C-Cl Bond Activation at Rotated vs Unrotated Dinuclear Site Related to [FeFe]-Hydrogenases

The novel dinuclear complex related to the [FeFe]-hydrogenases active site, [Fe2(μ-pdt)(κ2-dmpe)2(CO)2] (1), is highly reactive toward chlorinated compounds CHxCl4-x (x = 1, 2) affording selectively terminal or bridging chloro diiron isomers through a C-Cl bond activation. DFT calculations suggest a cooperative mechanism involving a formal concerted regioselective chloronium transfer depending on the unrotated or rotated conformation of two isomers of 1.

The Complex Reactivity of [(salen)Fe]2(μ-O) with HBpin and Its Implications in Catalysis

We report a detailed study into the method of precatalyst activation during alkyne cyclotrimerization. During these studies we have prepared a homologous series of Fe(III)-μ-oxo(salen) complexes and use a range of techniques including UV-vis, reaction monitoring studies, single crystal X-ray diffraction, NMR spectroscopy, and LIFDI mass spectrometry to provide experimental evidence for the nature of the on-cycle iron catalyst. These data infer the likelihood of ligand reduction, generating an iron(salan)-boryl complex as a key on-cycle intermediate. We use DFT studies to interrogate spin states, connecting this to experimentally identified diamagnetic and paramagnetic species. The extreme conformational flexibility of the salan system appears connected to challenges associated with crystallization of likely on-cycle species.

Unprecedented intact radical anions, closed shell anions, cluster ions, and traditional cations and radical cations by LIFDI-MS

Liquid injection field desorption ionization (LIFDI) proves the extraordinary softness of the ionization process combined with a convenient sample supply under the exclusion of moisture and air. LIFDI-mass spectrometry (MS) is used for organometallic and other seriously air-sensitive compounds forming intact ions without substantial fragmentation. Unprecedented molecular radical anions M^-• are presented along with well-known intact M^+• radical cations. Furthermore closed shell cations [C]⁺ and adduct ions like [M + H]⁺ or [M + Alkali]⁺ are gently transferred from the solid emitter surface into the gas phase. Anions [A]^- or [M - H]^- are accessible by LIFDI-MS at medium field strengths. Ion pairs [C]⁺[A]^- are separately detected by positive and negative mode LIFDI-MS, respectively. Here we give an overview of the different ion types accessible by LIFDI-MS. For the first time the field ionization/desorption of solar cell electron acceptor compounds is shown to deliver M^-• and M^2-• radical ions.

Bulky oxadithiolate-bridged [FeFe]‑hydrogenase mimics [Fe2(μ-R2odt)(CO)4(κ2-diphosphine)] (R = Ph and H) with chelating diphosphines

In order to develop an attractive generation of bulky oxadithiolate-bridged [FeFe]‑hydrogenase mimics with chelating diphosphines, two new series of asymmetrically diphosphine-substituted diiron model complexes [Fe₂(μ-R₂odt)(CO)₄(κ²-diphosphine)] (3-5) with bulky Ph₂odt bridge and their reference counterparts (6-8) with common odt bridge were obtained from the Me₃NO-assisted substitutions of diiron hexacarbonyl precursors [Fe₂(μ-R₂odt)(CO)₆] (R₂odt = (SCHR)₂O, R = Ph (1) and H (2)) with different diphosphines such as (Ph₂P)₂NBn (labelled PN^BnP, Bn = benzyl), (Ph₂PCH₂)₂NBn (PCN^BnCP), and (Ph₂PCH₂)₂CH₂ (DPPP)), respectively. All the as-prepared complexes have been characterized by elemental analysis, IR plus NMR spectroscopies, and particularly by X-ray crystallography for 3-8. It is interesting to note that complexes 3 and 6 chelating by small bite-angle PN^BnP diphosphine have the favorable dibasal isomer whereas analogues 4, 5 and 7, 8 chelating by flexible backbone PCN^BnCP or DPPP ligands possess the main apical-basal isomer in solution or in the solid state. Further, the electrochemical properties of two pairs of representative complexes 3, 6 and 5, 8 are explored and compared by cyclic voltammetry (CV) in the absence and presence of trifluoroacetic acid (CF₃CO₂H) as proton source, indicating that the complete protonations of 3, 6 and 5, 8 with higher concentration of CF₃CO₂H lead to two new catalytic waves for the electrocatalytic proton reduction to hydrogen (H₂).

