Fe Bond Distance Research Articles

Synthesis of lithium doped magnesium ferrites and their vibrational and magnetic properties: Correlation of experimental and density functional theory

Recently, there has been an increased focus on investigating the possible use of ferrite nanoparticles as magnetic memory devices. In this study, we report on the structural, microstructural, Raman, density functional theory and magnetic characteristics of Mg1-xLixFe2O4 (x = 0, 0.15, 0.3, 0.45, 0.5 and 0.75). Samples were synthesized by solution combustion synthesis method. Samples were subjected to characterize X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Raman Spectra. XRD reveals that all synthesized samples show a single phase formation and unit cell volume, crystallite size varied with an increase in Lithium-ion concentration. SEM micrographs confirmed the porous nature increases with the rise of Li concentration. Particles are agglomeration and spherical type of nature was confirmed by TEM images. Further, Raman Spectra confirms the spinel cubic structure of MgFe2O4 nanoparticles belongs to the Fd3m space group. Despite the fact that the full unit cell has 56 atoms (Z = 8), the smallest Bravais cell consists of only 14 atoms (Z = 2). Consequently, in order to understand this effect, the electronic structure of pure and doped models was investigated through the Band Structure and Density of States (D.O.S.) profiles. First the crystalline structure of pure and Li-doped MgFe2O4 was analyzed based on the optimized lattice parameters and M − O (M = Mg, Li and Fe) bond distances. The band-gap was computed to be 2.54 eV being a direct transition between two points, in an excellent agreement with the experimental measurements. To investigate the temperature dependent magnetic properties, the M − H loops of Li doped MgFe2O4 nanoparticles traced at below and above room temperature over the applied magnetic field ±60 kOe. The M − H loops recorded at above and below room temperature demonstrated that the ferromagnetic nature of the sample. Further, the magnetic properties was investigated using temperature dependent FC and ZFC magnetization measurements. Above room temperature, the ZFC and FC curves show irreversibility and split at certain temperatures, as well as a broad maximum in ZFC curves at blocking temperature (TB). Saturation magnetization (Ms) values are higher at 15 K compared to 300 K, attributed to reduced thermal fluctuation. Our results suggest that synthesized materials are useful for room temperature magnetic memory device application.

Selective hydroboration of terminal alkynes catalyzed by heterometallic clusters with uranium–metal triple bonds

Selective hydroboration of terminal alkynes catalyzed by heterometallic clusters with uranium–metal triple bonds

Synthesis and structural investigation of stoichiometric iron mononitride thin films

We report the synthesis of iron mononitride (FeN) thin films having zinc blende (ZB) type structure, deposited using a direct current magnetron sputtering process on amorphous quartz substrates. The effect of substrate temperature (Ts) on the structure of FeN thin films was systematically investigated by varying the Tsfrom 300 to 500 K(in close intervals of 25K). The long-range as well as the local structure of samples was studied using x-ray diffraction (XRD), x-ray absorption near edge spectroscopy (XANES) at N K-edge, Fe L and K-edge and extended x-ray absorption fine structure (EXAFS) at Fe K-edge. The lattice parameter (LP) obtained from XRD and the presence of an intense pre-edge feature in Fe K-edge XANES confirms the ZB-structure of FeN samples. It was found in this work that the stoichiometric FeN with a LP close to its theoretical value can only be obtained in a narrow temperature regime due to minimization of stresses generated during the growth near the thermodynamic equilibrium conditions. These findings were further validated by EXAFS results yielding Fe–N and Fe–Fe bond distances, in conformity with the theoretically predicted values for FeN in ZB type structure.

Origin of the magnetic ground state developed upon Si, Ge, and Sb-substitution at Sn site in Fe2TiSn

A nonmagnetic full Heusler alloy, Fe2TiSn, was substituted with Si, Ge, and Sb at the Sn site with a primary aim of investigating the physical properties of these compositions that are much anticipated to result in a high thermoelectric power factor and generate a half-metallic ground state. The X-ray diffraction (XRD) study carried out using the synchrotron source confirms the solubility limit of substituent atoms to be a mere 5% for Si and Ge and 60% for Sb that results in a pure L21 Heusler phase. Rietveld refinement of the XRD profiles yields the lattice parameters. Despite the fact that chemical substitution should lead to atomic disorder and an increase in the scattering of charge carriers, resistivity measurements show a significant decrease in the magnitude with as little as 5% substitution of Sn by Si or Ge. The weak localization present in Fe2TiSn seems to drastically reduce in the substituted compositions, and the long range magnetic order seems to affect the transport of the charge carriers. The origin of magnetic interactions is investigated using extended X-ray absorption fine structure spectroscopy. The changes in the local crystal structure in terms of the Fe–Fe bond distance are determined. The inference drawn from the experimental study is further complemented with exchange coupling parameter, Jij, and orbital projected density of states calculations, thus presenting an overall understanding of the origin of magnetic interactions in these technologically relevant compositions.

