Density functional theory (DFT) calculations on cubane-type metal-sulfido clusters MIr3S4 ligating N-2 (M = V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Ru, and W) have been performed for the proposal of new clusters that can highly activate N-2 beyond the RuIr3S4 cluster prepared by Mizobe and co-workers [Angew. Chem. Int. Ed. 2007, 46, 5431]. The degree of N-2 activation in the metal-N-2 complexes was evaluated based on the N-N bond distance and vibrational frequency and the gross atomic charge on N-2. The degree of N-2 activation strongly depends on the metal atoms at the N-2-binding site, and the MoIr3S4 and WIr3S4 clusters exhibit significant N-2-activation ability. The reactivity of the MIr3S4-N-2 complexes (M = Ru, Mo, and W) with a proton donor (lutidinium) has been discussed from a kinetic aspect by exploring a possible reaction pathway of proton transfer. The protonation of the Ru-N-2 complex would not occur due to a very high-activation barrier and to an instability of the Ru-NNH+ complex, which is consistent with our present experimental result that the Ru-N-2 complex has not been protonated at room temperature. On the other hand, the protonation of the Mo-N-2 and W-N-2 complexes would proceed smoothly from DFT criteria. The result of calculations indicates that the Mo and W clusters are best suited for the protonation of N-2, which is the first step toward nitrogen fixation.