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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.

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)).

Mechanisms for biological and bioinspired dihydrogen activation and production often invoke the intermediacy of diiron dithiolato dihydrides. The first example of such a Fe2(SR)2H2 species is provided by the complex [(term-H)(μ-H)Fe2(pdt)(CO)(dppv)2] ([H1H](0)). Spectroscopic and computational studies indicate that [H1H](0) contains both a bridging hydride and a terminal hydride, which, notably, occupies a basal site. The synthesis begins with [(μ-H)Fe2(pdt)(CO)2(dppv)2](+) ([H1(CO)](+)), which undergoes substitution to afford [(μ-H)Fe2(pdt)(CO)(NCMe)(dppv)2](+) ([H1(NCMe)](+)). Upon treatment of [H1(NCMe)](+) with borohydride salts, the MeCN ligand is displaced to afford [H1H](0). DNMR (EXSY, SST) experiments on this complex show that the terminal and bridging hydride ligands interchange intramolecularly at a rate of 1 s(-1) at -40 °C. The compound reacts with D2 to afford [D1D](0), but not mixed isotopomers such as [H1D](0). The dihydride undergoes oxidation with Fc(+) under CO to give [1(CO)](+) and H2. Protonation in MeCN solution gives [H1(NCMe)](+) and H2. Carbonylation converts [H1H](0) into [1(CO)](0).

A lucky break: A combination of steric pressure and electron asymmetry has provided the first example of a diiron dithiolate that is both rotated and basic. The present work establishes the feasibility of a hydride free rotated structure for the Hred state of the enzyme., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

This investigation examines the protonation of diiron dithiolates, exploiting the new family of exceptionally electron-rich complexes Fe(2)(xdt)(CO)(2)(PMe(3))(4), where xdt is edt (ethanedithiolate, 1), pdt (propanedithiolate, 2), and adt (2-aza-1,3-propanedithiolate, 3), prepared by the photochemical substitution of the corresponding hexacarbonyls. Compounds 1-3 oxidize near -950 mV vs Fc(+/0). Crystallographic analyses confirm that 1 and 2 adopt C(2)-symmetric structures (Fe-Fe = 2.616 and 2.625 Å, respectively). Low-temperature protonation of 1 afforded exclusively [μ-H1](+), establishing the non-intermediacy of the terminal hydride ([t-H1](+)). At higher temperatures, protonation afforded mainly [t-H1](+). The temperature dependence of the ratio [t-H1](+)/[μ-H1](+) indicates that the barriers for the two protonation pathways differ by ∼4 kcal/mol. Low-temperature (31)P{(1)H} NMR measurements indicate that the protonation of 2 proceeds by an intermediate, proposed to be the S-protonated dithiolate [Fe(2)(Hpdt)(CO)(2)(PMe(3))(4)](+) ([S-H2](+)). This intermediate converts to [t-H2](+) and [μ-H2](+) by first-order and second-order processes, respectively. DFT calculations support transient protonation at sulfur and the proposal that the S-protonated species (e.g., [S-H2](+)) rearranges to the terminal hydride intramolecularly via a low-energy pathway. Protonation of 3 affords exclusively terminal hydrides, regardless of the acid or conditions, to give [t-H3](+), which isomerizes to [t-H3'](+), wherein all PMe(3) ligands are basal.

Described herein are preparations of synthetic models for the deactivated Ni(II)Fe(II) states of the [NiFe]-hydrogenases. Iodination of the S = ½ species [(dppe)Ni(pdt)Fe(CO)(3)](+) afforded the diamagnetic iodo complex [(dppe)Ni(pdt)IFe(CO)(3)](+). Crystallographic analysis of this species confirmed the presence of square-pyramidal Ni linked to an octahedral Fe centre. The NiFe separation of 3.018 Å indicated the absence of metal-metal bonding. This complex could be reduced to give (dppe)Ni(pdt)Fe(CO)(3) and, in the presence of iodide, decarbonylated to afford (dppe)Ni(pdt)FeI(2). Derivatives of the type [(diphosphine)Ni(dithiolate)XFe(CO)(2)L](+) (X = Cl, Br, I) were prepared by halogenation of mixed-valence precursors [(diphosphine)Ni(dithiolate)Fe(CO)(2)L](+) (diphosphine = dppe, dcpe; L = tertiary phosphine or CO). The Fe(CO)(2)(PR(3))-containing derivatives are more robust than the related tricarbonyl derivatives. Exploiting this greater stability, we characterised examples of chloride and bromide derivatives. Related fluorides could be prepared by F(-) abstraction from BF(4)(-). Spectroscopic evidence is presented for the hydroperoxide [(dppe)Ni(pdt)(OOH)Fe(CO)(2)L](+), which represents a model for the Ni-SU state.

