Monoanionic molybdenum and tungsten tris(dithiolene) complexes: a multifrequency EPR study

Numerous Mo and W tris(dithiolene) complexes in varying redox states have been prepared and representative examples characterized crystallographically: [M(S(2)C(2)R(2))(3)](z) [M = Mo, R = Ph, z = 0 (1) or 1- (2); M = W, R = Ph, z = 0 (4) or 1- (5); R = CN, z = 2-, M = Mo (3) or W (6)]. Changes in dithiolene C-S and C-C bond lengths for 1 versus 2 and 4 versus 5 are indicative of ligand reduction. Trigonal twist angles (Θ) and dithiolene fold angles (α) increase and decrease, respectively, for 2 versus 1, 5 versus 4. Cyclic voltammetry reveals generally two reversible couples corresponding to 0/1- and 1-/2- reductions. The electronic structures of monoanionic molybdenum tris(dithiolene) complexes have been analyzed by multifrequency (S-, X-, Q-band) EPR spectroscopy. Spin-Hamiltonian parameters afforded by spectral simulation for each complex demonstrate the existence of two distinctive electronic structure types. The first is [Mo(IV)((A)L(3)(5-?))](1-) ((A)L = olefinic dithiolene, type A), which has the unpaired electron restricted to the tris(dithiolene) unit and is characterized by isotropic g-values and small molybdenum superhyperfine coupling. The second is formulated as [Mo(V)((B)L(3)(6-))](1-) ((B)L = aromatic dithiolene, type B) with spectra distinguished by a prominent g-anisotropy and hyperfine coupling consistent with the (d(z(2)))(1) paramagnet. The electronic structure disparity is also manifested in their electronic absorption spectra. The compound [W(bdt)(3)](1-) exhibits spin-Hamiltonian parameters similar to those of [Mo(bdt)(3)](1-) and thus is formulated as [W(V)((B)L(3)(6-))](1-). The EPR spectra of [W((A)L(3))](1-) display spin-Hamiltonian parameters that suggest their electronic structure is best represented by two resonance forms {[W(IV)((A)L(3)(5-?))](1-) ? [W(V)((A)L(3)(6-))](1-)}. The contrast with the corresponding [Mo(IV)((A)L(3)(5-?))](1-) complexes highlights tungsten's preference for higher oxidation states.     

Sulfur K-edge X-ray absorption spectroscopy and density functional theory calculations on superoxide reductase: role of the axial thiolate in reactivity

Superoxide reductase (SOR) is a non-heme iron enzyme that reduces superoxide to peroxide at a diffusion-controlled rate. Sulfur K-edge X-ray absorption spectroscopy (XAS) is used to investigate the ground-state electronic structure of the resting high-spin and CN- bound low-spin FeIII forms of the 1Fe SOR from Pyrococcus furiosus. A computational model with constrained imidazole rings (necessary for reproducing spin states), H-bonding interaction to the thiolate (necessary for reproducing Fe-S bond covalency of the high-spin and low-spin forms), and H-bonding to the exchangeable axial ligand (necessary to reproduce the ground state of the low-spin form) was developed and then used to investigate the enzymatic reaction mechanism. Reaction of the resting ferrous site with superoxide and protonation leading to a high-spin FeIII-OOH species and its subsequent protonation resulting in H2O2 release is calculated to be the most energetically favorable reaction pathway. Our results suggest that the thiolate acts as a covalent anionic ligand. Replacing the thiolate with a neutral noncovalent ligand makes protonation very endothermic and greatly raises the reduction potential. The covalent nature of the thiolate weakens the FeIII bond to the proximal oxygen of this hydroperoxo species, which raises its pKa by an additional 5 log units relative to the pKa of a primarily anionic ligand, facilitating its protonation. A comparison with cytochrome P450 indicates that the stronger equatorial ligand field from the porphyrin results in a low-spin FeIII-OOH species that would not be capable of efficient H2O2 release due to a spin-crossing barrier associated with formation of a high-spin 5C FeIII product. Additionally, the presence of the dianionic porphyrin pi ring in cytochrome P450 allows O-O heterolysis, forming an FeIV-oxo porphyrin radical species, which is calculated to be extremely unfavorable for the non-heme SOR ligand environment. Finally, the 5C FeIII site that results from the product release at the end of the O2- reduction cycle is calculated to be capable of reacting with a second O2-, resulting in superoxide dismutase (SOD) activity. However, in contrast to FeSOD, the 5C FeIII site of SOR, which is more positively charged, is calculated to have a high affinity for binding a sixth anionic ligand, which would inhibit its SOD activity.     

