Short Fe-Fe Distances Research Articles

N-Alkyl substituted triazenide-bridged homoleptic iron(II) dimers with an exceptionally short Fe-Fe bond.

Dinuclear transition metal complexes with direct metal-metal interactions have the potential to generate unique reactivities and properties. Using asymmetric triazine ligands HN3tBuR (R = Et, iPr, nBu) featuring different alkyl substituents at 1,3-N centers, we report here the first rational synthesis of 'tetragonal lantern' type Fe(II) triazenides [Fe2(N3tBuR)4] [R = Et (1), iPr (2), nBu (3)] having an exceptionally short Fe-Fe distance (2.167-2.174 Å). Unlike the previously reported lantern structures with related amidinate or guanidinate ligands, highly air-sensitive 1-3 show a lower spin ground state, as indicated by Mössbauer, 1H NMR and DFT studies.

Investigating Metal-Metal Bond Polarization in a Heteroleptic Tris-Ylide Diiron System.

This article describes the synthesis, characterization, and S-atom transfer reactivity of a series of C3v-symmetric diiron complexes. The iron centers in each complex are coordinated in distinct ligand environments, with one (FeN) bound in a pseudo-trigonal bipyramidal geometry by three phosphinimine nitrogens in the equatorial plane, a tertiary amine, and the second metal center (FeC). FeC is coordinated, in turn, by FeN, three ylidic carbons in a trigonal plane, and, in certain cases, by an axial oxygen donor. The three alkyl donors at FeC form through the reduction of the appended N═PMe3 arms of the monometallic parent complex. The complexes were studied crystallographically, spectroscopically (NMR, UV-vis, and Mössbauer), and computationally (DFT, CASSCF) and found to be high-spin throughout, with short Fe-Fe distances that belie weak orbital overlap between the two metals. Further, the redox nature of this series allowed for the determination that oxidation is localized to the FeC. S-atom transfer chemistry resulted in the formal insertion of a S atom into the Fe-Fe bond of the reduced diiron complex to form a mixture of Fe4S and Fe4S2 products.

Spectroscopic Properties of a Biologically Relevant [Fe2(μ-O)2] Diamond Core Motif with a Short Iron-Iron distance.

Diiron cofactors in enzymes perform diverse challenging transformations. The structures of high valent intermediates (Q in methane monooxygenase and X in ribonucleotide reductase) are debated since Fe-Fe distances of 2.5-3.4 Å were attributed to "open" or "closed" cores with bridging or terminal oxo groups. We report the crystallographic and spectroscopic characterization of a FeIII2(µ-O)2 complex (2) with tetrahedral (4C) centres and short Fe-Fe distance (2.52 Å), persisting in organic solutions. 2 shows a large Fe K-pre-edge intensity, which is caused by the pronounced asymmetry at the TD Fe(III) centres due to the short Fe-µ-O bonds. A ~2.5 Å Fe-Fe distance is unlikely for six-coordinate sites in Q or X, but for a Fe2(μ-O)2 core containing four-coordinate (or by possible extension five-coordinate) iron centres there may be enough flexibility to accommodate a particularly short Fe-Fe separation with intense pre-edge transition. This finding may broaden the scope of models considered for the structure of high-valent diiron intermediates formed upon O2 activation in biology.

Interplay of magnetic states and hyperfine fields of iron dimers on MgO(001)

Individual nuclear spin states can have very long lifetimes and could be useful as qubits. Progress in this direction was achieved on MgO/Ag(001) via detection of the hyperfine interaction (HFI) of Fe, Ti and Cu adatoms using scanning tunneling microscopy. Previously, we systematically quantified from first-principles the HFI for the whole series of 3d transition adatoms (Sc-Cu) deposited on various ultra-thin insulators, establishing the trends of the computed HFI with respect to the filling of the magnetic s- and d-orbitals of the adatoms and on the bonding with the substrate. Here we explore the case of dimers by investigating the correlation between the HFI and the magnetic state of free standing Fe dimers, single Fe adatoms and dimers deposited on a bilayer of MgO(001). We find that the magnitude of the HFI can be controlled by switching the magnetic state of the dimers. For short Fe-Fe distances, the antiferromagnetic state enhances the HFI with respect to that of the ferromagnetic state. By increasing the distance between the magnetic atoms, a transition toward the opposite behavior is observed. Furthermore, we demonstrate the ability to substantially modify the HFI by atomic control of the location of the adatoms on the substrate. Our results establish the limits of applicability of the usual hyperfine hamiltonian and we propose an extension based on multiple scattering processes.