Phosphine-substituted diiron complexes Fe2(μ-Rodt)(CO)6−n(PPh3)n (R = Ph, Me, H and n = 1, 2) featuring desymmetrized oxadithiolate bridges: structures, protonation, and electrocatalysis

The influence of desymmetrized dithiolates (Rodt) and phosphine coordination modes (PPh3) on the structural, protophilic, and electrocatalytic features of diiron complexes 4–6 and 7–9 is described.

[FeFe]-Hydrogenases: maturation and reactivity of enzymatic systems and overview of biomimetic models

[FeFe]-hydrogenases recieved increasing interest in the last decades. This review summarises important findings regarding their enzymatic reactivity as well as inorganic models applied as electro- and photochemical catalysts.

Triiron clusters derived from dinuclear complexes related to the active site of [Fe–Fe] hydrogenases: steric effect of the dithiolate bridge on redox properties, a DFT analysis

CV and DFT calculations reveal that electrochemical behaviours of triiron clusters [Fe₃(CO)₅(κ₂-dppe)(μ-pdt^R2)(μ-pdt)] depend on the nature of the dithiolate bridge.

Catalytic H2 evolution/oxidation in [FeFe]-hydrogenase biomimetics: account from DFT on the interplay of related issues and proposed solutions

A DFT overview on selected issues regarding diiron catalysts related to [FeFe]-hydrogenase biomimetic research, with implications for both energy conversion and storage strategies.

Can carbene decorated [FeFe]-hydrogenase model complexes catalytically produce dihydrogen? An insight from theory

Cyclic alkyl amino carbene (CAAC) anchored [FeFe]-hydrogenase model complex featuring rotated conformation at one of the iron centers are found to be promising candidate for effective production of dihydrogen. A stepwise comparison of the complete mechanism using the CAAC stabilized model complex [1]⁰ has been performed with that of an experimentally isolated one ([2]⁰). Interestingly, the reduction events involved in the catalytic cycles are found to be more favorable than those previously reported for a similar experimentally known system. Furthermore, the computed ΔpK_a values indicate that the distal iron center with a vacant coordination site is more basic compared to the amino nitrogen atom of the azadithiolate bridge. We also made an attempt to determine the oxidation states of the iron centers for the intermediates involved in the catalytic cycles on the basis of the computed Mössbauer isomer shift and Mulliken spin density values.

Protonation/reduction dynamics at the [4Fe-4S] cluster of the hydrogen-forming cofactor in [FeFe]-hydrogenases

The [FeFe]-hydrogenases of bacteria and algae are the most efficient hydrogen conversion catalysts in nature. Their active-site cofactor (H-cluster) comprises a [4Fe-4S] cluster linked to a unique diiron site that binds three carbon monoxide (CO) and two cyanide (CN^-) ligands. Understanding microbial hydrogen conversion requires elucidation of the interplay of proton and electron transfer events at the H-cluster. We performed real-time spectroscopy on [FeFe]-hydrogenase protein films under controlled variation of atmospheric gas composition, sample pH, and reductant concentration. Attenuated total reflection Fourier-transform infrared spectroscopy was used to monitor shifts of the CO/CN^- vibrational bands in response to redox and protonation changes. Three different [FeFe]-hydrogenases and several protein and cofactor variants were compared, including element and isotopic exchange studies. A protonated equivalent (HoxH) of the oxidized state (Hox) was found, which preferentially accumulated at acidic pH and under reducing conditions. We show that the one-electron reduced state Hred' represents an intrinsically protonated species. Interestingly, the formation of HoxH and Hred' was independent of the established proton pathway to the diiron site. Quantum chemical calculations of the respective CO/CN^- infrared band patterns favored a cysteine ligand of the [4Fe-4S] cluster as the protonation site in HoxH and Hred'. We propose that proton-coupled electron transfer facilitates reduction of the [4Fe-4S] cluster and prevents premature formation of a hydride at the catalytic diiron site. Our findings imply that protonation events both at the [4Fe-4S] cluster and at the diiron site of the H-cluster are important in the hydrogen conversion reaction of [FeFe]-hydrogenases.

