Spectroscopic Characterization of Mass‐Selected Heteronuclear Iron–Silver Carbonyl Cluster Anions

Heteronuclear iron–silver carbonyl anions AgFe(CO) _n ⁻ ( n  = 4–6) are produced via a laser vaporization supersonic cluster ion source and are studied by infrared photodissociation spectroscopy in the carbonyl stretching region. Their structures are determined by comparing experimental spectra with density‐functional theory calculations. The experimentally observed AgFe(CO) _n ⁻ ( n  = 4–5) anions possess (OC) ₄ Fe‐Ag(CO) _{n −4} structures, featuring a formal Fe–Ag σ‐type bond and an 18‐electron Fe(CO) ₄ ⁻ core. AgFe(CO) ₆ ⁻ is a CO‐tagged adduct with a physisorbed CO coexisting with a chemisorbed Ag–CO. Bonding analysis reveals multicenter electron‐sharing σ character. The AgCO unit in AgFe(CO) ₅ ⁻ retains a distinctly bent geometry, owing to silver’s large 4d‐5s energy separation, which quenches π backdonation, and to residual Ag 5s density that repels the CO 5σ lone pair. These factors also weaken the Ag–CO bond and limit CO saturation to n  = 5.