The mechanisms of dinitrogen hydrogenation by two different complexess[(eta(5)-C5Me4H)(2)Zr](2)-(eta(2),eta(2),eta(2)-N-2), synthesized by Chirik and co-workers [Nature 2004, 427, 527], and {[P2N2]Zr}(2)(mu(2),eta(2),eta(2)-N-2), where P2N2 = PhP(CH2SiMe2NSiMe2CH2)(2)PPh, synthesized by Fryzuk and co- workers [Science 1997, 275, 1445]-are compared with density functional theory calculations. The former complex is experimentally known to be capable of adding more than one H-2 molecule to the side-on coordinated H-2 molecule, while the latter does not add more than one H-2. We have shown that the observed difference in the reactivity of these dizirconium complexes is caused by the fact that the former ligand environment is more rigid than the latter. As a result, the addition of the first N-2 molecule leads to two different products: a non-H-bridged intermediate for the Chirik-type complex and a H-bridged intermediate for the Fryzuk-type complex. The non-H-bridged intermediate requires a smaller energy barrier for the second H-2 addition than the H-bridged intermediate. We have also examined the effect of different numbers of methyl substituents in [(eta(5)-C-5-MenH5-n)(2)Zr](2)(mu(2),mu(2),eta(2)-N-2) for n = 0, 4, and 5 (n = 5 is hypothetical) and [(eta(5)- C5H2-1,2,4-Me-3)(eta(5)-C5Me5)(2)-Zr] (2)(mu(2),eta(2),eta(2)-N-2) and have shown that all complexes of this type would follow a similar H-2 addition mechanism. We have also performed an extensive analysis on the factors (side-on coordination of N-2 to two Zr centers, availability of the frontier orbitals with appropriate symmetry, and inflexibility of the catalyst ligand environment) that are required for successful hydrogenation of the coordinated dinitrogen.