B-B Coupling and B-B Catenation: Computational Study of the Structure and Reactions of Metal-Bis(borylene) Complexes.

A detailed molecular orbital analysis of the metal-bis(borylene) complex [Fe(CO)3 {B(Dur)B(N(SiMe3 )2 )}] (Dur=2,3,5,6-tetramethylphenyl) (1 a) serves as a focal point of recent developments in this area of chemistry, such as B-B coupling and B-B catenation reactions. There is strong a π delocalization between the Fe(CO)3 and (B-Dur)(B-N(SiMe3 )2 ) units; the short B-B distance in 1 a is due to this π delocalization. The π-donor ligand N(SiMe3 )2 on the boron provides a decisive stability to the complex 1 a. The LUMO of 1 a has B-B σ-bonding character. Hence B-B coupling is facilitated by filling the LUMO. Strong σ-donating ligands, such as PMe3 or PCy3 , induce B-B coupling. Expulsion of one CO from 1 a followed by dimerization leads to [Fe(CO)2 {B(Dur)B(N(SiMe3 )2 )}]2 (3 a) with a short Fe-Fe distance of 2.355 Å. A detailed mechanism for the reaction of 3 a with CO to give the B-B catenation product 2 f is presented. The bonding of all intermediates is compared to their isolobal main-group analogues.

High-Resolution Extended X-ray Absorption Fine Structure Analysis Provides Evidence for a Longer Fe···Fe Distance in the Q Intermediate of Methane Monooxygenase.

Despite decades of intense research, the core structure of the methane C-H bond breaking diiron(IV) intermediate, Q, of soluble methane monooxygenase remains controversial, with conflicting reports supporting either a "diamond" diiron core structure or an open core structure. Early extended X-ray absorption fine structure (EXAFS) data assigned a short 2.46 Å Fe-Fe distance to Q (Shu et al. Science 1997, 275, 515 ) that is inconsistent with several theoretical studies and in conflict with our recent high-resolution Fe K-edge X-ray absorption spectroscopy (XAS) studies (Castillo et al. J. Am. Chem. Soc. 2017, 139, 18024 ). Herein, we revisit the EXAFS of Q using high-energy resolution fluorescence-detected extended X-ray absorption fine structure (HERFD-EXAFS) studies. The present data show no evidence for a short Fe-Fe distance, but rather a long 3.4 Å diiron distance, as observed in open core synthetic model complexes. The previously reported 2.46 Å feature plausibly arises from a background metallic iron contribution from the experimental setup, which is eliminated in HERFD-EXAFS due to the increased selectivity. Herein, we explore the origin of the short diiron feature in partial-fluorescent yield EXAFS measurements and discuss the diagnostic features of background metallic scattering contribution to the EXAFS of dilute biological samples. Lastly, differences in sample preparation and resultant sample inhomogeneity in rapid-freeze quenched samples for EXAFS analysis are discussed. The presented approaches have broad implications for EXAFS studies of all dilute iron-containing samples.

