Fe Isomer Shifts Research Articles

In situ synchrotron 57Fe Mössbauer spectroscopy of RFe2 (R=Y,Gd) hydrides synthesized under ultra-high-pressure hydrogen

• We report the high pressure Mössbauer studies of C15 Laves RFe 2 (R = Y, Gd) hydrides. • Two-step magnetic transition was found in C15 Laves RFe 2 under high pressure H 2 . • Pressure induced magnetic transition of RFe 2 hydride was not affected by R atom. In situ synchrotron radiation 57 Fe Mössbauer spectroscopy was used to study the C15 Laves phases RFe 2 (R = Y, Gd) under ultra-high-pressure hydrogen in a diamond anvil cell. In both cases, the 57 Fe isomer shift showed a rapid increase at a few GPa, and once again showed a small increase at above P ∼ 10 GPa. The quadrupole splitting also showed stepwise changes. As a marked effect, a two-step magnetic transition was clearly observed for RFe 2 (R = Y, Gd). The results indicate that two different hydride phases appear in the RFe 2 –H reaction system under the hydrogen pressure (up to 34 GPa).

Specific incorporation of chalcogenide bridge atoms in molybdenum/tungsten-iron-sulfur single cubane clusters.

An extensive series of heterometal-iron-sulfur single cubane-type clusters with core oxidation levels [MFe(3)S(3)Q](3+,2+) (M = Mo, W; Q = S, Se) has been prepared by means of a new method of cluster self-assembly. The procedure utilizes the assembly system [((t)Bu(3)tach)M(VI)S(3)]/FeCl(2)/Na(2)Q/NaSR in acetonitrile/THF and affords product clusters in 30-50% yield. The trisulfido precursor acts as a template, binding Fe(II) under reducing conditions and supplying the MS(3) unit of the product. The system leads to specific incorporation of a μ(3)-chalcogenide from an external source (Na(2)Q) and affords the products [((t)Bu(3)tach)MFe(3)S(3)QL(3)](0/1-) (L = Cl(-), RS(-)), among which are the first MFe(3)S(3)Se clusters prepared. Some 16 clusters have been prepared, 13 of which have been characterized by X-ray structure determinations including the incomplete cubane [((t)Bu(3)tach)MoFe(2)S(3)Cl(2)(μ(2)-SPh)], a possible trapped intermediate in the assembly process. Comparisons of structural and electronic features of clusters differing only in atom Q at one cubane vertex are provided. In comparative pairs of complexes differing only in Q, placement of one selenide atom in the core increases core volumes by about 2% over the Q = S case, sets the order Q = Se > S in Fe-Q bond lengths and Q = S > Se in Fe-Q-Fe bond angles, causes small positive shifts in redox potentials, and has an essentially nil effect on (57)Fe isomer shifts. Iron mean oxidation states and charge distributions are assigned to most clusters from isomer shifts. ((t)Bu(3)tach = 1,3,5-tert-butyl-1,3,5-triazacyclohexane).

Mono- and dinuclear iron complexes of bis(1-methylimidazol-2-yl)ketone (bik): structure, magnetic properties, and catalytic oxidation studies.

The newly synthesized dinuclear complex [Fe(III)(2)(μ-OH)(2)(bik)(4)](NO(3))(4) (1) (bik, bis(1-methylimidazol-2-yl)ketone) shows rather short Fe···Fe (3.0723(6) Å) and Fe-O distances (1.941(2)/1.949(2) Å) compared to other unsupported Fe(III)(2)(μ-OH)(2) complexes. The bridging hydroxide groups of 1 are strongly hydrogen-bonded to a nitrate anion. The (57)Fe isomer shift (δ = 0.45 mm s(-1)) and quadrupole splitting (ΔE(Q) = 0.26 mm s(-1)) obtained from Mössbauer spectroscopy are consistent with the presence of two identical high-spin iron(III) sites. Variable-temperature magnetic susceptibility studies revealed antiferromagnetic exchange (J = 35.9 cm(-1) and H = JS(1)·S(2)) of the metal ions. The optimized DFT geometry of the cation of 1 in the gas phase agrees well with the crystal structure, but both the Fe···Fe and Fe-OH distances are overestimated (3.281 and 2.034 Å, respectively). The agreement in these parameters improves dramatically (3.074 and 1.966 Å) when the hydrogen-bonded nitrate groups are included, reducing the value calculated for J by 35%. Spontaneous reduction of 1 was observed in methanol, yielding a blue [Fe(II)(bik)(3)](2+) species. Variable-temperature magnetic susceptibility measurements of [Fe(II)(bik)(3)](OTf)(2) (2) revealed spin-crossover behavior. Thermal hysteresis was observed with 2, due to a loss of cocrystallized solvent molecules, as monitored by thermogravimetric analysis. The hysteresis disappears once the solvent is fully depleted by thermal cycling. [Fe(II)(bik)(3)](OTf)(2) (2) catalyzes the oxidation of alkanes with t-BuOOH. High selectivity for tertiary C-H bond oxidation was observed with adamantane (3°/2° value of 29.6); low alcohol/ketone ratios in cyclohexane and ethylbenzene oxidation, a strong dependence of total turnover number on the presence of O(2), and a low retention of configuration in cis-1,2-dimethylcyclohexane oxidation were observed. Stereoselective oxidation of olefins with dihydrogen peroxide yielding epoxides was observed under both limiting oxidant and substrate conditions.