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)]BF(4) ([1]BF(4), where dppe = Ph(2)PCH(2)CH(2)PPh(2) and pdt(2-) = -SCH(2)CH(2)CH(2)S-) with P-donor ligands (L) afforded the substituted derivatives [(dppe)Ni(pdt)Fe(CO)(2)L]BF(4) incorporating L = PHCy(2) ([1a]BF(4)), PPh(NEt(2))(2) ([1b]BF(4)), P(NMe(2))(3) ([1c]BF(4)), P(i-Pr)(3) ([1d]BF(4)), and PCy(3) ([1e]BF(4)). The related precursor [(dcpe)Ni(pdt)Fe(CO)(3)]BF(4) ([2]BF(4), where dcpe = Cy(2)PCH(2)CH(2)PCy(2)) gave the more electron-rich family of compounds [(dcpe)Ni(pdt)Fe(CO)(2)L]BF(4) for L = PPh(2)(2-pyridyl) ([2a]BF(4)), PPh(3) ([2b]BF(4)), and PCy(3) ([2c]BF(4)). For bulky and strongly basic monophosphorus ligands, the salts feature distorted coordination geometries at iron: crystallographic analyses of [1e]BF(4) and [2c]BF(4) showed that they adopt "rotated" Fe(I) centers, in which PCy(3) 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(II)Fe(I) (S = ½) systems according to electron paramagnetic resonance spectroscopy, although with attenuated (31)P 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 PCy(3) complexes, rare examples of species featuring "rotated" Fe centers, both structurally and spectroscopically incorporate features from homobimetallic mixed-valence diiron dithiolates. Also, when the NiS(2)Fe 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-SI(a) state. In the case of L = PPh(2) (2-pyridyl), chelation of this ligand accompanies the second oxidation.

Understanding the catalytic process of the heterolytic splitting and formation of molecular hydrogen is one of the key topics for the development of a future hydrogen economy. With an interest in elucidating the enzymatic mechanism of the [Fe(2)(S(2)C(2)H(4)NH)(CN)(2)(CO)(2)(μ-CO)] active center uniquely found in [FeFe]hydrogenases, we present a detailed spectroscopic and theoretical analysis of its inorganic model [Fe(2)(S(2)X)(CO)(3)(dppv)(PMe(3))](+) [dppv = cis-1,2-bis(diphenylphosphino)ethylene] in two forms with S(2)X = ethanedithiolate (1edt) and azadithiolate (1adt). These complexes represent models for the oxidized mixed-valent Fe(I)Fe(II) state analogous to the active oxidized "H(ox)" state of the native H-cluster. For both complexes, the (31)P hyperfine interactions were determined by pulse electron paramagnetic resonance and electron nuclear double resonance (ENDOR) methods. For 1edt, the (57)Fe parameters were measured by electron spin-echo envelope modulation and Mössbauer spectroscopy, while for 1adt, (14)N and selected (1)H couplings could be obtained by ENDOR and hyperfine sublevel correlation spectroscopy. The spin density was found to be predominantly localized on the Fe(dppv) site. This spin distribution is different from that of the H-cluster, where both the spin and charge densities are delocalized over the two Fe centers. This difference is attributed to the influence of the "native" cubane subcluster that is lacking in the inorganic models. The degree and character of the unpaired spin delocalization was found to vary from 1edt, with an abiological dithiolate, to 1adt, which features the authentic cofactor. For 1adt, we find two (14)N signals, which are indicative for two possible isomers of the azadithiolate, demonstrating its high flexibility. All interaction parameters were also evaluated through density functional theory calculations at various levels.