Sterically Stabilized Terminal Hydride of a Diiron Dithiolate

The kinetically robust hydride [t-HFe₂(Me₂pdt)(CO)₂(dppv)₂]⁺ ([t-H1]⁺) (Me₂pdt^2- = Me₂C(CH₂S^-)₂; dppv = cis-1,2-C₂H₂(PPh₂)₂) and related derivatives were prepared with ⁵⁷Fe enrichment for characterization by NMR, FT-IR, and NRVS. The experimental results were rationalized using DFT molecular modeling and spectral simulations. The spectroscopic analysis was aimed at supporting assignments of Fe-H vibrational spectra as they relate to recent measurements on [FeFe]-hydrogenase enzymes. The combination of bulky Me₂pdt^2- and dppv ligands stabilizes the terminal hydride with respect to its isomerization to the 5-16 kcal/mol more stable bridging hydride ([μ-H1]⁺) with t_1/2(313.3 K) = 19.3 min. In agreement with the nOe experiments, the calculations predict that one methyl group in [t-H1]⁺ interacts with the hydride with a computed CH···HFe distance of 1.7 Å. Although [t-H⁵⁷1]⁺ exhibits multiple NRVS features in the 720-800 cm^-1 region containing the bending Fe-H modes, the deuterated [t-D⁵⁷1]⁺ sample exhibits a unique Fe-D/CO band at ∼600 cm^-1. In contrast, the NRVS spectra for [μ-H⁵⁷1]⁺ exhibit weaker bands near 670-700 cm^-1 produced by the Fe-H-Fe wagging modes coupled to Me₂pdt^2- and dppv motions.

[FeFe]-Hydrogenase H-Cluster Mimics with Various -S(CH2)nS- Linker Lengths (n = 2-8): A Systematic Study

The effect of the nature of the dithiolato ligand on the physical and electrochemical properties of synthetic H-cluster mimics of the [FeFe]-hydrogenase is still of significant concern. In this report we describe the cyclization of various alkanedithiols to afford cyclic disulfide, tetrasulfide, and hexasulfide compounds. The latter compounds were used as proligands for the synthesis of a series of [FeFe]-hydrogenase H-cluster mimics having the general formulas [Fe₂(CO)₆{μ-S(CH₂)_nS}] (n = 4-8), [Fe₂(CO)₆{μ-S(CH₂)_nS}]₂ (n = 6-8), and [Fe₂(CO)₆{(μ-S(CH₂)_nS)₂}] (n = 6-8). The resulting complexes were characterized by ¹H and ¹³C{¹H} NMR and IR spectroscopic techniques, mass spectrometry, and elemental analysis as well as X-ray analysis. The purpose of this research was to study the influence of the systematic increase of n from 2 to 7 on the redox potentials of the models and the catalytic ability in the presence of acetic acid (AcOH) by applying cyclic voltammetry.

Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides

Hydrogenase enzymes efficiently process H2 and protons at organometallic FeFe, NiFe, or Fe active sites. Synthetic modeling of the many H2ase states has provided insight into H2ase structure and mechanism, as well as afforded catalysts for the H2 energy vector. Particularly important are hydride-bearing states, with synthetic hydride analogues now known for each hydrogenase class. These hydrides are typically prepared by protonation of low-valent cores. Examples of FeFe and NiFe hydrides derived from H2 have also been prepared. Such chemistry is more developed than mimicry of the redox-inactive monoFe enzyme, although functional models of the latter are now emerging. Advances in physical and theoretical characterization of H2ase enzymes and synthetic models have proven key to the study of hydrides in particular, and will guide modeling efforts toward more robust and active species optimized for practical applications.

Synthesis of Diiron(I) Dithiolato Carbonyl Complexes

Virtually all organosulfur compounds react with Fe(0) carbonyls to give the title complexes. These reactions are reviewed in light of major advances over the past few decades, spurred by interest in Fe2(μ-SR)2(CO)x centers at the active sites of the [FeFe]-hydrogenase enzymes. The most useful synthetic route to Fe2(μ-SR)2(CO)6 involves the reaction of thiols with Fe2(CO)9 and Fe3(CO)12. Such reactions can proceed via mono-, di-, and triiron intermediates. The reactivity of Fe(0) carbonyls toward thiols is highly chemoselective, and the resulting dithiolato complexes are fairly rugged. Thus, many complexes tolerate further synthetic elaboration directed at the organic substituents. A second major route involves alkylation of Fe2(μ-S2)(CO)6, Fe2(μ-SH)2(CO)6, and Li2Fe2(μ-S)2(CO)6. This approach is especially useful for azadithiolates Fe2[(μ-SCH2)2NR](CO)6. Elaborate complexes arise via addition of the FeSH group to electrophilic alkenes, alkynes, and carbonyls. Although the first example of Fe2(μ-SR)2(CO)6 was prepared from ferrous reagents, ferrous compounds are infrequently used, although the Fe(II)(SR)2 + Fe(0) condensation reaction is promising. Almost invariably low-yielding, the reaction of Fe3(CO)12, S8, and a variety of unsaturated substrates results in C-H activation, affording otherwise inaccessible derivatives. Thiones and related C═S-containing reagents are highly reactive toward Fe(0), often giving complexes derived from substituted methanedithiolates and C-H activation.

