Infrared spectra of mass-selected homoleptic dinuclear iron carbonyl cluster anions Fe₂(CO)_n⁻ (n = 4–9) are measured via infrared photodissociation spectroscopy in the carbonyl stretching frequency region. The cluster anions are produced via a laser vaporization supersonic cluster source. Density functional calculations have been performed and the calculated vibrational spectra are compared to the experimental data to identify the gas-phase structures of the cluster anions. The experimentally observed Fe₂(CO)_n⁻ (n = 4–7) cluster anions are characterized to have unusual asymmetric (OC)₄Fe–Fe(CO)_n−4 structures, which also correspond to the computed lowest energy structures. The experimentally observed Fe₂(CO)₈⁻ cluster anion is determined to have an unbridged structure instead of the previously reported dibridged structure. The Fe₂(CO)₉⁻ cluster anion is determined to involve a Fe₂(CO)₈⁻ core anion that is solvated by an external CO molecule. Bonding analysis indicates that these anions each have a Fe–Fe single bond to satisfy the 18-electron configuration of one iron center. The results provide important new insight into the structure and bonding mechanisms of transition-metal carbonyl clusters.