A series of mixed-valence nickel-iron dithiolates is described. Oxidation of (diphosphine)Ni(dithiolate)Fe(CO)(3) complexes 1, 2, and 3 with ferrocenium salts affords the corresponding tricarbonyl cations [(dppe)Ni(pdt)Fe(CO)(3)](+) ([1](+)), [(dppe)Ni(edt)Fe(CO)(3)](+) ([2](+)) and [(dcpe)Ni(pdt)Fe(CO)(3)](+) ([3](+)), respectively, where dppe = Ph(2)PCH(2)CH(2)PPh(2), dcpe = Cy(2)PCH(2)CH(2)PCy(2), (Cy = cyclohexyl), pdtH(2) = HSCH(2)CH(2)CH(2)SH, and edtH(2) = HSCH(2)CH(2)SH. The cation [2](+) proved unstable, but the propanedithiolates are robust. IR and EPR spectroscopic measurements indicate that these species exist as C(s)-symmetric species. Crystallographic characterization of [3]BF(4) shows that Ni is square planar. Interaction of [1]BF(4) with P-donor ligands (L) afforded a series of substituted derivatives of type [(dppe)Ni(pdt)Fe(CO)(2)L]BF(4) for L = P(OPh)(3) ([4a]BF(4)), P(p-C(6)H(4)Cl)(3) ([4b]BF(4)), PPh(2)(2-py) ([4c]BF(4)), PPh(2)(OEt) ([4d]BF(4)), PPh(3) ([4e]BF(4)), PPh(2)(o-C(6)H(4)OMe) ([4f]BF(4)), PPh(2)(o-C(6)H(4)OCH(2)OMe) ([4g]BF(4)), P(p-tol)(3) ([4h]BF(4)), P(p-C(6)H(4)OMe)(3) ([4i]BF(4)), and PMePh(2) ([4j]BF(4)). EPR analysis indicates that ethanedithiolate [2](+) exists as a single species at 110 K, whereas the propanedithiolate cations exist as a mixture of two conformers, which are proposed to be related through a flip of the chelate ring. Mössbauer spectra of 1 and oxidized S = 1/2 [4e]BF(4) are both consistent with a low-spin Fe(I) state. The hyperfine coupling tensor of [4e]BF(4) has a small isotropic component and significant anisotropy. DFT calculations using the BP86, B3LYP, and PBE0 exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the SOMOs in complexes of the present type are localized in an Fe(I)-centered d(z(2)) orbital. The DFT calculations allow an assignment of oxidation states of the metals and rationalization of the conformers detected by EPR spectroscopy. Treatment of [1](+) with CN(-) and compact basic phosphines results in complex reactions. With dppe, [1](+) undergoes quasi-disproportionation to give 1 and the diamagnetic complex [(dppe)Ni(pdt)Fe(CO)(2)(dppe)](2+) ([5](2+)), which features square-planar Ni linked to an octahedral Fe center.

Described are new derivatives of the type [HNiFe(SR)(2)(diphosphine)(CO)(3)](+), which feature a Ni(diphosphine) group linked to a Fe(CO)(3) group by two bridging thiolate ligands. Previous work had described [HNiFe(pdt)(dppe)(CO)(3)](+) ([1H](+)) and its activity as a catalyst for the reduction of protons (J. Am. Chem. Soc. 2010, 132, 14877). Work described in this paper focuses on the effects on properties of NiFe model complexes of the diphosphine attached to nickel as well as the dithiolate bridge, 1,3-propanedithiolate (pdt) vs 1,2-ethanedithiolate (edt). A new synthetic route to these Ni-Fe dithiolates is described, involving reaction of Ni(SR)(2)(diphosphine) with FeI(2)(CO)(4) followed by in situ reduction with cobaltocene. Evidence is presented that this route proceeds via a metastable μ-iodo derivative. Attempted isolation of such species led to the crystallization of NiFe(Me(2)pdt)(dppe)I(2), which features tetrahedral Fe(II) and square planar Ni(II) centers (H(2)Me(2)pdt = 2,2-dimethylpropanedithiol). The new tricarbonyls prepared in this work are NiFe(pdt)(dcpe)(CO)(3) (2, dcpe = 1,2-bis(dicyclohexylphosphino)ethane), NiFe(edt)(dppe)(CO)(3) (3), and NiFe(edt)(dcpe)(CO)(3) (4). Attempted preparation of a phenylthiolate-bridged complex via the FeI(2)(CO)(4) + Ni(SPh)(2)(dppe) route gave the tetrametallic species [(CO)(2)Fe(SPh)(2)Ni(CO)](2)(μ-dppe)(2). Crystallographic analysis of the edt-dcpe compund [2H]BF(4) and the edt-dppe compound [3H]BF(4) verified their close resemblance. Each features pseudo-octahedral Fe and square pyramidal Ni centers. Starting from [3H]BF(4) we prepared the PPh(3) derivative [HNiFe(edt)(dppe)(PPh(3))(CO)(2)]BF(4) ([5H]BF(4)), which was obtained as a ∼2:1 mixture of unsymmetrical and symmetrical isomers. Acid-base measurements indicate that changing from Ni(dppe) (dppe = Ph(2)PCH(2)CH(2)PPh(2)) to Ni(dcpe) decreases the acidity of the cationic hydride complexes by 2.5 pK(a)(PhCN) units, from ∼11 to ∼13.5 (previous work showed that substitution at Fe leads to more dramatic effects). The redox potentials are more strongly affected by the change from dppe to dcpe, for example the [2](0/+) couple occurs at E(1/2) = -820 for [2](0/+) vs -574 mV (vs Fc(+/0)) for [1](0/+). Changes in the dithiolate do not affect the acidity or the reduction potentials of the hydrides. The acid-independent rate of reduction of CH(2)ClCO(2)H by [2H](+) is about 50 s(-1) (25 °C), twice that of [1H](+). The edt-dppe complex [2H](+) proved to be the most active catalyst, with an acid-independent rate of 300 s(-1).