Preparation and Protonation of Fe2(pdt)(CNR)6, Electron-Rich Analogues of Fe2(pdt)(CO)6

The complexes Fe2(pdt)(CNR)6 (pdt(2-) = CH2(CH2S(-))2) were prepared by thermal substitution of the hexacarbonyl complex with the isocyanides RNC for R = C6H4-4-OMe (1), C6H4-4-Cl (2), Me (3). These complexes represent electron-rich analogues of the parent Fe2(pdt)(CO)6. Unlike most substituted derivatives of Fe2(pdt)(CO)6, these isocyanide complexes are sterically unencumbered and have the same idealized symmetry as the parent hexacarbonyl derivatives. Like the hexacarbonyls, the stereodynamics of 1-3 involve both turnstile rotation of the Fe(CNR)3 as well as the inversion of the chair conformation of the pdt ligand. Structural studies indicate that the basal isocyanide has nonlinear CNC bonds and short Fe-C distances, indicating that they engage in stronger Fe-C π-backbonding than the apical ligands. Cyclic voltammetry reveals that these new complexes are far more reducing than the hexacarbonyls, although the redox behavior is complex. Estimated reduction potentials are E1/2 ≈ -0.6 ([2](+/0)), -0.7 ([1](+/0)), and -1.25 ([3](+/0)). According to DFT calculations, the rotated isomer of 3 is only 2.2 kcal/mol higher in energy than the crystallographically observed unrotated structure. The effects of rotated versus unrotated structure and of solvent coordination (THF, MeCN) on redox potentials were assessed computationally. These factors shift the redox couple by as much as 0.25 V, usually less. Compounds 1 and 2 protonate with strong acids to give the expected μ-hydrides [H1](+) and [H2](+). In contrast, 3 protonates with [HNEt3]BAr(F)4 (pKa(MeCN) = 18.7) to give the aminocarbyne [Fe2(pdt)(CNMe)5(μ-CN(H)Me)](+) ([3H](+)). According to NMR measurements and DFT calculations, this species adopts an unsymmetrical, rotated structure. DFT calculations further indicate that the previously described carbyne complex [Fe2(SMe)2(CO)3(PMe3)2(CCF3)](+) also adopts a rotated structure with a bridging carbyne ligand. Complex [3H](+) reversibly adds MeNC to give [Fe2(pdt)(CNR)6(μ-CN(H)Me)](+) ([3H(CNMe)](+)). Near room temperature, [3H](+) isomerizes to the hydride [(μ-H)Fe2(pdt)(CNMe)6](+) ([H3](+)) via a first-order pathway.

Effect of Bridgehead Steric Bulk on the Intramolecular C-H Heterolysis of [FeFe]-Hydrogenase Active Site Models Containing a P2N2 Pendant Amine Ligand

A series of pendant amine-containing [FeFe]-hydrogenase models, [X(CH2S-μ)2{Fe(CO)3}{Fe(CO)(P2(Ph)N2(Bn))}] (1H, X = CH2; 2Me, C(CH3)2; 3Et, C(CH2CH3)2; and P2(Ph)N2(Bn) = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) with different groups at the bridgehead carbon of the S-to-S linker were synthesized. The oxidations of these complexes as well as the reverse reduction reaction were studied by cyclic voltammetry and in situ IR spectroscopy. Regardless of the bridgehead steric bulk, all three complexes demonstrate intramolecular iron-mediated C(sp(3))-H bond heterolytic cleavage with the assistance of the pendant amine base within the chelating diphosphine ligand in the two-electron oxidation process. X-ray crystallographic analysis shows that the doubly oxidized products, [1'H](+), [2'Me](+), and [3'Et](+), all have a rigid FeSC three-membered ring at the open apical site of the rotated iron center. The most noticeable difference in structures of the oxidized complexes is that the single CO ligand of the rotated Fe(P2(Ph)N2(Bn))(CO) unit in [1'H](+) and [2'Me](+) is found below the Fe···Fe vector, while in [3'Et](+) an unusually rotated Fe(P2(Ph)N2(Bn))(CO) moiety positions one of the P donors within the bidentate ligand under the Fe···Fe vector. The starting Fe(I)Fe(I) complexes can be recovered from their corresponding doubly oxidized complexes by reduction in the presence of Brönsted acid.

