The conversion of metal nitrides to NH3 is an essential step in dinitrogen fixation, but there is limited knowledge of the reactivity of nitrides with protons (H+). Herein, we report comparative studies for the reactions of H+ and NH3 with uranium nitrides, containing different types of ancillary ligands. We show that the differences in ancillary ligands, leads to dramatically different reactivity. The nitride group, in nitride-bridged cationic and anionic diuranium(iv) complexes supported by -N(SiMe3)(2) ligands, is resistant toward protonation by weak acids, while stronger acids result in ligand loss by protonolysis. Moreover, the basic -N(SiMe3)(2) ligands promote the N-H heterolytic bond cleavage of NH3, yielding a "naked" diuranium complex containing three bridging ligands, a nitride (N3-) and two NH2 ligands. Conversely, in the nitride-bridged diuranium(iv) complex supported by -OSi((OBu)-Bu-t)(3) ligands, the nitride group is easily protonated to afford NH3, which binds the U(iv) ion strongly, resulting in a mononuclear U-NH3 complex, where NH3 can be displaced by addition of strong acids. Furthermore, the U-OSi((OBu)-Bu-t)(3) bonds were found to be stable, even in the presence of stronger acids, such as NH4BPh4, therefore indicating that -OSi((OBu)-Bu-t)(3) supporting ligands are well suited to be used when acidic conditions are required, such as in the H+/e(-) mediated catalytic conversion of N-2 to NH3.