Despite the critical role Ru and Os complexes have played in the development of transition metal dinitrogen chemistry, they have not been shown to mediate catalytic N-2-to-NH3 conversion (N2RR), nor have M-N Hy complexes been derived from protonation of their M-N-2 precursors. To help delineate factors for N2RR catalysis, we report on isostructural tris(phosphino)silyl Ru and Os complexes that mediate catalytic N2RR, and compare their activities with an isostructural Fe complex. The Os system is most active, and liberates more than 120 equiv NH3 per Os center in a single batch experiment using Cp*Co-2 and [H2NPh2] [OTf] as reductant and acid source. Isostructural Ru and Fe complexes generate little NH3 under the same conditions. Protonation of Os-N-2 affords a structurally characterized Os=NNH2+ hydrazido species that mediates NH3 generation, suggesting it is a plausible intermediate of the catalysis. Inactive Os hydrides are characterized that form during catalysis.