Origin of structural and magnetic transitions in BaFeRuxAs2 materials

Using the experimentally measured temperature- and doping-dependent structural parameters on Ru-doped BaFe2As2, orbital-dependent reconstruction of the electronic structure across the magnetostructural transition is found, through first principles simulations. Below structural transition there exist two distinct Fe–Fe bond distances which modify the Fe–dxy orbital largely due to its planar spatial extension leading to Lifshitz transition, while the otherwise degenerate Fe–dxz and dyz orbitals become non-degenerate, giving rise to orbital order. The orbital order follows the temperature dependence of orthorhombocity and is also the cause of two distinct Fe–Fe bond distances. Doping-dependent Fermi surfaces show nearly equal expansion of both the electron and the hole-like Fermi surfaces, whereas the hole Fermi surface shrinks with increasing temperature and the electron Fermi surface expands comparatively slowly. The observed structural transition in this compound is electronic in origin, and occurs close to the Lifshitz transition, whereas the suppression of the concurrent magnetic transition is due to the loss of temperature dependent nesting of Fermi surface.

Facile synthesis, X-ray analysis, and spectroscopic studies of di-iron propanedithiolate complexes with tris(aromatic)phosphine ligands

Three new propanedithiolate-type iron–sulfur complexes containing tris(aromatic)phosphine ligands, [{(μ-SCH2)2CH2}Fe2(CO)5L] (L = P(PhOMe-p)3, 1; P(PhMe-p)3, 2; P(PhF-p)3, 3), have been prepared through carbonyl substitution in the presence of Me3NO. The new complexes 1–3 were characterized by elemental analysis, IR, 1H, 13C{1H}, and 31P{H} NMR spectra. The molecular structures of 1–3 were unequivocally determined by single crystal X-ray diffraction, in which the tris(aromatic)phosphine coordinated to Fe resides in an apical position of the pseudo-square-pyramidal geometry. IR spectroscopy and X-ray crystallographic analysis for 1–3 have indicated that the highly electron rich tris(aromatic)phosphine ligands (where the corresponding electron-donating abilities display the following order of P(PhOMe-p)3 > P(PhMe-p)3 > P(PhF-p)3) result in a considerable red shift of the CO-stretching frequencies and a clear change of the Fe–Fe bond distances in 1–3.

Structures and reactivity of doubly-bridged dicyclopentadienyl dinuclear iron complexes

The molecular structure of the doubly-bridged bis(cyclopentadienyl) diiron complex [(η5-C5H3)2(SiMe2)2]Fe2(CO)2(μ-CO)2 (1b) was determined, and compared with that of the other analogue [(η5-C5H3)2(CMe2)(SiMe2)]Fe2(CO)4 (1a). The former involves two bridging and two terminal carbonyls, and a normal Fe–Fe bond distance, while the latter contains all-terminal carbonyls and a longer than normal Fe–Fe bond distance. The possible reason for different coordination modes is discussed. Complexes (1a–b) react with I2 to give the corresponding diiodo products [(η5-C5H3)2(EMe2)(SiMe2)]Fe2(CO)4I2 (E=C, 2a; E=Si, 2b) and with phenylacetylene to give the bridging μ-CCHPh products [(η5-C5H3)2(EMe2)(SiMe2)]Fe2(CO)2(μ-CO)(μ-CCHPh) (3a–b). Reaction of 1a with Na/Hg, then with MeI yields the unexpected desilylated product [(η5-C5H4)2(CMe2)][Fe(CO)2Me]2 (5a). Reaction of 1b with LiBHEt3 affords the bridging methylene product [(η5-C5H3)2(SiMe2)2]Fe2(CO)2(μ-CO)(μ-CH2) (6b). The molecular structures of 2a and 3b were also determined by X-ray diffraction.