A density functional study of oxygen atom transfer reactions between biological oxygen atom donors and molybdenum(IV) bis(dithiolene) complexes

Density functional calculations have been used to investigate oxygen atom transfer reactions from the biological oxygen atom donors trimethylamine N-oxide (Me(3)NO) and dimethyl sulfoxide (DMSO) to the molybdenum(IV) complexes [MoO(mnt)(2)](2-) and [Mo(OCH(3))(mnt)(2)](-) (mnt = maleonitrile-1,2-dithiolate), which may serve as models for mononuclear molybdenum enzymes of the DMSO reductase family. The reaction between [MoO(mnt)(2)](2-) and trimethylamine N-oxide was found to have an activation energy of 72 kJ/mol and proceed via a transition state (TS) with distorted octahedral geometry, where the Me(3)NO is bound through the oxygen to the molybdenum atom and the N-O bond is considerably weakened. The computational modeling of the reactions between dimethyl sulfoxide (DMSO) and [MoO(mnt)(2)](2-) or [Mo(OCH(3))(mnt)(2)](-) indicated that the former is energetically unfavorable while the latter was found to be favorable. The addition of a methyl group to [MoO(mnt)(2)](2-) to form the corresponding des-oxo complex not only lowers the relative energy of the products but also lowers the activation energy. In addition, the reaction with [Mo(OCH(3))(mnt)(2)](-) proceeds via a TS with trigonal prismatic geometry instead of the distorted octahedral TS geometry modeled for the reaction between [MoO(mnt)(2)](2-) and Me(3)NO.     

Electronic structures of tris(dioxolene)chromium and tris(dithiolene)chromium complexes of the electron-transfer series [Cr(dioxolene)(3)](z) and [Cr(dithiolene)(3)](z) (z = 0, 1-, 2-, 3-). A combined experimental and density functional theoretical study

From the reaction mixture of 3,6-di-tert-butylcatechol, H2[3,6L(cat)], [CrCl3(thf)3], and NEt3 in CH3CN in the presence of air, the neutral complex [CrIII(3,6L*(sq))3] (S = 0) (1) was isolated. Reduction of 1 with [Co(Cp)2] in CH2Cl2 yielded microcrystals of [Co(Cp)2][CrIII(3,6L*(sq))2(3,6L(cat))] (S = 1/2) (2) where (3,6L*(sq)(1-) is the pi-radical monoanionic o-semiquinonate of the catecholate dianion (3,6Lcat)(2-). Electrochemistry demonstrated that both species are members of the electron-transfer series [Cr(3,6LO,O)]z (z = 0, 1-, 2-, 3-). The corresponding tris(benzo-1,2-dithiolato)chromium complex [N(n-Bu)4][CrIII(3,5L*S,S)2(3,5LS,S)] (S = 1/2) (3) has also been isolated; (3,5LS,S)(2-) represents the closed-shell dianion 3,5-di-tert-butylbenzene-1,2-dithiolate(2-), and (3,5L*S,S)(1-) is its monoanionic pi radical. Complex 3 is a member of the electron-transfer series [Cr(3,5L(S,S))3]z (z = 0, 1-, 2-, 3-). It is shown by Cr K-edge and S K-edge X-ray absorption, UV-vis, and EPR spectroscopies, as well as X-ray crystallography, of 1 and 3 that the oxidation state of the central Cr ion in each member of both electron-transfer series remains the same (+III) and that all redox processes are ligand-based. These experimental results have been corroborated by broken symmetry density functional theoretical calculations by using the B3LYP functional.     

