The electronic and geometrical structure of the ground and excited states of FenCO, FenCO-, and FenCO+ (n=2-6) are computed using density functional theory (DFT). Several hybrid and pure DFT methods are tested on FeCO and Fe2CO where experimental data are available. The pure DFT methods are superior to hybrid methods in reproduction of experimental spectroscopic data, except for the dissociation energy of FeCO. CO is twofold coordinated in Fe2CO, Fe3CO, and Fe4CO, threefold coordinated in Fe5CO, and fourfold in Fe6CO. While the Boudouard-type disproportionation reaction FenCO+CO-->FenC+CO2 is endothermic by 1.59, 1.10, and 0.55 eV for FeCO, Fe2CO, and Fe3CO, respectively, it becomes exothermic beginning with n=4. The reaction barrier decreases from 3.5 eV for FeCO to 1.3 eV for Fe4CO. The most exothermic (by 0.42 eV) reaction is Fe6CO++CO-->Fe6C++CO2. It is found that the catalytic ability increases with clusters size due to the decreasing FenC-O (or increasing Fe-n-C) bond strength. The Fe-n-CO binding energy varies slowly with cluster size and therefore does not significantly influence the variation in the energetics of the Boudouard-type reaction with cluster size. (C) 2003 American Institute of Physics.