Using the thermally stable salts of [Fe(2)(SR)(2)(CO)(3)(PMe(3))(dppv)]BAr(F)(4), we found that the azadithiolates [Fe(2)(adtR)(CO)(3)(PMe(3))(dppv)](+) react with high pressures of H(2) to give the hydride [Fe(2)(mu-H)(adtR)(CO)(3)(dppv)(PMe(3))]BAr(F)(4). The related oxadithiolate and propanedithiolate complexes are unreactive toward H(2). Molecular hydrogen is proposed to undergo heterolysis assisted by the amine followed by isomerization of an initially formed terminal hydride. Use of H(2) and D(2)O gave the deuteride as well as the hydride, implicating protic intermediates.

The ferrous dithiolato carbonyl Fe(S(2)C(3)H(6))(CO)(2)(dppe) (1) condenses with NiCl(2)(dppe) to give [FeNi(pdt)(mu-Cl)(CO)(dppe)(2)]BF(4) ([2Cl](+)). The corresponding reaction of the ditosylate Ni(OTs)(2)(dppe) gave the dication [(CO)(2)(dppe)Fe(pdt)Ni(dppe)](OTs)(2) ([2(CO)](OTs)(2)) (pdt = 1,3-propanedithiolate; dppe = 1,2-C(2)H(4)(PPh(2))(2); OTs(-) = CH(3)C(6)H(4)-4-SO(3)(-)). Reduction of the bimetallic dicarbonyl with borohydride salts afforded impure, thermally stable hydride, [(CO)(dppe)Fe(pdt)(mu-H)Ni(dppe)](+) ([2H](+)). A reliable route to NiFe(SR)(2)H species entailed protonation of (CO)(3)Fe(pdt)Ni(dppe) to give [(CO)(3)Fe(pdt)(mu-H)Ni(dppe)](+) ([3H](+)). The iron-nickel dithiolato hydride, [3H]BF(4), was characterized crystallographically: as anticipated by biophysical studies, the hydride ligand is bridging, the Fe center is octahedral, and the Ni center is pentacoordinate. Solutions of [3H]BF(4) undergo substitution by dppe to give [2H](+). The hydride undergoes rapid deprotonation and is an electrocatalyst for hydrogen evolution from trifluoroacetic acid. Oxidation of 3 gives a mixed valence species [3](+), a potential model for the Ni-L state.

This tutorial review is aimed at chemical scientists interested in understanding and exploiting the remarkable catalytic behavior of the hydrogenases. The key structural features are analyzed for the active sites of the two most important hydrogenases. Reactivity is emphasized, focusing on mechanism and catalysis. Through this analysis, gaps are identified in the synthesis of functional replicas of these fascinating and potentially useful enzymes.