Determining relative f and d orbital contributions to M-Cl covalency in MCl6(2-) (M = Ti, Zr, Hf, U) and UOCl5(-) using Cl K-edge X-ray absorption spectroscopy and time-dependent density functional theory

Chlorine K-edge X-ray absorption spectroscopy (XAS) and ground-state and time-dependent hybrid density functional theory (DFT) were used to probe the electronic structures of O(h)-MCl(6)(2-) (M = Ti, Zr, Hf, U) and C(4v)-UOCl(5)(-), and to determine the relative contributions of valence 3d, 4d, 5d, 6d, and 5f orbitals in M-Cl bonding. Spectral interpretations were guided by time-dependent DFT calculated transition energies and oscillator strengths, which agree well with the experimental XAS spectra. The data provide new spectroscopic evidence for the involvement of both 5f and 6d orbitals in actinide-ligand bonding in UCl(6)(2-). For the MCl(6)(2-), where transitions into d orbitals of t(2g) symmetry are spectroscopically resolved for all four complexes, the experimentally determined Cl 3p character per M-Cl bond increases from 8.3(4)% (TiCl(6)(2-)) to 10.3(5)% (ZrCl(6)(2-)), 12(1)% (HfCl(6)(2-)), and 18(1)% (UCl(6)(2-)). Chlorine K-edge XAS spectra of UOCl(5)(-) provide additional insights into the transition assignments by lowering the symmetry to C(4v), where five pre-edge transitions into both 5f and 6d orbitals are observed. For UCl(6)(2-), the XAS data suggest that orbital mixing associated with the U 5f orbitals is considerably lower than that of the U 6d orbitals. For both UCl(6)(2-) and UOCl(5)(-), the ground-state DFT calculations predict a larger 5f contribution to bonding than is determined experimentally. These findings are discussed in the context of conventional theories of covalent bonding for d- and f-block metal complexes.     

IR spectroscopy and density functional theory of small V+(N2)n complexes

V+(N2)n clusters are generated in a pulsed nozzle laser vaporization source. Clusters in the size range of n = 3-7 are mass selected and investigated via infrared photodissociation spectroscopy in the N-N stretch region. The IR forbidden N-N stretch of free nitrogen becomes strongly IR active when the molecule is bound to the metal ion. Photodissociation proceeds through the elimination of intact N2 molecules for all cluster sizes, and the fragmentation patterns reveal the coordination number of V+ to be six. The dissociation process is enhanced on vibrational resonances and the IR spectrum is obtained by monitoring the fragmentation yield as a function of wavelength. Vibrational bands are red-shifted with respect to the free nitrogen N-N stretch, in the same way seen for the C-O stretch in transition metal carbonyls. Comparisons between the measured IR spectra and the predictions of density functional theory provide new insight into the structure and bonding of these metal ion complexes.     

Electronic structure and bonding of {Fe(PhNO2)}6 complexes: a density functional theory study

Reduction of nitro-aromatic compounds (NACs) proceeds through intermediates with a partial electron transfer into the nitro group from a reducing agent. To estimate the extent of such a transfer and, therefore, the activity of various model ferrous-containing reductants toward NAC degradation, the unrestricted density functional theory (DFT) in the basis of paired L?wdin-Amos-Hall orbitals has been applied to complexes of nitrobenzene (NB) and model Fe(II) hydroxides including cationic [FeOH]+, then neutral Fe(OH)2, and finally anionic [Fe(OH)3]-. Electron transfer is considered to be a process of unpairing electrons (without the change of total spin projection Sz) that reveals itself in a substantial spin contamination of the unrestricted solution. The unrestricted orbitals are transformed into localized paired orbitals to determine the orbital channels for a particular electron-transfer state and the weights of idealized charge-transfer and covalent electron structures. This approach allows insight into the electronic structure and bonding of the {Fe(PhNO2)}6 unit (according to Enemark and Feltham notation) to be gained using model nitrobenzene complexes. The electronic structure of this unit can be expressed in terms of pi-type covalent bonding [Fe+2(d6, S = 2) - PhNO2(S = 0)] or charge-transfer configuration [Fe+3(d5, S = 5/2) - {PhNO2}- ((pi*)1, S = 1/2)].     

