Double σ-Aromaticity in a Surface-Deposited Cluster: Pd4 on TiO2 (110).

We report double σ-aromaticity in a surface-deposited cluster, Pd4, on the TiO2 (110) surface. In the gas phase, Pd4 adopts a tetrahedral structure. However, surface binding promotes a flat, σ-aromatic cluster. This is the first time aromaticity has been found in surface-deposited clusters. The concept of aromaticity is expected to become instrumental in predicting and interpreting properties of such systems, much like it is for isolated molecules and clusters.