Fe-mediated biological nitrogen fixation is thought to proceed via either a sequence of proton and electron transfer steps, concerted H atom transfer steps, or some combination thereof. Regardless of the specifics and whether the intimate mechanism for N-2-to-NH3 conversion involves a distal pathway, an alternating pathway, or some hybrid of these limiting scenarios, Fe NxPly intermediates are implicated that feature reactive N-H bonds. Thermodynamic knowledge of the N-H bond strengths of such species is scant, and is especially difficult to obtain for the most reactive early stage candidate intermediates (e.g., Fe-N=NH, Fe=N-NH2, Fe-NH=NH). Such knowledge is essential to considering various mechanistic hypotheses for biological (and synthetic) nitrogen fixation and to the rational design of improved synthetic N-2 fixation catalysts. We recently reported several reactive complexes derived from the direct protonation of Fe N-2 and Fe-CN species at the terminal N atom (e.g., Fe=N-NH2, Fe C equivalent to NH, Fe equivalent to C-NH2). These same Fe-N-2 and Fe-CN systems are functionally active for N-2-to-NH3 and CN-to-CH4/NH3 conversion, respectively, when subjected to protons and electrons, and hence provide an excellent opportunity for obtaining meaningful N H bond strength data. We report here a combined synthetic, structural, and spectroscopic/analytic study to estimate the N-H bond strengths of several species of interest. We assess the reactivity profiles of species featuring reactive N H bonds and estimate their homolytic N H bond enthalpies (BDEN_(H)) via redox and acidity titrations. Very low N-H bond dissociation enthalpies, ranging from 65 (Fe-C equivalent to NH) to <= 37 kcal/mol (Fe N=NH), are determined. The collective data presented herein provide insight into the facile reactivity profiles of early stage protonated Fe-N2 and Fe-CN species.