The reaction mechanism of the reduction of dinitrogen coordinated side-on to a binuclear Zr complex, [(eta(5)-C5Me4H)(2)Zr](2)(mu(2),eta(2),eta(2)-N-2,)(T1),was investigated theoretically using a model complex, [(eta(5)-C5H5)(2)Zr](2)(mu(2),eta(2),eta(2)-N-2)(A1), employing density functional theory calculations. The effectiveness of Al in describing T1 was confirmed by comparing the structures, distributions of charge, and frontier molecular orbitals. Our calculations showed that Al has a twisted structure, resembling that of T1, which results in similar properties. The calculations for A(1) and its derivatives on H-2 addition clearly explain the reaction mechanism and the reaction path that T1 follows, as well as the experimentally required reaction conditions. The immediate reaction of the first and second H-2 additions produces [(eta(5)-C5Me4H)(2)ZrH](2)(mu(2),eta(2),eta(2) -N2H2) (T2), and this is explained by the barrier heights of the reaction, which were calculated to be 20.4 and 10.9 kcal/mol, respectively. The latter barrier was below that of A1 + 2H(2). Complex T2 may be the initial complex for further H-2 addition under proper conditions, or it could lose one H-2 molecule followed by H migration from the Zr site to the N site. Both reactions are expected to occur, because of the closeness of the barrier heights (25.1 and 36.5 kcal/mol, respectively). Gentle warming is required for further H-2 additions, which can be understood from the energetics as well. The high reactivity of T1 with H-2 has been discussed by the comparison of the calculation of Al and another complex with different ligands, presenting an interesting indication on the effects of the ligands. These theoretical results and discussion explaining the experiment should provide insight into the nature of the hydrogenation mechanism.