We collected solid-state infrared (IR) data of all three crystallized complexes (Figure middle) and observed a strong sharp band at 2116.3 cm -1 in FeN2 that is characteristic for a νNN stretching vibration. The band is not present for the FeNP complex. The high wavenumber of 2116.3 cm -1 in comparison to other transition metal dinitrogen complexes is indicative for a weak Fe-N bond. For FeNP we observed a shoulder signal at 1270.7 cm -1 that is overlapping with another signal but clearly not present in FeN2. We assign this signal to the νNP stretching mode that is red shifted by roughly Δν = 52 cm -1 in comparison to free PN (νNP = 1323 cm -1 ). A new band appeared for isotopically labeled Fe 15 NP at 1238.3 cm -1 . The experimental isotope shift for the νNP stretching vibration is Δνexp = 32.4 cm -1 that is in good agreement with a computed harmonic shift of Δνcalc = 34.6 cm -1 at the PBE0-D3(BJ)/cc-pVTZ level of theory. We recorded 57 Fe Mößbauer spectra of FeN2 and FeNP at 80 K and fitted the data with two Lorentzian functions as included in the WMOSS software (Figure and). The spectra of the two complexes show a single doublet which clearly establishes the spectroscopic purity of the samples in line with our other analytical characterization methods (see elemental analysis in Methods section). The line broadening of both complexes (Γ = 0.266 mm s -1 for FeN2 and Γ = 0.269 mm s -1 for FeNP) is similar to the previously reported [(dppe)Fe(Cp*)][PF6] complex (Γ = 0.257 mm s -1 ). The isomer shift (δ) for FeN2 is δ = 0.395 mm s -1 and for FeNP is δ = 0.299 mm s -1 . The quadrupole splitting (ΔEQ) for FeN2 is ΔEQ = 2.018 mm s -1 and for FeNP is ΔEQ = 1.696 mm s -1 . The experimental values are in good agreement with our calculations of the corresponding cations; δ = 0.409 mm s -1 and ΔEQ = 2.008 mm s -1 for FeN2 + and δ = 0.317 mm s -1 and ΔEQ = 1.712 mm s -1 for FeNP + (see SI for further details). The experimental difference of roughly 0.1 mm s -1 for the isomer shift is a hint for a greater covalency in FeNP. Similar observations have been reported by the Peters group. In the [K(benzo-15-crown-2)2][((Ph2PCH2SiMe2)3CH)Fe(L)] system the isomer shift for L = CO is 0.18 mm s -1 lower than that for L = N2, consistent with stronger CO π backbonding. Our computed reduced Mulliken (FeN2 + : 0.84, FeNP + : 0.94) and Löwdin (FeN2 + : 0.88, FeNP + : 0.97) orbital charges show a significant higher charge for the Fe dyz orbital for the FeNP + system. The higher orbital charge is related with the experimentally observed lower isomer shift and stronger π backbonding in FeNP (vide infra).