Models of the Ni-L and Ni-SIa States of the [NiFe]-Hydrogenase Active Site

A new class of synthetic models for the active site of [NiFe]-hydrogenases are described. The Ni(I/II)(SCys)2 and Fe(II)(CN)2CO sites are represented with (RC5H4)Ni(I/II) and Fe(II)(diphos)(CO) modules, where diphos = 1,2-C2H4(PPh2)2(dppe) or cis-1,2-C2H2(PPh2)2(dppv). The two bridging thiolate ligands are represented by CH2(CH2S)2(2-) (pdt(2-)), Me2C(CH2S)2(2-) (Me2pdt(2-)), and (C6H5S)2(2-). The reaction of Fe(pdt)(CO)2(dppe) and [(C5H5)3Ni2]BF4 affords [(C5H5)Ni(pdt)Fe(dppe)(CO)]BF4 ([1a]BF4). Monocarbonyl [1a]BF4 features an S = 0 Ni(II)Fe(II) center with five-coordinated iron, as proposed for the Ni-SIa state of the enzyme. One-electron reduction of [1a](+) affords the S = 1/2 derivative [1a](0), which, according to density functional theory (DFT) calculations and electron paramagnetic resonance and Mössbauer spectroscopies, is best described as a Ni(I)Fe(II) compound. The Ni(I)Fe(II) assignment matches that for the Ni-L state in [NiFe]-hydrogenase, unlike recently reported Ni(II)Fe(I)-based models. Compound [1a](0) reacts with strong acids to liberate 0.5 equiv of H2 and regenerate [1a](+), indicating that H2 evolution is catalyzed by [1a](0). DFT calculations were used to investigate the pathway for H2 evolution and revealed that the mechanism can proceed through two isomers of [1a](0) that differ in the stereochemistry of the Fe(dppe)CO center. Calculations suggest that protonation of [1a](0) (both isomers) affords Ni(III)-H-Fe(II) intermediates, which represent mimics of the Ni-C state of the enzyme.

Diiron azadithiolates as models for the [FeFe]-hydrogenase active site and paradigm for the role of the second coordination sphere

