Polar Intermetallics Pr₅ Co₂ Ge₃ and Pr₇ Co₂ Ge₄ with Planar Hydrocarbon-Like Metal Clusters

Planar hydrocarbon-like metal clusters may foster new insights linking organic molecules with conjugated π-π bonding interactions and inorganic structures in terms of their bonding characteristics. However, such clusters are uncommon in polar intermetallics. Herein, we report two polar intermetallic phases, Pr₅ Co₂ Ge₃ and Pr₇ Co₂ Ge₄ , both of which feature such planar metal clusters, namely, ethylene-like [Co₂ Ge₄ ] clusters plus the concatenated forms and polyacene-like [Co₂ Ge₂ ]_n ribbons in Pr₅ Co₂ Ge₃ , and 1,2,4,5-tetramethylbenzene-like [Co₄ Ge₆ ] cluster in Pr₇ Co₂ Ge₄ . Just as in the related planar organic structures, these metal-metalloid species are dominated by covalent bonding interactions. Both compounds magnetically order at low temperature with net ferromagnetic components: Pr₅ Co₂ Ge₃ through a series of transitions below 150 K and Pr₇ Co₂ Ge₄ through a single ferromagnetic transition at 19 K. Spin-polarized electronic structure calculations for Pr₇ Co₂ Ge₄ reveal strong spin-orbit coupling within Pr and considerable magnetic contributions from Co atoms. This work suggests that similar structural chemistry can emerge for other rare-earth/late-transition-metal/main-group systems.