Connecting [NiFe]- and [FeFe]-Hydrogenases: Mixed-Valence Nickel–Iron Dithiolates with Rotated Structures

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 ([<b>1</b>]­BF₄, where dppe = Ph₂PCH₂CH₂PPh₂ and pdt^2– = −SCH₂CH₂CH₂S−) with P-donor ligands (L) afforded the substituted derivatives [(dppe)­Ni­(pdt)­Fe­(CO)₂L]­BF₄ incorporating L = PHCy₂ ([<b>1a</b>]­BF₄), PPh­(NEt₂)₂ ([<b>1b</b>]­BF₄), P­(NMe₂)₃ ([<b>1c</b>]­BF₄), P­(<i>i</i>-Pr)₃ ([<b>1d</b>]­BF₄), and PCy₃ ([<b>1e</b>]­BF₄). The related precursor [(dcpe)­Ni­(pdt)­Fe­(CO)₃]­BF₄ ([<b>2</b>]­BF₄, where dcpe = Cy₂PCH₂CH₂PCy₂) gave the more electron-rich family of compounds [(dcpe)­Ni­(pdt)­Fe­(CO)₂L]­BF₄ for L = PPh₂(2-pyridyl) ([<b>2a</b>]­BF₄), PPh₃ ([<b>2b</b>]­BF₄), and PCy₃ ([<b>2c</b>]­BF₄). For bulky and strongly basic monophosphorus ligands, the salts feature distorted coordination geometries at iron: crystallographic analyses of [<b>1e</b>]­BF₄ and [<b>2c</b>]­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 Ni^IIFe^I (<i>S</i> = ¹/₂) systems according to electron paramagnetic resonance spectroscopy, although with attenuated ³¹P 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 [<b>1e</b>]⁺ is mostly localized in a Fe^I-centered d­(<i>z</i>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)₂L]^+/2+. The resulting unsaturated 32e^– dications represent the closest approach to modeling the highly electrophilic Ni–SI_a state. In the case of L = PPh₂ (2-pyridyl), chelation of this ligand accompanies the second oxidation