An activated side-on-bound ansa-zirconocene dinitrogen complex, [Me2Si(eta(5)-C5Me4)(eta(5)-C5H3-3-Bu-t)Zr](2)(mu(2),eta(2),eta(2)-N-2), has been prepared by sodium amalgam reduction of the corresponding dichloride precursor under an atmosphere of N-2. Both solution spectroscopic and X-ray diffraction data establish diastereoselective formation of the syn homochiral dizirconium dimer. Addition of 1 atm of H-2 resulted in rapid hydrogenation of the N-2 ligand to yield one diastereomer of the hydrido zirconocene diazenido complex. Kinetic measurements have yielded the barrier for H-2 addition and in combination with isotopic labeling studies are consistent with a 1,2-addition pathway. In the absence of H-2, the hydrido zirconocene diazenido product undergoes swift diazene dehydrogenation to yield an unusual hydrido zirconocene dinitrogen complex. The NN bond length of 1.253(5) angstrom determined by X-ray crystallography indicates that the side-on-bound N-2 ligand is best described as a two-electron reduced [N-2](2-) fragment. Comparing the barrier for deuterium exchange with [Me2Si(eta(5)-C5Me4)(eta(5)-C5H3-3-Bu-t)ZrH](2)(mu(2),eta(2),eta(2)-N2H2 ) to diazene dehydrogenation is consistent with rapid 1,2-elimination of dihydrogen followed by rate-determining hydride migration to the zirconium. This mechanistic proposal is also corroborated by H-2 inhibition and the observation of a normal, primary kinetic isotope effect for dehydrogenation.