Dispersive x-ray absorption studies at the Fe K-edge on the iron chalcogenide superconductor FeSe under pressure

The local structure and the electronic properties of FeSe under hydrostatic pressure were studied by means of dispersive x-ray absorption measurements at the Fe K-edge. The pressure dependence of the x-ray absorption near edge structure features seems to follow the behavior of the superconducting transition temperature Tc. The local structure, that has an important impact on the superconducting properties, appears to fall into two regimes: the pressure dependence of the Fe–Fe bond distance shows a clear change in the compressibility at p ∼ 5 GPa; in contrast, the Fe–Se bond distance decreases continuously with increasing pressure with a lower compressibility than the Fe–Fe bond. The results suggest that the pressure dependent changes in Tc of FeSe are closely related to the changes in local structure.

Effect of silicic acid on arsenate and arsenite retention mechanisms on 6-L ferrihydrite: A spectroscopic and batch adsorption approach

The competitive adsorption of arsenate and arsenite with silicic acid at the ferrihydrite–water interface was investigated over a wide pH range using batch sorption experiments, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) modeling. Batch sorption results indicate that the adsorption of arsenate and arsenite on the 6-L ferrihydrite surface exhibits a strong pH-dependence, and the effect of pH on arsenic sorption differs between arsenate and arsenite. Arsenate adsorption decreases consistently with increasing pH; whereas arsenite adsorption initially increases with pH to a sorption maximum at pH 7–9, where after sorption decreases with further increases in pH. Results indicate that competitive adsorption between silicic acid and arsenate is negligible under the experimental conditions; whereas strong competitive adsorption was observed between silicic acid and arsenite, particularly at low and high pH. In situ, flow-through ATR-FTIR data reveal that in the absence of silicic acid, arsenate forms inner-sphere, binuclear bidentate, complexes at the ferrihydrite surface across the entire pH range. Silicic acid also forms inner-sphere complexes at ferrihydrite surfaces throughout the entire pH range probed by this study (pH 2.8–9.0). The ATR-FTIR data also reveal that silicic acid undergoes polymerization at the ferrihydrite surface under the environmentally-relevant concentrations studied (e.g., 1.0mM). According to ATR-FTIR data, arsenate complexation mode was not affected by the presence of silicic acid. EXAFS analyses and DFT modeling confirmed that arsenate tetrahedra were bonded to Fe metal centers via binuclear bidentate complexation with average As(V)-Fe bond distance of 3.27Å. The EXAFS data indicate that arsenite forms both mononuclear bidentate and binuclear bidentate complexes with 6-L ferrihydrite as indicated by two As(III)–Fe bond distances of ∼2.92–2.94 and 3.41–3.44Å, respectively. The As–Fe bond distances in both arsenate and arsenite EXAFS spectra remained unchanged in the presence of Si, suggesting that whereas Si diminishes arsenite adsorption preferentially, it has a negligible effect on As–Fe bonding mechanisms.

Dinitrosyl iron adducts of (N2S2)M(NO) complexes (M = Fe, Co) as metallodithiolate ligands

Adducts of the dinitrosyl iron unit and mono-nitrosyl bis-mercaptoethanediazacycloheptane (bme-dach) iron and cobalt are described in terms of the (bme-dach)M(NO) as metallodithiolate ligands. With M=Fe, a diamagnetic (N2S2)Fe(NO)·Fe(NO)2 diiron complex, in which the (N2S2)Fe(NO) unit serves as a bidentate ligand to the Fe(NO)2 unit is derived by use of a trinitrosyliron cation as precursor to the DNIC unit as well as a NO source to cleave the [(bme-dach)Fe]2 dimer (J. Am. Chem. Soc., 2010, 132, 14118). By a similar approach of [(bme-dach)Co]2 dimer cleavage, the Co–Fe bimetallic contains (N2S2)Co(NO) as a monodentate metallothiolate ligand to Fe(NO)2L, L=I-Mes, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene. Characterization of this paramagnetic cation includes infrared and EPR spectroscopies, and X-ray diffraction analysis. The latter finds a Co⋯Fe distance of 3.697Å in contrast to the Fe–Fe bond distance of 2.7857(8)Å in the homobimetallic complex.

