by Raman spectroscopy. For the first time Raman modes typical for endohedrally filled [Pd@Sn₉]^4– clusters are observed and in accordance with quantum-chemical calculations. In addition Raman spectroscopy shows also the presence of filled Pd@Sn₉ clusters in a phase of nominal composition “Na₁₂Pd₂Sn₁₇”. The results are discussed with respect to the volume increase due the incorporation of transition metal atoms.