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 _<i>n</i> ^± (<i>n</i> = 1-35) and Pt _<i>n</i> ^± (<i>n</i> = 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 "<i>Aā</i>" 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.