The dithiolate cofactor for the [FeFe]-hydrogenase models, Fe(2)(xdt)(CO)(2)(dppv)(2) (where xdt = 1,3-propanedithiolate (pdt), azadithiolate (adt), (SCH(2))(2)NH, and oxadithiolate (odt), (SCH(2))(2)O; dppv = cis-1,2-bis(diphenylphosphino)ethylene) have been probed for their functionality as proton relays enabling formation and deprotonation of terminal hydrides. Compared to the propanedithiolate derivative, the azadithiolate and oxaditiholate show enhanced rates of proton transfer between solution and the terminal site on one Fe center. The results are consistent with the heteroatom of the dithiolate serving a gating role for both protonation and deprotonation. The pK(a) of the transiently formed ammonium (pK(CD(2))(Cl(2)) 5.7-8.2) or oxonium (pK(CD(2))(Cl(2)) -4.7-1.6) regulates the proton transfer. As a consequence, only the azadithiolate is capable of yielding the terminal hydride from weak acids. The aza- and oxadithiolates manifested the advantages of proton relays: the odt derivative proved to be a faster catalyst for hydrogen evolution than the pdt derivative as indicated from cyclic voltammetry plots of i(c)/i(p) vs [H(+)]. The adt derivative was capable of proton reduction from the weak acid [HPMe(2)Ph]BF(4) (pK(CD(2))(Cl(2)) = 5.7). The proton relay function does not apply to the isomeric bridged-hydrides [Fe(2)(xdt)(mu-H)(CO)(2)(dppv)(2)](+), where the hydride is too distant and too basic to interact to be affected by the heteroatomic relay site. None of these mu-H species can be deprotonated.

Nitrosyl derivatives of diiron dithiolato carbonyls have been prepared starting from the precursor Fe(2)(S(2)C(n)H(2n))(dppv)(CO)(4) (dppv = cis-1,2-bis(diphenylphosphinoethylene). These studies expand the range of substituted diiron(I) dithiolato carbonyl complexes. From [Fe(2)(S(2)C(2)H(4))(CO)(3)(dppv)(NO)]BF(4) ([1(CO)(3)]BF(4)), the following compounds were prepared: [1(CO)(2)(PMe(3))]BF(4), [1(CO)(dppv)]BF(4), NEt(4)[1(CO)(CN)(2)], and 1(CO)(CN)(PMe(3)). Some of these substitution reactions occur via the addition of 2 equiv of the nucleophile followed by the dissociation of one nucleophile and decarbonylation. Such a double adduct was characterized crystallographically in the case of [Fe(2)(S(2)C(2)H(4))(CO)(3)(dppv)(NO)(PMe(3))(2)]BF(4). This result shows that the addition of two ligands causes scission of the Fe-Fe bond and one Fe-S bond. When cyanide is the nucleophile, nitrosyl migrates away from the Fe(dppv) site, yielding a Fe(CN)(2)(NO) derivative. Compounds [1(CO)(3)]BF(4), [1(CO)(2)(PMe(3))]BF(4), and [1(CO)(dppv)]BF(4) were also prepared by the addition of NO(+) to the di-, tri-, and tetrasubstituted precursors. In these cases, the NO(+) appears to form an initial 36e(-) adduct containing terminal Fe-NO, followed by decarbonylation. Several complexes were prepared by the addition of NO to the mixed-valence Fe(I)Fe(II) derivatives. The diiron nitrosyl complexes reduce at mild potentials and in certain cases form weak adducts with CO. IR and EPR spectra of 1(CO)(dppv), generated by low-temperature reduction of [1(CO)(dppv)]BF(4) with Co(C(5)Me(5))(2), indicates that the SOMO is located on the FeNO subunit.