A density functional theory study of uranium(VI) nitrate monoamide complexes

Density functional theory calculations were performed on uranyl complexed with nitrate and monoamide ligands (L) [UO(2)(NO(3))(2)·2L]. The obtained results show that the complex stability is mainly governed by two factors: (i) the maximization of the polarizability of the coordinating ligand and (ii) the minimization of the steric hindrance effects. Furthermore, the electrostatic interaction between ligands and uranium(vi) was found to be a crucial parameter for the complex stability. These results pave the way to the definition of (quantitative) property/structure relationships for the in silico screening of monoamide ligands with improved extraction efficiency of uranium(vi) in nitrate acidic solution.     

X-ray emission spectroscopy and density functional study of CO/Fe(100)

We report x-ray emission and absorption spectroscopy studies of the electronic structure of the predissociative α(3) phase of CO bound at hollow sites of Fe(100) as well as of the on-top bound species in the high-coverage α(1) phase. The analysis is supported by density functional calculations of structures and spectra. The bonding of "lying down" CO in the hollow site is well described in terms of π to π? charge transfer made possible through bonding interaction also at the oxygen in the minority spin-channel. The on-top CO in the mixed, high-coverage α(1) phase is found to be tilted due to adsorbate-adsorbate interaction, but still with bonding mainly characteristic of "vertical" on-top adsorbed CO similar to other transition-metal surfaces.     

Catalyzing aldehyde hydrosilylation with a molybdenum(VI) complex: a density functional theory study

[MoCl(2)O(2)] catalyzes the hydrosilylation reaction of aldehydes and ketones, as well as the reduction of other related groups, in apparent contrast to its known behavior as an oxidation catalyst. In this work, the mechanism of this reaction is studied by means of density functional theory calculations using the B3LYP functional complemented by experimental data. We found that the most favorable pathway to the first step, the Si--H activation, is a [2+2] addition to the Mo=O bond, in agreement with previous and related work. The stable intermediate that results is a distorted-square-pyramidal hydride complex. In the following step, the aldehyde approaches this species and coordinates weakly through the oxygen atom. Two alternative pathways can be envisaged: the classical reduction, in which a hydrogen atom migrates to the carbon atom to form an alkoxide, which then proceeds to generate the final silyl ether, or a concerted mechanism involving migration of a hydrogen atom to a carbon atom and of a silyl group to an oxygen atom to generate the silyl ether weakly bound to the molybdenum atom. In this Mo(VI) system, the gas-phase free energies of activation for both approaches are very similar, but if solvent effects are taken into account and HSiMe(3) is used as a source of silicon, the classical mechanism is favored. Several unexpected results led us to search for still another route, namely a radical path. The energy involved in this and the classical pathway are similar, which suggests that hydrosilylation of aldehydes and ketones catalyzed by [MoCl(2)O(2)] in acetonitrile may follow a radical pathway, in agreement with experimental results.     

X-ray spectroscopy of enzyme active site analogues and related molecules: bis(dithiolene)molybdenum(IV) and -tungsten(IV,VI) complexes with variant terminal ligands

