Neutral All-Metal σ-Aromaticity in a Rhombic Cluster

Aromaticity is a cornerstone concept in chemistry, playing a crucial role in understanding molecular stability and reactivity. Traditionally, aromaticity has been primarily associated with cyclic planar conjugated organic molecules composed solely of carbon, but it has recently expanded to include metal-containing systems. However, metal-only aromatics remain extremely scarce. Here, we present the first neutral all-metal aromatic cluster with a rhombic geometry. X-ray crystallography reveals that the rhombic Al₂Pd₂ core is stabilized by an innovative double-layer N-P ligand framework, featuring essentially the same Al-Pd bond lengths (2.4706(4) and 2.4636(4) Å) and two planar tetracoordinate Al centers. Quantum chemical calculations provide compelling evidence for the two-electron σ-aromaticity in this molecule. Further research on the reactivity of the σ-aromatic Al₂Pd₂ cluster reveals that it can accept lone-pair electron coordination and act as a two-electron reducing agent. This study not only extends the concept of aromaticity to neutral all-metal rhombic systems but also opens new horizons for the synthesis and exploration of novel all-metal aromatic clusters.