Methylborabenzene ligands in binuclear iron carbonyl derivatives: High spin states and iron–iron multiple bonding

The experimentally known methylborabenzene iron carbonyl (C5H5BCH3)2Fe2(CO)4 and its decarbonylation products (C5H5BCH3)2Fe2(CO)n (n = 3, 2) have been studied by density functional theory for comparison with their cyclopentadienyl analogs. The lowest energy (η6-C5H5BCH3)2Fe2(CO)4 structures are the experimentally known singlet doubly bridged cis-(η6-C5H5BCH3)2Fe2(CO)2(μ-CO)2 and the corresponding trans isomer similar to the corresponding (η5-C5H5)2Fe2(CO)2(μ-CO)2 system. Also the triplet triply bridged (η6-C5H5BCH3)2Fe2(μ-CO)3 is the lowest energy tricarbonyl structure similar to the cyclopentadienyl system. However, significant differences between the methylborabenzene and cyclopentadienyl derivatives are found in the dicarbonyl systems. A singlet (η5-C5H5)2Fe2(μ-CO)2 structure with a short FeFe distance of ∼2.1 Å was previously found to be the lowest energy structure in the cyclopentadienyl system. An analogous methylborabenzene structure (η6-C5H5BCH3)2Fe2(μ-CO)2 is found as the lowest energy singlet structure. However, an unsymmetrical quintet (η6-C5H5BCH3)FeFe(CO)2(η6-C5H5BCH3) structure is predicted to lie ∼22 kcal/mol below this singlet structure. The existence of this low-energy, high-spin (η6-C5H5BCH3)2Fe2(CO)2 structure makes (η6-C5H5BCH3)2Fe2(μ-CO)3 disfavored with respect to disproportionation into (η6-C5H5BCH3)2Fe2(μ-CO)4 + (η6-C5H5BCH3)2Fe2(μ-CO)2 unlike its cyclopentadienyl analog.

Unsaturation in binuclear cyclopentadienyliron carbonyl thiocarbonyls: Four-electron donor bridging thiocarbonyl groups versus metal–metal multiple bonds

Theoretical studies on the binuclear cyclopentadienyliron carbonyl thiocarbonyl derivatives Cp2Fe2(CO)2(μ-CS)(μ-CO) and Cp2Fe2(CO)2(μ-CS)2 indicate that the trans and cis isomers are nearly degenerate in energy, consistent with experiment. Structures with bridging CS groups are of lower energy than corresponding structures with bridging CO groups. The corresponding unbridged Cp2Fe2(CS)(CO)3 and Cp2Fe2(CS)2(CO)2 isomers are predicted to lie 11 and 16kcal/mol, respectively, above their global minima, indicating increasing activation energies for the cis/trans interconversion as bridging CO groups are replaced by bridging CS groups. The unsaturated species Cp2Fe2(μ-CS)(μ-CO)2 and Cp2Fe2(μ-CS)2(μ-CO) are predicted to have triply bridged triplet spin state structures with FeFe double bonds of lengths 2.26Å, analogous to the experimentally known triplet (Me5C5)2Fe2(μ-CO)3. However, low-lying singlet Cp2Fe2(CS)(CO)2 and Cp2Fe2(CS)2(CO) structures with four-electron donor bridging η2-μ-CS groups and formal Fe–Fe single bonds are also found. The lowest lying Cp2Fe2(CS)(CO) and Cp2Fe2(CS)2 structures have two bridging groups and very short FeFe distances of ∼2.14Å, suggesting formal triple bonds. Several higher energy four-electron donor η2-μ-CS bridged structures are also found for Cp2Fe2(CS)(CO) and Cp2Fe2(CS)2. In addition, singlet and triplet structures are found for Cp2Fe2(CS)2 in which the two CS ligands have coupled to form a bridging SCCS group with a carbon–carbon bond. Only a η2-μ-CS bridged singlet structure is predicted for Cp2Fe2(CS), rather than the normal bridged structure with a FeFe quadruple bond such as that predicted for the carbonyl analog Cp2Fe2(CO).