Spectroscopic Evidence for Ternary Complex Formation between Arsenate and Ferric Iron Complexes of Humic Substances

Formation of ternary complexes between arsenic (As) oxyanions and ferric iron (Fe) complexes of humic substances (HS) is often hypothesized to represent a major mechanism for As-HS interactions under oxic conditions. However, direct evidence for this potentially important binding mechanism is still lacking. To investigate the molecular-scale interaction between arsenate, As(V), and HS in the presence of Fe(III), we reacted fulvic and humic acids with Fe(III) (1 wt %) and equilibrated the Fe(III)-HS complexes formed with As(V) at pH 7 (molar Fe/As ~10). The local (<5 Å) coordination environments of As and Fe were subsequently studied by means of X-ray absorption spectroscopy. Our results show that 4.5-12.5 μmol As(V)/g HS (25-70% of total As) was associated with Fe(III). At least 70% of this As pool was bound to Fe(III)-HS complexes via inner-sphere complexation. Results obtained from shell fits of As K-edge extended X-ray absorption fine structure (EXAFS) spectra were consistent with a monodentate binuclear ((2)C) and monodentate mononuclear ((1)V) complex stabilized by H-bonds (R(As-Fe) = 3.30 Å). The analysis of Fe K-edge EXAFS spectra revealed that Fe in Fe(III)-HS complexes was predominantly present as oligomeric Fe(III) clusters at neutral pH. Shell-fit results complied with a structural motif in which three corner-sharing Fe(O,OH)(6) octahedra linked by a single μ(3)-O bridge form a planar Fe trimer. In these complexes, the average Fe-C and Fe-Fe bond distances were 2.95 Å and 3.47 Å, respectively. Our study provides the first spectroscopic evidence for ternary complex formation between As(V) and Fe(III)-HS complexes, suggesting that this binding mechanism is of fundamental importance for the cycling of oxyanions such as As(V) in organic-rich, oxic soils and sediments.

Highly Unsaturated Binuclear Butadiene Iron Carbonyls: Quintet Spin States, Perpendicular Structures, Agostic Hydrogen Atoms, and Iron-Iron Multiple Bonds

The highly unsaturated binuclear butadiene iron carbonyls (C4H6)2Fe2(CO)n (n = 2, 1) have been examined using density functional theory. For (C4H6)2Fe2(CO)n (n = 2, 1), both coaxial and perpendicular structures are found. The global minima of (C4H6)2Fe2(CO)n (n = 2, 1) are the perpendicular structures 2Q-1 and 1Q-1, respectively, with 17- and 15-electron configurations for the iron atoms leading to quintet spin states. The Fe=Fe distance of 2.361 Å (M06-L) in the (C4H6)2Fe2(CO)2 structure 2Q-1 suggests a formal double bond. The Fe≡Fe bond distance in the (C4H6)2Fe2(CO) structure 1Q-1 is even shorter at 2.273 Å (M06-L), suggesting a triple bond. Higher energy (C4H6)2Fe2(CO)n (n = 2, 1) structures include structures in which a bridging butadiene ligand is bonded to one of the iron atoms as a tetrahapto ligand and to the other iron atom through two agostic hydrogen atoms from the end CH2 groups. Singlet (C4H6)2Fe2(CO) structures with formal Fe–Fe quadruple bonds of lengths ∼2.05 Å were also found but at very high energies (∼47 kcal/mol) relative to the global minimum.

Spectroscopic studies of arsenic retention onto biotite

Biotite is a constituent Fe-bearing mineral of Delta subsoils in India and Bangladesh and has been hypothesized as a primary source of arsenic (As). The adsorption behavior of As onto structural Fe(II, III)-bearing biotite fractions (<50μm) was investigated in the pH range of 4–8 under a CO2-free, anoxic condition (O2<1ppmv) using X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS). The batch experiments indicate that As adsorption is strongly pH dependent and that As(V) adsorbs more efficiently than As(III). X-ray Absorption Near-Edge Structure (XANES) spectra show no oxidation or reduction of As by biotite after reaction with As(III) or As(V) solutions for 3days. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopic results suggest that As(III) forms bidentate mononuclear edge-sharing (2E) and bidentate binuclear corner-sharing (2C) surface complexes at pH 7.7 as indicated by average As–Fe bond distances at 3.00±0.02Å and 3.37±0.03Å respectively. The surface speciation retrieved by XPS does not provide any evidence of reduction of As(V) on biotite after 30days further confirming the thermodynamic prediction and the XANES results. This study has therefore significant environmental implications for As contaminated areas, where biotite retards the release of As into reducing groundwater. The changes in soil redox conditions and weathering of biotite may likely contribute to the occurrence of high As in groundwater.