The X-ray absorption spectra at the molybdenum and selenium K-edges and the tungsten L2,3-edges are acquired for a set of 14 Mo(IV) and W(IV,VI) bis(dithiolene) complexes related to the active sites of molybdo- and tungstoenzymes. The set includes square pyramidal [MoIVL(S2C2Me2)2]- (L = O2-, R3SiO-, RO-, RS-, RSe-) and [WIV(OR)(S2C2Me2)2]-, distorted trigonal prismatic [MoIV(CO)(SeR)(S2C2Me2)2]- and [WIV(CO)L(S2C2Me2)2]- (L = RS-, RSe-), and distorted octahedral [WVIO(OR)(S2C2Me2)2]-. The dithiolene simulates the pterin-dithiolene cofactor ligand, and L represents a protein ligand. Bond lengths are determined by EXAFS analysis using the GNXAS protocol. Normalized edge spectra, non-phase-shift-corrected Fourier transforms, and EXAFS data and fits are presented. Bond lengths determined by EXAFS and X-ray crystallography agree to < or = 0.02 A as do the M-Se distances determined by both metal and selenium EXAFS. The complexes [MoIV(QR)(S2C2Me2)2]- simulate protein ligation by the DMSO reductase family of enzymes, including DMSO reductase itself (Q = O), dissimilatory nitrate reductase (Q = S), and formate dehydrogenase (Q = Se). Edge shifts of these complexes correlate with the ligand electronegativities. Terminal ligand binding is clearly distinguished in the presence of four Mo-S(dithiolene) interactions. Similarly, five-coordinate [ML(S2C2Me2)2]- and six-coordinate [M(CO)L(S2C2Me2)2]- are distinguishable by edge and EXAFS spectra. This study expands a previous XAS investigation of bis(dithiolene)metal(IV,V,VI) complexes (Musgrave, K. B.; Donahue, J. P.; Lorber, C.; Holm, R. H.; Hedman, B.; Hodgson, K. O. J. Am. Chem. Soc. 1999, 121, 10297) by including a larger inventory of molecules with variant physiologically relevant terminal ligation. The previous and present XAS results should prove useful in characterizing and refining metric features and structures of enzyme sites.     

Time dependent density functional theory study of electronic absorption properties of lead(II) complexes with a series of hydroxyflavones

The structural changes occurring with the chelation of lead(II) to 3-hydroxyflavone, 5-hydroxyflavone, and 3',4'-dihydroxyflavone have been investigated by the density functional theory (DFT) method with the B3LYP functional and the 6-31G(d,p) basis set. The two effective core potentials Lanl2dz (Los Alamos) and MWB78 (Stuttgart/Dresden) were used for the Pb ion. Only the 3',4'-dihydroxyflavone ligand shows minor geometrical modifications upon chelation, whereas the two other ligands present important changes of their chromone moiety. The time dependent density functional theory (TD-DFT) has been employed to calculate the electronic absorption spectra of the 1:1 complexes of lead(II) with the three hydroxyflavones, as well in a vacuum as in methanol. The solvent effect is modeled using the self-consistent reaction field (SCRF) method with the polarized continuum model (PCM). Comparison with experimental data allows a precise assessment of the performances of the method, which appears competitive and suitable to reproduce the spectral measurements when the solvent effect is taken into account. These calculations and the molecular orbital analysis have allowed an explanation of the different behaviors of the three ligands toward Pb(II) and particularly the fact that no bathochromic shift is observed with the addition of lead(II) to a 5-hydroxyflavone solution. A complete assignment of the electronic absorption spectra of both free and complexed ligands has been carried out.     

Electronic emission and absorption spectroscopy of the rhenium complexes Re(CO)3Cl(phosphine)2

Summary The compounds Re(CO)₃Cl(L)₂,L=triphenylphosphine, tri-p-tolyphosphine, and Re(CO)₃-Cl(L),L=1,2-bis(diethylphosphinoethane) are luminescent in solution and in crystalline form when excited between 351 nm and 514 nm at temperatures ranging from 10 K to room temperature. The absorption spectra contain a weak (E 10M ^–1 cm^–1) band in the visible region of the spectrum between 400 and 500 nm. The lowest energy transition giving rise to these spectroscopic features is assigned to a d-d transition.

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Elektronenemissions- und Absorptionsspektroskopie der Rheniumkomplexe Re(CO)₃Cl(Phoshin)₂

Zusammenfassung Die Verbindungen Re(CO)₃Cl(L)₂ mitL=Triphenylphosphin, Tri-p-tolylphosphin und Re(CO)₃Cl(L)₂ mitL=1,2-Bis(diethylphosphinoethan) sind in Lösung und im kristallinen Zustand bei Anregung zwischen 351 und 514 nm im Temperaturbereich von 10 K bis Raumtemperatur lumineszent. Die Absorptionsspektren enthalten im sichtbaren Bereich zwischen 400 und 500 nm eine schwache Bande von 10M ^–1 cm^–1. Der energetisch tiefstliegende Übergang, der für dieses Verhalten verantwortlich ist, wird einem d-d-Übergang zugeordnet.

