Substrate selectivity in reductive multielectron/proton catalysis with small molecules such as N-2, CO2, and O-2 is a major challenge for catalyst design, especially where the competing hydrogen evolution reaction (HER) is thermodynamically and kinetically competent. In this study, we investigate how the selectivity of a tris(phosphine)borane iron(I) catalyst, (P3Fe+)-Fe-B, for catalyzing the nitrogen reduction reaction (N2RR, N-2-to-NH3 conversion) versus HER changes as a function of acid pK(a). We find that there is a strong correlation between pKa and N2RR efficiency. Stoichiometric studies indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of N-2 reduction (e.g., Fe(NNH) or Fe(NNH2)) in the presence of the metallocene reductant Cp*Co-2. This suggests that the interaction of acid and reductant is playing a critical role in N-H bond-forming reactions. DFT studies identify a protonated metallocene species as a strong PCET donor and suggest that it should be capable of forming the early stage N-H bonds critical for N2RR. Furthermore, DFT studies also suggest that the observed pK(a) effect on N2RR efficiency is attributable to the rate and thermodynamics of Cp*Co-2 protonation by the different anilinium acids. Inclusion of Cp*Co-2(+) as a cocatalyst in controlled potential electrolysis experiments leads to improved yields of NH3. The data presented provide what is to our knowledge the first unambiguous demonstration of electrocatalytic nitrogen fixation by a molecular catalyst (up to 6.7 equiv of NH3 per Fe at -2.1 V vs Fc(+/0)).