Privileged metal cluster complexes

Clusters are a prominent subject of interest in modern chemistry research, bridging atoms and materials or catalysts. Metal coordination and metal-metal bonding are crucial in determining the chemical structures and properties of metal clusters; however, formulating a universal principle for assessing their electronic activity remains challenging. Utilizing self-developed mass spectrometry, this study examines the gas-phase reactions of rhodium and platinum clusters with common ligand molecules in forming metal complexes. We find that the Rh _n ^± (n = 1-35) and Pt _n ^± (n = 3-20) clusters readily react with CO and NO to form highly-selective products of cluster complexes. This illustrates the size-dependent saturable effect of sequential coordination, which is rooted in cluster stability alongside concurrent electron delocalization and local bonding. We introduce a new electronic rule, termed the electronic "Aā" rule, to understand the adaptive balance of electron delocalization and averaged local bonding in stabilizing metal clusters, whether they are coordinated with ligands or not.