Photoinitiated Singlet and Triplet Electron Transfer across a Redesigned [Myoglobin, Cytochromeb5] Interface

We describe a strategy by which reactive binding of a weakly bound, 'dynamically docked (DD)' complex without a known structure can be strengthened electrostatically through optimized placement of surface charges, and discuss its use in modulating complex formation between myoglobin (Mb) and cytochrome b(5) (b(5)). The strategy employs paired Brownian dynamics (BD) simulations, one which monitors overall binding, the other reactive binding, to examine [X --> K] mutations on the surface of the partners, with a focus on single and multiple [D/E --> K] charge reversal mutations. This procedure has been applied to the [Mb, b(5)] complex, indicating mutations of Mb residues D44, D60, and E85 to be the most promising, with combinations of these showing a nonlinear enhancement of reactive binding. A novel method of displaying BD profiles shows that the 'hits' of b(5) on the surfaces of Mb(WT), Mb(D44K/D60K), and Mb(D44K/D60K/E85K) progressively coalesce into two 'clusters': a 'diffuse' cluster of hits that are distributed over the Mb surface and have negligible electrostatic binding energy and a 'reactive' cluster of hits with considerable stability that are localized near its heme edge, with short Fe-Fe distances favorable to electron transfer (ET). Thus, binding and reactivity progressively become correlated by the mutations. This finding relates to recent proposals that complex formation is a two-step process, proceeding through the formation of a weakly bound encounter complex to a well-defined bound complex. The design procedure has been tested through measurements of photoinitiated ET between the Zn-substituted forms of Mb(WT), Mb(D44K/D60K), and Mb(D44K/D60K/E85K) and Fe(3+)b(5). Both mutants convert the complex from the DD regime exhibited by Mb(WT), in which the transient complex is in fast kinetic exchange with its partners, k(off) >> k(et), to the slow-exchange regime, k(et) >> k(off), and both mutants exhibit rapid intracomplex ET from the triplet excited state to Fe(3+)b(5) (rate constant, k(et) approximately 10(6) s(-1)). The affinity constants of the mutant Mbs cannot be derived through conventional analysis procedures because intracomplex singlet ET quenching causes the triplet-ground absorbance difference to progressively decrease during a titration, but this effect has been incorporated into a new procedure for computing binding constants. Most importantly, these measurements reveal the presence of fast photoinduced singlet ET across the protein-protein interface, (1)k(et) approximately 2 x 10(8) s(-1).

The crystal-structure and vacancy distribution in 6C pyrrhotite

The crystal structure of a 6 C pyrrhotite from Mponeng Mine, South Africa, has been refined using starting atomic parameters postulated by Koto et al. (1975). This pyrrhotite is monoclinic and is described in the non-standard space group Fd , so that the metrically orthorhombic unit-cell shape is preserved. The cell symmetry is monoclinic with dimensions a = 6.897(2), b = 11.954(3), c = 34.521(7), and with β = 90.003(4)°. The structure has been refined with anisotropic displacement parameters to R = 0.029 using 1493 observable reflections with I > 2σ( I ) and R = 0.034 for all 1800 reflections. The internal R int is 0.023 for the symmetrically equivalent reflection data. The composition of the crystal, as determined by electron microprobe analysis is Fe 10.91 Ni 0.05 S 12 . The crystal structure resembles that of the 5 C pyrrhotite in that the atomic positions of the Fe and S atoms are arranged in a very similar fashion, the only real difference being the arrangement of partially occupied iron sites. The coordination of the iron atoms is octahedral and short Fe-Fe distances along the c -axis are also encountered in this structure. The vacancy distribution is similar to that postulated by Koto et al. (1975) and is characterized by the stacking of two approximately half-occupied sites, followed by an essentially fully occupied layer. This is however a simplification and results in a composition that is too metal-rich. Two other slightly defect sites with occupancies of 0.90 and 0.87 are also present in the structure, and all layers contain both fully occupied and partially occupied sites. Refinement of the occupancies of all these sites gives rise to an atomic distribution that resembles the measured composition most closely, and is refined as Fe 10.99 S 12 . The powder diffraction pattern of this structure is compared to the very similar pattern of the 5 C structure. The crystal structure is also given in the more conventional Cc setting so as to be compatible with the available crystallographic software that cannot normally accommodate the Fd space group.

