CO Activation on Silver Clusters: How Valence Electrons, Geometry, and p Back-Donation Dictate Reactivity

While CO adsorption on transition metal clusters holds promise for environmental remediation, how silver cluster properties─such as size-dependent atomic packing, molecular orbitals, and superatomic character─govern site-specific binding remains unclear. Here, we combine time-of-flight mass spectrometry (TOF-MS) and density functional theory (DFT) to investigate gas-phase CO reactions with Ag_n^- (n = 10-25) clusters. Charge effects are assessed via CO adsorption on Ag_n^+,0 (n = 14, 16, 18, 20) clusters. We find enhanced CO reactivity below 18 valence electrons, showing odd-even oscillations in adsorption energy, a decline from 18 to 22 electrons, and a rebound beyond 23. Key factors include (1) the stabilization of high-coordination Ag* sites by electron donation; (2) orbital interactions involving σ donation, π back-donation, and newly identified p back-donation requiring matched HOMO/LUMO symmetry; (3) hybridization between C-Ag* σ bonds and adjacent d orbitals favoring icosahedral or oblate over cage structures; and (4) jellium-based electron counting, where open shells enhance reactivity relative to closed ones. These results provide critical atomic-scale insights into CO adsorption mechanisms on silver clusters.