We present IR spectra and quantum chemical calculations for anionic iron-CO2 clusters of the form [Fe(CO2)n](-) (n= 3-7). All observed clusters have at least two CO, units strongly bound to the metal atom. These strongly bound iron-CO2 complexes form the core ions of the clusters, and are solvated by additional, weakly bound CO, molecules. Larger clusters show clear infrared signatures of core ion isomers with three CO, moieties as well. Dominant structural motifs are based on bidentate CO, ligands with Fe-O/Fe-C bonds, oxalate ligands, and metal insertion into a CO bond.