Tri-μ-ethanethiolato-bis{[η5-1,2,3,4-tetramethyl-5-(trimethylsilyl)cyclopentadienyl]iron(II,III)}(FeII–FeIII)

The title complex, [Fe2(C2H5S)3(C12H21Si)2], has an unusual Fe2S3 core. The two 1,2,3,4-tetra­methyl-5-(trimethyl­silyl)cyclo­penta­dienyl (Cp′) ligands coordinate to the Fe atoms with their C5 planes perpendicular [dihedral angles = 88.23 (7) and 88.55 (7)°] to the Fe—Fe vector, building two Cp′Fe subunits. These two subunits are bridged by three thiol­ate ligands. There are no significant differences in the coordination geometries between the two Fe atoms. The short Fe—Fe distance of 2.7842 (5) Å is clear evidence of an inter­metallic bond. Such a diiron–sulfur structure might act as a model of active sites in some metalloproteins.

Unsaturation in Binuclear Cyclopentadienyliron Carbonyls

The binuclear cyclopentadienyliron carbonyls Cp2Fe2(CO)n (n = 4, 3, 2, 1; Cp = eta(5)-C5H5) have been studied by density functional theory (DFT) using the B3LYP and BP86 methods. The trans- and cis-Cp2Fe2(CO)2(mu-CO)2 isomers of Cp2Fe2(CO)4 known experimentally are predicted by DFT methods to be genuine minima with no significant imaginary vibrational frequencies. The energies of these two Cp2Fe2(CO)2(mu-CO)2 structures are very similar, consistent with the experimental observation of an equilibrium between these isomers in solution. An intermediate between the interconversion of the trans- and cis-Cp2Fe2(CO)2(mu-CO)2 dibridged isomers of Cp2Fe2(CO)4 can be the trans unbridged isomer of Cp2Fe2(CO)4 calculated to be 2.3 kcal/mol (B3LYP) or 9.1 kcal/mol (BP86) above the global minimum trans-Cp2Fe2(CO)2(mu-CO)2. For the unsaturated Cp2Fe2(CO)3, the known triplet isomer Cp2Fe2(mu-CO)3 with an Fe=Fe double bond similar to the O=O double bond in O2 is found to be the global minimum. The lowest-energy structure for the even more unsaturated Cp2Fe2(CO)2 is a dibridged structure Cp2Fe2(mu-CO)2, with a short Fe-Fe distance suggestive of the Fe[triple bond]Fe triple bond required to give both Fe atoms the favored 18-electron configuration. Singlet and triplet unbridged structures for Cp2Fe2(CO)2 were also found but at energies considerably higher (20-50 kcal/mol) than that of the global minimum Cp2Fe2(mu-CO)2. The lowest-energy structure for Cp2Fe2(CO) is the triplet unsymmetrically bridged structure Cp2Fe2(mu-CO), with a short Fe-Fe distance (approximately 2.1 A) suggestive of the sigma + 2pi + (2/2)delta Fe[quadruple bond]Fe quadruple bond required to give both Fe atoms the favored 18-electron rare gas configuration.

Density functional study of a micro-1,1-carboxylate bridged Fe(III)-O-Fe(IV) model complex. 2. Comparison with ribonucleotide reductase intermediate X.

Using broken-symmetry density functional theory, we have studied an experimentally proposed model for ribonucleotide reductase (RNR) intermediate X, which contains a single oxo bridge, one terminal H(2)O or OH(-) ligand, a bidentate carboxylate from Glu115, and a mono-oxygen bridge provided by Glu238. For the models proposed here, the terminal H(2)O/OH(-) ligand binds to site Fe1 which is closer to Tyr122. The diiron centers are assigned as high-spin Fe(III)Fe(IV) and antiferromagnetically coupled to give the S(total) = (1)/(2) ground state. Calculations show that the model with a terminal hydroxide in the antiferromagnetic [S(Fe1) = 2, S(Fe2) = (5)/(2)] state (Fe1 = Fe(IV), Fe2 = Fe(III)) is the lowest energy state, and the calculated isomer shift and quadrupole splitting values for this cluster are also the best among the four clusters studied here when compared with the experimental values. However, the DFT-calculated (1)H proton and (17)O hyperfine tensors for this state do not show good agreement with the experiments. The calculated Fe1-Fe2 distances for this and the other three clusters at >2.9 A are much longer than the 2.5 A which was predicted by the EXAFS measurements. The mono-oxygen bridge provided by Glu238 tends to be closer to one of the Fe sites in all clusters studied here, and it does not function as a bridge in helping to produce a short Fe-Fe distance. Overall, the models tested here are not likely to represent the core structure of RNR intermediate X. The model with the terminal OH(-) binding to the Fe1(III) center shows the best calculated (1)H proton and (17)O hyperfine tensors compared with the experimental values. This supports the earlier proposal based on analysis of ENDOR spectra (Willems et al.(16)) that the terminal oxygen group binds to the Fe(III) site in RNR-X.