     

Characterization by 27Al NMR, X-ray absorption spectroscopy, and density functional theory techniques of the species responsible for benzene hydrogenation in Y zeolite-supported carburized molybdenum catalysts

Carburized molybdenum catalysts supported on a dealuminated NaH-Y zeolite were prepared by carburization under a 20% methane in hydrogen flow of two precursors obtained by adsorption of molybdenum hexacarbonyl, one containing 5 wt % and the other 10 wt % Mo, and a third one was prepared by impregnation with aqueous ammonium heptamolybdate, containing 5 wt % Mo. The three catalysts displayed very distinct behaviors in the benzene hydrogenation reaction at atmospheric pressure and 363 K. By using XANES spectroscopy at the molybdenum L edge, EXAFS and XANES spectroscopy at the molybdenum K edge, and 27Al solid-state NMR spectroscopy, it was shown that different carburized molybdenum species exist in each sample. In the catalyst containing 10 wt % Mo, formation of molybdenum carbide nanoparticles was observed, with an estimated diameter of 1.8 nm. In the catalyst containing 5 wt % Mo and prepared by carburization of adsorbed molybdenum hexacarbonyl, formation of molybdenum oxycarbide dimers is proposed. In the latter case, density functional theory calculations have led to a dimer structure which is compatible with EXAFS results. In the catalyst prepared by impregnation with ammonium heptamolybdate solution followed by carburization, the molybdenum seems to interact with extraframework alumina to produce highly disordered mixed molybdenum-aluminum oxycarbides.     

DFT study of H--H activation by Cp(2) LnH d(0) complexes.

The energy required to activate the H--H bond in the entire series of Cp(2)LnH complexes has been calculated by DFT (B3PW91) methods. The activation energies have been calculated to vary from 0.5 to 8.0 kcal x mol(-1), indicating an overall facile reaction. The electronegativity of the lanthanide in its most stable oxidation state is suggested to be a leading factor for interpreting the trends in activation energy. The geometry of the transition state is best viewed as an almost linear H(3)(-) ligand with short H--H distances and strong M--H interaction, through the wingtip H centers, with Ln. The exchange reaction is thus established to be a sigma bond metathesis reaction.     

Phenyl-coinage metal (Ag, Au) complexes: an anion photoelectron spectroscopy and density functional study

The important intermediate phenyl-coinage metal complexes (Ag(m)C6H5(-), Au(m)C6H5(-)), which are produced from the reactions between coinage metal clusters formed by laser ablation and the benzene molecules seeded in argon carrier gas, are studied by PES (photoelectron spectroscopy) and DFT (density functional theory). The EAs (adiabatic electron affinities) of these complexes are obtained from PES at both 308 and 193 nm photon energies and show odd-even alternation. Calculations with DFT are carried out on the structural and electronic properties of Ag(m)C6H5(-) and Au(m)C6H5(-); the adiabatic detachment energy and the calculated DOS (density of states) for the ground state of a given anion are in good agreement with the experimental PES results. The observed spectra are also compared with those of the pure coinage metal clusters, which reveal that there are some similarities between them and the phenyl acts like an additional metal atom in the clusters. Furthermore, the bonding between phenyl and metal is analyzed, suggesting that phenyl group binds perpendicularly on metal clusters through C-M sigma bond.     

