Mossbauer spectroscopy and density functional theory (DFT) calculations are reported for the mononuclear Fe-nitrosyl complex [Fe(N,N'-bis(2-mercaptoethyl)-1,4-diazacycloheptane)NO] {[Fe(bme-dach) (NO)] (1)} and the series of dithiolate-bridged dinuclear complexes M-Fe(CO)Cp [M = Fe(bme-dach)(NO) (1-A), Ni(bme-dach) (2-A), and Co(bme-dach)(NO) (3-A)], in which M is a metallo-ligand to Fe(CO)Cp+ (Fe'(Cp)). The latter is an organometallic fragment in which Fe is coordinated by one CO and one cyclopentadienyl ligand. Complexes 1-A and 2-A were previously shown to have electrocatalytic hydrogen evolution activity. Mononuclear {Fe-NO}(7) complex 1, with overall spin of 1/2, has an isomer shift of 0.23(2) mm/s [Delta EQ = 1.37(2) mm/s] and magnetic hyperfine couplings of {-38 T, -26.8 T, 8.6 T}. In complexes 2-A and 3-A, Fe'(CO)Cp+ has a diamagnetic ground state and delta = 0.33(2) mm/s (Delta E-Q approximate to 1.78 mm/s), consistent with a low-spin F-II site. In contrast, in complex 1-A, M = Fe(bme-dach)(NO) (i.e., complex 1) the magnetic hyperfine interactions of both metallo-ligand, M, and low-spin Fe'(Cp) are perturbed and Fe'(Cp) exhibits small magnetic hyperfine interactions, although its isomer shift and quadrupole splittings are largely unaltered. The DFT calculations for 1-A are in agreement with the paramagnetism observed for the Fe'(CO)Cp+ iron site.