A short Fe-Fe distance in peroxodiferric ferritin: control of Fe substrate versus cofactor decay?

The reaction of oxygen with protein diiron sites is important in bioorganic syntheses and biomineralization. An unusually short Fe-Fe distance of 2.53 angstroms was found in the diiron (mu-1,2 peroxodiferric) intermediate that forms in the early steps of ferritin biomineralization. This distance suggests the presence of a unique triply bridged structure. The Fe-Fe distances in the mu-1, 2 peroxodiferric complexes that were characterized previously are much longer (3.1 to 4.0 angstroms). The 2.53 angstrom Fe-Fe distance requires a small Fe-O-O angle (approximately 106 degrees to 107 degrees). This geometry should favor decay of the peroxodiferric complex by the release of H2O2 and mu-oxo or mu-hydroxo diferric biomineral precursors rather than by oxidation of the organic substrate. Geometrical differences may thus explain how diiron sites can function either as a substrate (in ferritin biomineralization) or as a cofactor (in O2 activation).

Theoretical Model Studies of the Iron Dimer Complex of MMO and RNR.

Hybrid DFT calculations are used to study the structures and chemistry of the diiron complex of MMO and RNR. The chemical model used is larger than the ones used previously and contains the full first ligand sphere. New types of structures are suggested for the intermediates observed in the reactions of these enzymes. To obtain the very short Fe-Fe distances recently measured by EXAFS, it is suggested that both compound Q of MMO and compound X of RNR have two bridging carboxylates like the reduced diferrous complex but unlike the diferric complex. Antiferromagnetic coupling is shown to have significant effects on the electronic structure of the complexes and is also important for achieving the short Fe-Fe distance. The new model is also used to study the activation of methane, and a pure hydrogen abstraction transition state is located where a hydrogen of methane is abstracted by a bridging &mgr;-oxo ligand. The combination of the methyl radical with the bridging &mgr;-OH ligand formed occurs without a barrier and is extremely exothermic. This reaction should therefore be very fast. A symmetric transition state for the activation of O(2) is also proposed leading from a suggested compound P to compound Q. In this process, one of the carboxylate bridges of the reduced complex has to move away from O(2) toward one of the irons. These results are discussed in relation to recent experiments.

Crystal structure of the hydroxylase component of methane monooxygenase from Methylosinus trichosporium OB3b.

Methane monooxygenase (MMO), found in aerobic methanotrophic bacteria, catalyzes the O2-dependent conversion of methane to methanol. The soluble form of the enzyme (sMMO) consists of three components: a reductase, a regulatory "B" component (MMOB), and a hydroxylase component (MMOH), which contains a hydroxo-bridged dinuclear iron cluster. Two genera of methanotrophs, termed Type X and Type II, which differ markedly in cellular and metabolic characteristics, are known to produce the sMMO. The structure of MMOH from the Type X methanotroph Methylococcus capsulatus Bath (MMO Bath) has been reported recently. Two different structures were found for the essential diiron cluster, depending upon the temperature at which the diffraction data were collected. In order to extend the structural studies to the Type II methanotrophs and to determine whether one of the two known MMOH structures is generally applicable to the MMOH family, we have determined the crystal structure of the MMOH from Type II Methylosinus trichosporium OB3b (MMO OB3b) in two crystal forms to 2.0 A resolution, respectively, both determined at 18 degrees C. The crystal forms differ in that MMOB was present during crystallization of the second form. Both crystal forms, however, yielded very similar results for the structure of the MMOH. Most of the major structural features of the MMOH Bath were also maintained with high fidelity. The two irons of the active site cluster of MMOH OB3b are bridged by two OH (or one OH and one H2O), as well as both carboxylate oxygens of Glu alpha 144. This bis-mu-hydroxo-bridged "diamond core" structure, with a short Fe-Fe distance of 2.99 A, is unique for the resting state of proteins containing analogous diiron clusters, and is very similar to the structure reported for the cluster from flash frozen (-160 degrees C) crystals of MMOH Bath, suggesting a common active site structure for the soluble MMOHs. The high-resolution structure of MMOH OB3b indicates 26 consecutive amino acid sequence differences in the beta chain when compared to the previously reported sequence inferred from the cloned gene. Fifteen additional sequence differences distributed randomly over the three chains were also observed, including D alpha 209E, a ligand of one of the irons.

