Using a density-functional approach, we investigate the reactivity of supported metal nanoclusters. We focus on the sequential adsorption of N-2 molecules on a Fe nanocluster supported by an MgO substrate. For an increasing number of N atoms preadsorbed on the nanocluster, we found that the binding energy of the N-2 molecule increases, and can become higher than that of its dissociation products, in marked contrast with the behavior at the respective metallic surface. We identify the electrostatic interaction as a primary factor determining this behavior, and discuss the observed trends in terms of interactions with highest occupied and lowest unoccupied molecular orbitals.