Cover Picture: ZAAC ‐ Journal of Inorganic and General Chemistry 15/2010

The cover picture shows the crystal structure of a diiron hexacarbonyl complex containing bridging phenanthrene‐4,5‐dithiolate ligand. The complex is a model for the active site of the [Fe–Fe] hydrogenase enzyme. From the X‐ray crystallographic studies, the complex is butterfly shaped with an Fe–Fe bond distance of 2.5365 Å close to that of the enzyme (2.6 Å). The infrared spectrum of the complex (top left) contains three peaks at 2001, 2040, and 2075 cm−1 corresponding to ν(CO). The cyclic voltammogram of the complex (bottom right) contains one quasi‐irreversible reduction (E1/2 = −0.84 V vs Ag/AgCl) tentatively assigned to the one‐electron transfer process; [FeI–FeI] → [FeI–Fe0]. More details are discussed in the Article by C. Mebi et al. on pp. 2550 ff.

Effect of citrate on the local Fe coordination in ferrihydrite, arsenate binding, and ternary arsenate complex formation

In oxic environments contaminated with arsenate (As(V)), small polyhydroxycarboxylates such as citrate may impact the structure of precipitating ferrihydrite (Fh) and thus the surface speciation of As(V). In this study, ‘2-line’ Fh was precipitated from ferric nitrate solutions that were neutralized to pH 6.5 in the presence of increasing citrate concentrations and in the absence or presence of As(V). The initial citrate/Fe and As/Fe ratios were 0–50 mol% and 5 mol%, respectively. The reaction products, enriched with up to 0.32 mol citrate per mole Fe, were characterized by X-ray diffraction, transmission electron microscopy, and Fe and As K-edge X-ray absorption spectroscopy. Citrate decreased the particle size of Fh by impairing the polymerization of Fe(O,OH) 6 octahedra via edge and corner linkages. In the presence of citrate and As(V), coordination numbers of Fe decreased by up to 28% relative to pure Fh. Citrate significantly reduced the static disorder of Fe–O bonds, implying a decreased octahedral distortion in Fh. Mean bond distances in Fh were not affected by citrate and remained constant within error at 1.98 Å for Fe–O, 3.03 Å for Fe–Fe1, and 3.45 Å for Fe–Fe2. Likewise, citrate had no effect on the As–Fe (3.31 Å) bond distance in As(V) coprecipitated with Fh. The As K-edge EXAFS data comply with the formation of (i) only monodentate binuclear ( 2C) As(V) surface complexes and (ii) combinations of 2C, monodentate mononuclear ( 1V), and outersphere As(V) surface complexes. Our results suggest that increasing citrate concentrations led to a decreasing 1V/ 2C ratio and/or that citrate increasingly impaired the formation of outersphere As(V) complexes. Moreover, citrate stabilized colloidal suspensions of Fh (pH 4.3–6.6, I ∼0.45 M) and reduced Fh formation at the expense of soluble Fe(III)-citrate complexes. At initial citrate/Fe ratios ⩾25 mol%, between 8% and 41% of total Fe was bound in Fe(III)-citrate complexes after Fh formation. Polynuclear Fe(III)-citrate species were found to bind As(V) via surface complexes indistinguishable by EXAFS from those of As(V) adsorbed to or coprecipitated with Fh. Our study implies that low molecular weight polyhydroxycarboxylates may enhance the mobility of As(V) in aqueous systems of high ionic strength (e.g., neutralizing acid mine drainage) by colloidal stabilization of suspended Fh particles and the formation of ternary As(V) complexes.

Synthesis and structures of substituted tetramethylcyclopentadienyl dinuclear metal carbonyl compounds

Cyclopentadienes (C5Me4R) [R = Allyl (1), n-Butyl (2), Benzyl (3), and PhMe-2 (4)] reacted with Fe(CO)5 in refluxing xylene to give new substituted tetramethylcyclopentadienyl dinuclear iron carbonyl complexes [(η 5-C5Me4R)Fe(CO)(μ-CO)]2 [R = Allyl (5), n-Butyl (6), Benzyl (7), and PhMe-2 (8)], respectively. The four new complexes 5–8 were characterized by elemental analysis, IR, and 1H NMR spectra. The crystal structures of complexes 5–7 were determined using single crystal X-ray diffraction. The crystal structure of complex 5 showed that allyl underwent isomerization to give the corresponding methyl-vinyl. A possible mechanism is discussed. The X-ray crystal structures of complexes 5, 6, and 7 confirm the structure with bridging and terminal CO groups. They show that the steric effects of substituents influence the Fe–Fe bond distances of the complexes.

