New mixed-valence iron-nickel dithiolates are described that exhibit structures similar to those of mixed-valence diiron dithiolates. The interaction of tricarbonyl salt [(dppe)Ni(pdt)Fe(CO)(3)]BF4 ([1]BF4, where dppe = Ph2PCH2CH2PPh2 and pdt(2-) = -SCH2CH2CH2S-) with P-donor ligands (L) afforded the substituted derivatives [(dppe)Ni(pdt)Fe(CO)(2)L]BF4 incorporating L = PHCy2 ([1a]BF4), PPh-(NEt2)(2) ([1b]BF4), P(NMe2)(3) ([1c]BF4), P(i-Pr)(3) ([1d]BF4), and PCy3 ([1e]BF4). The related precursor [(dcpe)Ni(pdt)Fe(CO)(3)]BF4 ([2]BF4, where dcpe = Cy2PCH2CH2PCy2) gave the more electron-rich family of compounds [(dcpe)Ni(pdt)Fe(CO)(2)L]BF4 for L = PPh2(2-pyridyl) ([2a]BF4), PPh3 ([2b]BF4), and PCy3 ([2c]BF4). For bulky and strongly basic monophosphorus ligands, the salts feature distorted coordination geometries at iron: crystallographic analyses of [1e]BF4 and [2c]BF4 showed that they adopt "rotated" Fe-I centers, in which PCy3 occupies a basal site and one CO ligand partially bridges the Ni and Fe centers. Like the undistorted mixed-valence derivatives, members of the new class of complexes are described as (NiFeI)-Fe-II (S = 1/2) systems according to electron paramagnetic resonance spectroscopy, although with attenuated P-31 hyperfine interactions. Density functional theory calculations using the BP86, B3LYP, and PBE0 exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the spin for [1e](+) is mostly localized in a Fe-I-centered d(z(2)) orbital, orthogonal to the Fe-P bond. The PCy3 complexes, rare examples of species featuring "rotated" Fe centers, both structurally and spectroscopically incorporate features from homobimetallic mixed-valence diiron dithiolates. Also, when the NiS2Fe core of the [NiFe]-hydrogenase active site is reproduced, the "hybrid models" incorporate key features of the two-major classes of hydrogenase. Furthermore, cyclic voltammetry experiments suggest that the highly basic phosphine ligands enable a second oxidation corresponding to the couple [(dxpe)Ni(pdt)Fe(CO)(2)L](+/2+). The resulting unsaturated 32e(-) dications represent the closest approach to modeling the highly electrophilic Ni-SIa state. In the case of L = PPh2 (2-pyridyl), chelation of this ligand accompanies the second oxidation.