The addition of acids to ferrous dinitrogen complexes [FeX(N-2)((PNPEt)-N-Et-P-Me)(dmpm)](+) (X = H, Cl, or Br; (PNPEt)-N-Et-P-Me = Et2PCH2N(Me)CH2PEt2; and dmpm = Me2PCH2PMe2) gives protonation at the pendent amine of the diphosphine ligand rather than at the dinitrogen ligand. This protonation increased the nu(N2) band of the complex by 25 cm(-1) and shifted the Fe(II/I) couple by 0.33 V to a more positive potential. A similar IR shift and a slightly smaller shift of the Fe(II/I) couple (0.23 V) was observed for the related carbonyl complex [FeH(CO)((PNPEt)-N-Et-P-Me)(dmpm)](+). [FeH((PNPEt)-N-Et-P-Me)(dmpm)](+) was found to bind N-2 about three times more strongly than NH3. Computational analysis showed that coordination of N-2 to Fe(II) centers increases the basicity of N-2 (vs free N-2) by 13 and 20 pK(a) units for the trans halides and hydrides, respectively. Although the iron center increases the basicity of the bound N2 ligand, the coordinated N-2 is not sufficiently basic to be protonated. In the case of ferrous dinitrogen complexes containing a pendent methylamine, the amine site was determined to be the most basic site by 30 pK(a) units compared to the N-2 ligand. The chemical reduction of these ferrous dinitrogen complexes was performed in an attempt to increase the basicity of the N-2 ligand enough to promote proton transfer from the pendent amine to the N-2 ligand. Instead of isolating a reduced Fe(0)-N-2 complex, the reduction resulted in isolation and characterization of HFe(Et2PC(H)N(Me)CH2PEt2)((PNPEt)-N-Et-P-Me), the product of oxidative addition of the methylene C-H bond of the (PNPEt)-N-Et-P-Me ligand to Fe.