The [FeFe] hydrogenases (H2ases) catalyze the redox reaction that interconverts protons and H2. This area of biocatalysis has attracted attention because the metal-based chemistry is unusual, and the reactions have practical implications. The active site consists of a [4Fe-4S] cluster bridged to a [Fe2(μ-dithiolate)(CN)2(CO)3](z) center (z = 1- and 2-). The dithiolate cofactor is [HN(CH2S)2](2-), called the azadithiolate ([adt(H)](2-)). Although many derivatives of Fe2(SR)2(CO)6-xLx are electrocatalysts for the hydrogen evolution reaction (HER), most operate by slow nonbiomimetic pathways. Biomimetic hydrogenogenesis is thought to involve intermediates, wherein the hydride substrate is adjacent to the amine of the adt(H), being bonded to only one Fe center. Formation of terminal hydride complexes is favored when the diiron carbonyl models contain azadithiolate. Although unstable in the free state, the adt cofactor is stable once it is affixed to the Fe2 center. It can be prepared by alkylation of Fe2(SH)2(CO)6 with formaldehyde in the presence of ammonia (to give adt(H) derivatives) or amines (to give adt(R) derivatives). Weak acids protonate Fe2(adt(R))(CO)2(PR3)4 to give terminal hydrido (term-H) complexes. In contrast, protonation of the related 1,3-propanedithiolate (pdt(2-)) complexes Fe2(pdt)(CO)2(PR3)4 requires strong acids. The amine in the azadithiolate is a kinetically fast base, relaying protons to and from the iron, which is a kinetically slow base. The crystal structure of the doubly protonated model [(term-H)Fe2(Hadt(H))(CO)2(dppv)2](2+) confirms the presence of both ammonium and terminal hydrido centers, which interact through a dihydrogen bond (dppv = cis-C2H2(PPh2)2). DFT calculations indicate that this H---H interaction is sensitive to the counterions and is strengthened upon reduction of the diiron center. For the monoprotonated models, the hydride [(term-H)Fe2(adt(H))(CO)2(dppv)2](+) exists in equilibrium with the ammonium tautomer [Fe2(Hadt(H))(CO)2(dppv)2](+). Both [(term-H)Fe2(Hadt(H))(CO)2(dppv)2](2+) and [(term-H)Fe2(adt(H))(CO)2(dppv)2](+) are highly active electrocatalysts for HER. Catalysis is initiated by reduction of the diferrous center, which induces coupling of the protic ammonium center and the hydride ligand. In contrast, the propanedithiolate [(term-H)Fe2(pdt)(CO)2(dppv)2](+) is a poor electrocatalyst for HER. Oxidation of H2 has been demonstrated, starting with models for the oxidized state ("Hox"), for example, [Fe2(adt(H))(CO)3(dppv)(PMe3)](+). Featuring a distorted Fe(II)Fe(I) center, this Hox model reacts slowly with high pressures of H2 to give [(μ-H)Fe2(adt(H))(CO)3(dppv)(PMe3)](+). Highlighting the role of the proton relay, the propanedithiolate [Fe2(pdt)(CO)3(dppv)(PMe3)](+) is unreactive toward H2. The Hox-model + H2 reaction is accelerated in the presence of ferrocenium salts, which simulate the role of the attached [4Fe-4S] cluster. The redox-complemented complex [Fe2(adt(Bn))(CO)3(dppv)(FcP*)](n+) catalyzes both proton reduction and hydrogen oxidation (FcP* = (C5Me5)Fe(C5Me4CH2PEt2)).

A sterically stabilized FeI–FeI semi-rotated conformation of [FeFe] hydrogenase subsite model

Semi-rotated state – As the first example so far a [Fe^IFe^I] H₂ase model complex with a bulky silicon-containing dithiolate bridge is reported showing a semi-rotated geometry without the need of stabilization via agostic interactions.

Ligand effects on the electrochemical behavior of [Fe2(CO)5(L){μ-(SCH2)2(Ph)PO}] (L = PPh3, P(OEt)3) hydrogenase model complexes

In this paper we study the influence of substituting one CO ligand in [Fe₂(CO)₆{μ-(SCH₂)₂(Ph)PO}] (1) by better σ-donors (PPh₃(2) and P(OMe)₃(3)) in relation to the electrochemical behavior.

Steric effect of the dithiolato linker on the reduction mechanism of [Fe2(CO)6{μ-(XCH2)2CRR′}] hydrogenase models (X = S, Se)

Investigations of the electrochemical properties of [Fe₂(CO)₆{μ-(XCH₂)₂CRR′}] (X = S, Se; R or R′ = H or Me) showed that the complex with the bulkiest CMe₂moiety undergoes reduction with potential inversion.

Protonation of nickel-iron hydrogenase models proceeds after isomerization at nickel

Theory and experiment indicate that the protonation of reduced NiFe dithiolates proceeds via a previously undetected isomer with enhanced basicity. In particular, it is proposed that protonation of (OC)₃Fe(pdt)Ni(dppe) (1; pdt^2– = ^–S(CH₂)₃S^–; dppe = Ph₂P(CH₂)₂PPh₂) occurs at the Fe site of the two-electron mixed-valence Fe(0)Ni(II) species, not the Fe(I)-Ni(I) bond for the homovalence isomer of 1. The new pathway, which may have implications for protonation of other complexes and clusters, was uncovered through studies on the homologous series L(OC)₂Fe(pdt)M(dppe), where M = Ni, Pd (2), and Pt (3) and L = CO, PCy₃. Similar to 1, complexes 2 and 3 undergo both protonation and 1e^– oxidation to afford well-characterized hydrides ([2H]⁺ and [3H]⁺) and mixed-valence derivatives ([2]⁺ and [3]⁺), respectively. Whereas the Pd site is tetrahedral in 2, the Pt site is square-planar in 3, indicating that this complex is best described as Fe(0)Pt(II). In view of the results on 2 and 3, the potential energy surface of 1 was reinvestigated with density functional theory. These calculations revealed the existence of an energetically accessible and more basic Fe(0)Ni(II) isomer with a square-planar Ni site.