Recombination of two amidogen radicals, NH2 (X(2)Bl), is relevant to hydrazine formation, ammonia oxidation and pyrolysis, nitrogen reduction (fixation), and a variety of other N/H/X combustion, environmental, and interstellar processes. We have performed a comprehensive analysis of the N2H4 potential energy surface, using a variety of theoretical methods, with thermochemical kinetic analysis and master equation simulations used to treat branching to different product sets in the chemically activated NH2 + NH2 process. For the first time, iminoammonium ylide (NH3NH), the less stable isomer of hydrazine, is involved in the kinetic modeling of N2H4. A new, low-energy pathway is identified for the formation of NH3 plus triplet NH. via initial production of NH3NH followed by singlet-triplet intersystem crossing. This new reaction channel results in the formation of dissociated products at a relatively rapid rate at even moderate temperatures and above. A further novel pathway is described for the decomposition of activated N2H4, which eventually leads to the formation of the simple products N-2 + 2H(2), via H-2 elimination to cis-N2H2. This process, termed as "dihydrogen catalysis", may have significant implications in the formation and decomposition chemistry of hydrazine and ammonia in diverse environments. In this mechanism, stereoselective attack of cis-N2H2 by molecular hydrogen results in decomposition to N-2 with a fairly low barrier. The reverse termolecular reaction leading to the gas-phase formation of cis-N2H2 + H-2 achieves non-heterogeneous catalytic nitrogen fixation with a relatively low activation barrier (77 kcal mol(-1)), much lower than the 125 kcal mol(-1) barrier recently reported for bimolecular addition of H-2 to N-2. This termolecular reaction is an entropically disfavored path, but it does describe a new means of activating the notoriously unreactive N-2. We design heterogeneous analogues of this reaction using the model compound (CH3)(2)FeH2 as a source of the H-2 catalyst and apply it to the decomposition of cis-diazene. The reaction is seen to proceed via a topologically similar transition state, suggesting that our newly described mechanism is general in nature.