Investigation on stabilities and spectroscopy of C80O2 based on C80 (D5d) using density functional theory

The relative stabilities of the 17 possible isomers for C₈₀O₂ based on C₈₀ (D_5d) were studied using Becke three parameters plus Lee, Yang, and Parr's (B3LYP) method and 6‐31G (d) basis set in density functional theory. The most stable geometry of C₈₀O₂ was predicted to be 23,24,27,28‐C₈₀O₂ (A) with annulene‐like structures, where the additive bonds are those between two hexagons (6/6 bonds) near the equatorial belt of C₈₀ (D_5d). Electronic spectra of C₈₀O₂ isomers were calculated based on the optimized geometries using intermediate neglect of differential overlap (INDO) calculation. Compared with those of C₈₀ (D_5d), the first absorptions in the electronic spectra of C₈₀O₂ are blue‐shifted owing to the wide energy gaps. ¹³C nuclear magnetic resonance spectra and nucleus independent chemical shifts of the C₈₀O₂ isomers were computed at B3LYP/6‐31G level. The chemical shifts of the bridged carbon atoms in the epoxy structures of C₈₀O₂ compared with those of the bridged carbon atoms in the annulene‐like structures are changed upfield. Generally, the isomers with the annulene‐like structures of C₈₀O₂ are more aromatic than those with the epoxy structures. The addition of the oxygen atoms near the pole of C₈₀ (D_5d) is favorable to improving the aromaticities of C₈₀O₂. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Discrimination of mononuclear and dinuclear dinitrosyl iron complexes (DNICs) by S K-edge X-ray absorption spectroscopy: insight into the electronic structure and reactivity of DNICs

In addition to probing the formation of dinitrosyl iron complexes (DNICs) by the characteristic Fe K-edge pre-edge absorption energy ranging from 7113.4 to 7113.8 eV, the distinct S K-edge pre-edge absorption energy and pattern can serve as an efficient tool to unambiguously characterize and discriminate mononuclear DNICs and dinuclear DNICs containing bridged-thiolate and bridged-sulfide ligands. The higher Fe-S bond covalency modulated by the stronger electron-donating thiolates promotes the Fe → NO π-electron back-donation to strengthen the Fe-NO bond and weaken the NO-release ability of the mononuclear DNICs, which is supported by the Raman ν(Fe-NO) stretching frequency. The Fe-S bond covalency of DNICs further rationalizes the binding preference of the {Fe(NO)(2)} motif toward thiolates following the trend of [SEt](-) > [SPh](-) > [SC(7)H(4)SN](-). The relative d-manifold energy derived from S K-edge XAS as well as the Fe K-edge pre-edge energy reveals that the electronic structure of the {Fe(NO)(2)}(9) core of the mononuclear DNICs [(NO)(2)Fe(SR)(2)](-) is best described as {Fe(III)(NO(-))(2)}(9) compared to [{Fe(III)(NO(-))(2)}(9)-{Fe(III)(NO(-))(2)}(9)] for the dinuclear DNICs [Fe(2)(μ-SEt)(μ-S)(NO)(4)](-) and [Fe(2)(μ-S)(2)(NO)(4)](2-).     

Structural study on highly oxidized states of a water oxidation complex [RuIII(bpy)2(H2O)]2(mu-O)4+ by ruthenium K-edge X-ray absorption fine structure spectroscopy

The oxidation-induced structural change of a water-oxidizing diruthenium complex, [(bpy)(2)(H(2)O)Ru(III)(micro-O)Ru(III)(OH(2))(bpy)(2)](4+) (bpy = 2,2'-bipyridine), was investigated by means of X-ray absorption spectroscopy. Ru K-edge XANES (X-ray absorption near-edge structure) spectra from the acidic solution and solid precipitates obtained by oxidation showed that the absorption edge shifts toward higher energy with a preedge feature slightly more enhanced than those of the lower oxidation states. This indicates that the higher oxidation state has a lower symmetry due to shortening of the Ru-O bonds that originated from the water ligands. The EXAFS (extended X-ray absorption fine structure) spectra were similar to those of the lower oxidation states, whose analysis revealed the existence of short Ru-O double bonds and an almost linear Ru-O-Ru angle (169 +/- 2 degrees ). Ab initio EXAFS simulations for several possible structural models suggest that the dimeric structure is maintained during the water oxidation reaction.