Electronic Structure, Heisenberg Coupling Constants, and Metal−Metal Bond in Dimeric Iron(II) Organometallics with the Metal Centers at Variable Distances: A Density Functional Approach

Density functional calculations were performed to investigate the electronic structure, the magnetic coupling, and the bonding in Fe2(NCH2)2(NCH)2(C6H5)2, Fe2(C6H5)4, and Fe2(HNCH)4 model systems. The corresponding real compounds are characterized by strongly reduced magnetic moments, while metal−metal distances vary in a wide range from uncommonly short to slightly long. The results show the absence of a direct Fe(II)Fe(II) bond for the first two compounds: both the electronic structure studies and the Heisenberg coupling analysis give indications in this direction. The reduced magnetic moment results essentially from strong antiferromagnetic interactions via bridging ligands. Heisenberg coupling constants were calculated for each complex, which show an antiferromagnetic coupling of four unpaired electrons on each center. In the third compound there is a relevant direct Fe−Fe interaction, although the short FeFe distance results mainly from a strong metal-bridging ligand interaction.

A structural analysis and some magnetic properties of the R2Fe17Hx series

A structural and magnetic characterization of the compounds R 2 Fe 17 H x through the R series (except for R = Eu, Yb) is presented. The starting alloys crystallize with the rhombohedral ( Th 2 Zn 17- type R-3 m) or the hexagonal ( Th 2 Ni 17- type P6 3/ mmc) structure for the light and heavy rare earths, respectively. It is seen that the host alloy symmetry is retained upon hydrogenation. Hydrogen is found to accomodate rare earth-rich octahedral and tetrahedral sites. The maximum hydrogen uptake ranges from 3 to 5 hydrogen atoms per formula unit for the heavier and lighter rare earths, respectively. The highly anisotropic cell expansion upon hydrogenation is analyzed. A filling scheme for hydrogen is proposed. The effect of hydrogenation on intrinsic magnetic parameters such as T C and the total magnetization has been studied. It is correlated to the variation of short Fe-Fe distances within the structure and discussed in terms of variations of the exchange interaction. It is also shown that hydrogenation modifies the thermal variation of the anisotropy constants in some compounds.

Reflected light examination of Marcasite and l�llingite

The reflectance spectra for the isotypic orthorhombic minerals marcasite, FeS2 and lollingite, FeAs2 have been measured in air and in immersion oil between 400 and 800 nm, and the indices of refraction and absorption indices have been calculated. Marcasite shows a practically “isotropic” behaviour of the optical constants, lollingite is characterized by a “uniaxial” optical behaviour with maximum absorption occurring perpendicular to the relatively short Fe-Fe distances.

Magnetic properties of the R-Fe-B ternary compounds

Due to short Fe-Fe distances in the R 2Fe 14B structure, 3d interactions are reduced and strongly distance dependent. The value of the Fe moment depends on local environment. The 4f moment is determined by dominant 3d-4f interactions and CEF interactions. High order CEF terms must be considered in the 4f anisotropy. The structure of the R 1+ϵFe 4B 4 compounds is composed of two distinct, but interacting, sublattices. The rare-earth sublattice is ferromagnetic at low temperatures.