This study probes the impact of electronic asymmetry of diiron(I) dithiolato carbonyls. Treatment of Fe2(S2C(n)H(2n))(CO)(6-x)(PMe3)x compounds (n = 2, 3; x = 1, 2, 3) with NOBF4 gave the derivatives [Fe2(S2C(n)H(2n))(CO)(5-x)(PMe3)x(NO)]BF4, which are electronically unsymmetrical because of the presence of a single NO(+) ligand. Whereas the monophosphine derivative is largely undistorted, the bis(PMe3) derivatives are distorted such that the CO ligand on the Fe(CO)(PMe3)(NO)(+) subunit is semibridging. Two isomers of [Fe2(S2C3H6)(CO)3(PMe3)2(NO)]BF4 were characterized spectroscopically and crystallographically. Each isomer features electron-rich Fe(CO)2PMe3 and electrophilic Fe(CO)(PMe3)(NO)(+) subunits. These species are in equilibrium with an unobserved isomer that reversibly binds CO (DeltaH = -35 kJ/mol, DeltaS = -139 J mol(-1) K(-1)) to give the symmetrical adduct [Fe2(S2C3H6)(mu-NO)(CO)4(PMe3)2]BF4. In contrast to Fe2(S2C3H6)(CO)4(PMe3)2, the bis(PMe3) nitrosyl complexes readily undergo CO substitution to give the (PMe3)3 derivatives. The nitrosyl complexes reduce at potentials that are approximately 1 V milder than their carbonyl counterparts. Results of density functional theory calculations, specifically natural bond orbital analysis, reinforce the electronic resemblance of the nitrosyl complexes to the corresponding mixed-valence diiron complexes. Unlike other diiron dithiolato carbonyls, these species undergo reversible reductions at mild potentials. The results show that the novel structural and chemical features associated with mixed-valence diiron dithiolates (the so-called H(ox) models) can be replicated in the absence of mixed-valency by the introduction of electronic asymmetry.

Oxidation of the electron-rich (E(1/2) = -175 vs Ag/AgCl) ethanedithiolato complex Fe2(S2C2H4)(CO)2(dppv)2 (1) under a CO atmosphere yielded [Fe2(S2C2H4)(mu-CO)(CO)2(dppv)2](+) ([1(CO)](+)), a model for the H(ox)(CO) state of the [FeFe]-hydrogenases. This complex exists as two isomers: a kinetically favored unsymmetrical derivative, unsym-[1(CO)](+), and a thermodynamically favored isomer, sym-[1(CO)](+), wherein both diphosphines span apical and basal sites. Crystallographic characterization of sym-[1(CO)](+) confirmed a C2-symmetric structure with a bridging CO ligand and an elongated Fe-Fe bond of 2.7012(14) A, as predicted previously. Oxidation of sym-[1(CO)](+) and unsym-[1(CO)](+) again by 1e(-) oxidation afforded the respective diamagnetic diferrous derivatives where the relative stabilities of the sym and unsym isomers are reversed. DFT calculations indicate that the stabilities of sym and unsym isomers are affected differently by the oxidation state of the diiron unit: the mutually trans CO ligands in the sym isomer are more destabilizing in the mixed-valence state than in the diferrous state. EPR analysis of mixed-valence complexes revealed that, for [1](+), the unpaired spin is localized on a single iron center, whereas for unsym/sym-[1(CO)](+), the unpaired spin was delocalized over both iron centers, as indicated by the magnitude of the hyperfine coupling to the phosphine ligands trans to the Fe-Fe vector. Oxidation of 1 by 2 equiv of acetylferrocenium afforded the dication [1](2+), which, on the basis of low-temperature IR spectrum, is structurally similar to [1](+). Treatment of [1](2+) with CO gives unsym-[1(CO)](2+).

The new complexes Fe2(S2CnH2n)(CO)2(dppv)2 (n = 2, 3; dppv = cis-1,2-C2H2(PPh2)2) form adducts with AlBr3 and B(C6F5)3, which adopt the "rotated structure" proposed for the active site of the Fe-only hydrogenases--the propanedithiolate is significantly more Lewis basic due to nonbonded interactions between the dithiolate strap and the ligands on Fe.

