A Purely σ-Aromatic, Planar {Ge₄}²⁺-Ring

Aromaticity is a fundamental concept traditionally used to explain the structure, stability, and reactivity of cyclic organic compounds. Heavier analogs have also been investigated for their aromatic character, but their electronic structure was shown to differ significantly depending on the substituents. Herein, we report the synthesis and isolation of a germanium analog of cyclobutadiene Y^N₂Ge₄ (Y^N = Ph₂P(NMes)-C-PPh₃), featuring two iminophosphinoyl-tethered ylide ligands. The Ge₄ compound is obtained through reduction of either chlorogermylene Y^NGeCl or digermylene Y^NGe-GeY^N and exhibits a planar Ge₄ core with a perpendicular coordination of the ylide groups. Detailed computational analyses revealed an aromatic character, as evidenced by several aromaticity descriptors. This aromaticity arises exclusively from the delocalization of the σ-electrons, while any π-bonding is lost due to the coordination of the ligands in the π-plane. This electronic structure resembles that of all-metal clusters, thereby extending the concept of σ- and all-metal aromaticity to cyclobutadiene-like systems.