We present infrared spectra and density functional theory calculations of mass selected [Sn(CO2) n]- cluster anions ( n = 2-6). The spectra and structures of these clusters exhibit less structural diversity than those of analogous clusters with first-row transition metals, but are more complex than those for the heavy coinage metals or for the related [Bi(CO2) n]- clusters. The most favorable core ion structure for all cluster sizes can be characterized as a Sn-oxalate complex, Sn[C2O4]-. Higher energy isomers based on a bidentate η2-(C,O) CO2 ligand tightly bound to the metal atom in SnCO2- complexes are also observed, even for the largest cluster sizes studied here. For n = 2, another high energy isomer is found, featuring a CO2 ligand weakly bound to the metal atom in a SnCO2- ion.