The reaction of Fe2(S2C2H4)(CO)6 with cis-Ph2PCH=CHPPh2 (dppv) yields Fe2(S2C2H4)(CO)4(dppv), 1(CO)4, wherein the dppv ligand is chelated to a single iron center. NMR analysis indicates that in 1(CO)4, the dppv ligand spans axial and basal coordination sites. In addition to the axial-basal isomer, the 1,3-propanedithiolate and azadithiolate derivatives exist as dibasal isomers. Density functional theory (DFT) calculations indicate that the axial-basal isomer is destabilized by nonbonding interactions between the dppv and the central NH or CH2 of the larger dithiolates. The Fe(CO)3 subunit in 1(CO)4 undergoes substitution with PMe3 and cyanide to afford 1(CO)3(PMe3) and (Et4N)[1(CN)(CO)3], respectively. Kinetic studies show that 1(CO)4 reacts faster with donor ligands than does its parent Fe2(S2C2H4)(CO)6. The rate of reaction of 1(CO)4 with PMe3 was first order in each reactant, k = 3.1 x 10(-4) M(-1) s(-1). The activation parameters for this substitution reaction, DeltaH = 5.8(5) kcal/mol and DeltaS = -48(2) cal/deg.mol, indicate an associative pathway. DFT calculations suggest that, relative to Fe2(S2C2H4)(CO)6, the enhanced electrophilicity of 1(CO)4 arises from the stabilization of a "rotated" transition state, which is favored by the unsymmetrically disposed donor ligands. Oxidation of MeCN solutions of 1(CO)3(PMe3) with Cp2FePF6 yielded [Fe2(S2C2H4)(mu-CO)(CO)2(dppv)(PMe3)(NCMe)](PF6)2. Reaction of this compound with PMe3 yielded [Fe2(S2C2H4)(mu-CO)(CO)(dppv)(PMe3)2(NCMe)](PF6)2.

Low-temperature oxidation of Fe(2)(S(2)C(n)H(2n)(CNMe)(6-x)(CO)x (n = 2, 3; x = 2, 3) affords a family of mixed carbonyl-isocyanides of the type [Fe(2)(S(2)C(n)H(2n)(CO)x(CNMe)(7-x)](2+). The degree of substitution is controlled by the RNC/Fe ratio, as well as the degree of initial substitution at iron, with tricarbonyl derivatives favoring more highly carbonylated products. The structures of the monocarbonyl derivatives [Fe(2)(S(2)C(n)H(2n))(mu-CO)(CNMe)(6)](PF(6))(2) (n = 2,3) established crystallographically and spectroscopically, are quite similar, with Fe---Fe distances of ca. 2.5 A, although the mu-CO is unsymmetrical in the propanedithiolate derivative. Isomeric forms of [Fe(2)(S(2)C(3)H(6))(CO)(CNMe)(6)](PF(6))(2) were characterized where the CO is bridging or terminal, the greatest structural difference being the 0.1 A elongation of the Fe---Fe distance when MeNC (vs CO) is bridging. In the dicarbonyl species, [Fe(2)(S(2)C(2)H(4))(mu-CO)(CO)(CNMe)(5)](PF(6))(2), the terminal CO ligand is situated at one of the basal sites, not trans to the Fe---Fe vector. Oxidation of Fe(2)(S(2)C(2)H(4))(CNMe)(3)(CO)(3) under 1 atm CO gives the deep pink tricarbonyl [Fe(2)(S(2)C(2)H(4))(CO)(3)(CNMe)(4)](PF(6))(2). DFT calculations show that a bridging CO or MeNC establishes a 3-center, 2-electron bond within the two Fe(II) centers, which would otherwise be nonbonding.

The azadithiolate (SCH2NHCH2S) cofactor proposed to occur in the Fe-only hydrogenases forms efficiently by the condensation of Fe2(SH)2(CO)6 (1), formaldehyde, and ammonia (as (NH4)2CO3). The resulting Fe2[(SCH2)2NH](CO)6 reacts with Et4NCN to give (Et4N)2[Fe2[(SCH2)2NH](CO)4(CN)2], for which crystallographic characterization confirmed an axial N-H and an elongated C-S bond of 1.858(3) A. Primary amines RNH2 (R = Ph, t-Bu) also participate in the condensation reaction, and Fe3S2(CO)9 can be employed in place of 1. Mechanistically, the Fe2[(SCH2)2NH] moiety is shown to arise via two pathways: (i) via the intermediacy of Fe2[(SCH2OH)2](CO)6, which was detected and shown to react with amines, and (ii) via the reaction of 1 with cyclic imines (CH2)3(NR)3 (R = Ph, Me). The reaction of 1 with (CH2)6N4 (hexamethylenetetramine) gives Fe2[(SCH2)2NH](CO)6. Trace amounts of Fe2[(SCH2)2N-t-Bu](CO)6 arise via the reaction of aqueous FeSO4, formaldehyde, NaSH, and t-BuNH2 under an atmosphere of CO.