In this work, we investigate with density functional methods mechanistic details of catalytic dinitrogen reduction mediated by Schrock's molybdenum complex under ambient conditions. We explicitly take into account the full HIPTN3N ligand without approximating it by model systems. Our data show that replacement of the bulky HIPT substituent by smaller groups leads to deviations in energy of up to 100 kJ mol(-1). Alternatives to the Chatt-like mechanism are also investigated. It turns out that for the generation of the first molecule of ammonia, protonation of the ligand plays a crucial role. With an increasing number of hydrogens on the terminal nitrogen atom, the reduction becomes more difficult. The energetically most feasible step is the generation of the first molecule of ammonia, while the preceding transfer of the second electron and proton is the most difficult one. Reaction energies are not only reported for decamethyl chromocene as in previous studies but also for a series of other metallocenes. Furthermore, results are provided in a way to allow for a convenient estimation of the thermochemical boundary conditions of catalysis with an arbitrary combination of acid and reductant. We demonstrate that the [Mo](NNH3)(+) complex easily loses ammonia even in the absence of a reductant. For some complexes, spin states with higher multiplicity are the ground state instead of those with lower spin multiplicity.