The N-N bond activation of the dinitrogen ligand in the cubane-type mixed-metal sulfido cluster, [(Cp*Ir)(3){Ru(tmeda)(N-2)}(mu(3)-S)(4)] (tmeda = Me2NCH2CH2NMe2), is investigated by using DFT calculations at the B3LYP level of theory. The elongated N-N bond distance, red-shifted N-N stretching, and negatively charged N2 ligand indicate that the dinitrogen is reductively activated by complexation. The degree of the N-N bond activation is classified into the "moderately activated" category, [Studt, F.; Tuczek, F. J. Comput. Chem. 2006, 27, 1278] as in the Mo-triamidoamine complex that can catalyze N-2 reduction [Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76]. Availability of the RuIr3S4 cluster as a catalyst for N-2 reduction is discussed by optimizing possible intermediates in a catalytic cycle analogous to that proposed by Yandulov and Schrock. A calculated energy profile of the catalytic cycle demonstrates that the RuIr3S4 cluster can transform dinitrogen into ammonia in the presence of lutidinium cation and CP*Co-2 as proton and electron sources, respectively. The RuIr3S4 clusters with an NNHx (x = 1-3) ligand, which are intermediates in the catalytic cycle, have a significantly bent Ru-N-N linkage, although precedent NNHx complexes generally adopt a linear M-N-N array. The unique structures of the nitrogenous ligands in these intermediates are interpreted in terms of the bonding interaction between the hydrogen atom bonded to the N-2 ligand and the adjacent iridium atom in the cuboidal RuIr3S4 framework.