Unsaturation in Binuclear Cyclobutadiene Iron Carbonyls: Triplet Structures, Four-Electron Bridging Carbonyl Groups, and Perpendicular Structures

In view of experimental work on (η4-C4H4)2Fe2(μ-CO)3 the full range of binuclear cyclobutadieneiron carbonyls (η4-C4H4)2Fe2(CO)n (n = 5, 4, 3, 2, 1) have been studied by density functional theory using the B3LYP and BP86 functionals. The pentacarbonyl (η4-C4H4)2Fe2(CO)5 is predicted to have a singly bridged (η4-C4H4)2Fe2(CO)4(μ-CO) structure with an Fe−Fe single-bond distance of 2.743 A (BP86). For (η4-C4H4)2Fe2(CO)4 cis and trans doubly CO-bridged singlet structures are found as well as two types of doubly CO-bridged triplet structures. One type has Fe−Fe bond distances in the single-bond range 2.64−2.66 A and 17-electron iron configurations. The other type of triplet structure has Fe═Fe bond distances in the range 2.38−2.39 A for a σ + 2/2π double bond like that in dioxygen and 18-electron iron configurations. For (η4-C4H4)2Fe2(CO)3 the global minimum is the experimentally observed triply CO-bridged structure with an Fe≡Fe distance of 2.148 A (BP86), corresponding to the triple bond required for 18-elec...

Synthesis and structures of cycloalkylidene-bridged biscyclopentadienyl-tetracarbonyldiiron complexes C(CH2) n [( η 5-C5H4)Fe(CO)]2( μ-CO)2(n = 4, 5 and 6)

A series of cycloalkylidene-bridged biscyclopentadienyldiiron complexes, C(CH2) n [(η5-C5H4)Fe(CO)]2(μ-CO)2 (n = 4, 5 and 6) have been synthesized by the reacting C(CH2) n Cp2 (Cp = C5H5) with Fe(CO)5 in refluxing xylene. The molecular structures of C(CH2)5[(η5-C5H4)Fe(CO)]2(μ-CO)2(2) and trans-C(CH2)4[(η5-t-BuC5H3)Fe(CO)]2(μ-CO)2(4t) have been determined by X-ray diffraction. The Fe—Fe bond distance [2.466 A], in (2) is the shortest reported to date for bridged biscyclopentadienyldiiron complexes.

Experimental and theoretical XANES and EXAFS study of tetra-ferriphlogopite

Experimental and theoretical EXAFS and XANES data on Fe coordination and bond distances have been obtained for a natural tetra-ferriphlogopite. XANES data show a predominance of trivalent iron in tetrahedral coordination. The use of theoretical multiple scattering calculations helps distinguish the contributions to the total spectrum coming from the different iron atoms. EXAFS spectra were reduced and analyzed using the GNXAS programs and Fe has been confirmed to be in tetrahedral coordination, with refined structural data Fe-O = 1.86 A, Fe-Si = 3.24 A, Fe-O-Si = 138°. Moreover, a further contribution from Fe is detected corresponding to a Fe-O distance equal to 2.22 A. This latter contribution (20 mol % of the total Fe ca. ) is interpreted to arise from divalent Fe in octahedral coordination, in excellent agreement with the structural and chemical data.

Theoretical Study of Bonding, Structure, and Vibrational Spectra of the [Fe2(CO)8]2-Anion and Its Derivatives

A theoretical study of the structural versatility of the di-iron carbonylate, [Fe2(CO)8]2-, and of its adducts with electrophiles is presented. The geometries of three energy minima and four transition states of [Fe2(CO)8]2- have been characterized, and the relative energies of several alternative structures have been evaluated. The calculated vibrational spectra in the Fe−Fe and the C⋮O stretching regions are discussed for the three isomers, and a good correlation between the Fe−Fe stretching force constant and the Fe−Fe bond distance is found for both theoretical and experimental data. The effect of the orientation of the terminal ligands on the Fe−Fe bond and the rearrangement of such ligands by the formation of adducts with electrophiles are also addressed.