A set of flat carbon clusters and ultrashort carbon nanotubes in different anion-radical states was investigated by density functional theory and complete-active space self-consistent field methods. It was found that carbon nanoparticles with pervasively oxidized edges are extremely strong oxidants, and their ground states in catalytic studies should be considered as negatively charged rather than neutral, as it is traditionally done. Negative charging renders initially diamagnetic/semiconducting types of carbon nanoparticles into half-metallic, which can also be achieved by doping with transition metals.