The [MoFe3S4]2+ Oxidation State: Synthesis, Substitution Reactions, and Structures of Phosphine-Ligated Cubane-Type Clusters with the S = 2 Ground State

The cluster [(Tp)MoFe(3)S(4)(PEt(3))(3)](1+) containing the cubane-type [MoFe(3)(μ(3)-S)(4)](2+) reduced core undergoes facile ligand substitution reactions at the iron sites leading to an extensive set of mono- and disubstituted species [(Tp)MoFe(3)S(4)(PEt(3))(3-n)L(n)](1-n) with L = halide, N(3)(-), PhS(-), PhSe(-), R(3)SiO(-), and R(3)SiS(-) and n = 1 and 2. Structures of 10 members of the set are reported. For two representative clusters, Curie behavior at 2-20 K indicates a spin-quintet ground state. Zero-field Mössbauer spectra consist of two doublets in a 2:1 intensity ratio. (57)Fe isomer shifts are consistent with the mean oxidation state Fe(3)(2.33+) arising from electron delocalization of the mixed-valence oxidation state description [Mo(3+)Fe(3+)Fe(2+)(2)]. Reaction of [(Tp)MoFe(3)S(4)(PEt(3))(2)Cl] with (Me(3)Si)(2)S affords [(Tp)MoFe(3)S(4)(PEt(3))(2)(SSiMe(3))], a likely first intermediate in the formation of the tricluster compound {[(Tp)MoFe(3)S(4)(PEt)(2)](3)S}(BPh(4)) from the reaction of [(Tp)MoFe(3)S(4)(PEt(3))(3)](BPh(4)) and NaSSiMe(3) in tetrahydrofuran (THF). The tricluster consists of three cluster units bound to a central μ(3)-S atom in a species of overall C(3) symmetry. Relatively few clusters in the [MoFe(3)S(4)](2+) oxidation state have been prepared compared to the abundance of clusters in the oxidized [MoFe(3)S(4)](3+) state. This work is the first comprehensive study of the [MoFe(3)S(4)](2+) state, one conspicuous feature of which is its ability to bind hard and soft σ-donors and strong to weak π-acid ligands. (Tp = tris(pyrazolyl)hydroborate(1-)).

Stabilization of 3:1 Site-Differentiated Cubane-Type Clusters in the [Fe4S4]1+ Core Oxidation State by Tertiary Phosphine Ligation: Synthesis, Core Structural Diversity, and S = 1/2 Ground States

An extensive series of 3:1 site-differentiated cubane-type clusters [Fe(4)S(4)(PPr(i)(3))(3)L] (L = Cl(-), Br(-), I(-), RO(-), RS(-), RSe(-)) has been prepared in 40-80% yield by two methods: ligand substitution of [Fe(4)S(4)(PPr(i)(3))(4)](1+) in tetrahydrofuran (THF)/acetonitrile by reaction with monoanions, and reductive cleavage of ligand substrates (RSSR, RSeSeR, I(2)) by the all-ferrous clusters [Fe(8)S(8)(PPr(i)(3))(6)]/[Fe(16)S(16)(PPr(i)(3))(8)] in THF. These neutral clusters are stable and do not undergo ligand redistribution reactions involving charged species in benzene and THF solutions. X-ray structural studies confirm the cubane stereochemistry but with substantial and variable distortions of the [Fe(4)S(4)](1+) core from idealized cubic core geometry. Based on Fe-S bond lengths, seven clusters were found to have compressed tetragonal distortions (4 short and 8 long bonds), and the remaining seven display other types of distortions with different combinations of long, short, and intermediate bond lengths. These results further emphasize the facile deformabililty of this core oxidation state previously observed in [Fe(4)S(4)(SR)(4)](3-) clusters. The Fe(2.25+) mean oxidation state was demonstrated from (57)Fe isomer shifts, and the appearance of two quadrupole doublets arises from the spin-coupled |9/2,4,1/2> state. The S = 1/2 ground state was further supported by electron paramagnetic resonance spectra and magnetic susceptibility data.

Calibration of 57Fe isomer shift from ab initio calculations: can theory and experiment reach an agreement?

Using linear response approach to the Mössbauer isomer shift, the calibration constant alpha((57)Fe) was obtained from high level ab initio calculations carried out for a representative set of iron compounds. The importance of the effects of relativity and electron correlation for an accurate description of the (57)Fe isomer shift is demonstrated on the basis of the Hartree-Fock, coupled cluster with singles and doubles and of the double hybrid density functional calculations. A reliable value of the calibration constant (alpha((57)Fe) = -0.306 +/- 0.009 mm s(-1)) was obtained with the use of the B2-PLYP double hybrid density functional. This value is in good agreement with the experimentally estimated constant of -0.31 +/- 0.04 a(0)(3) mm s(-1) and can be recommended for theoretical modeling of (57)Fe isomer shifts.

Density Functional Theory Study of the Structural, Electronic, and Magnetic Properties of a μ-oxo Bridged Dinuclear FeIV Complex Based on a Tetra-Amido Macrocyclic Ligand

Recently, the synthesis, crystallographic structure, and Mossbauer characterization of the first example of an [(Fe(IV)TAML)2O](2-) (TAML = tetra-amido microcyclic ligand) complex were reported. Here, we elucidate the prominent structural, electronic, and magnetic properties of this complex on the basis of density functional theory (DFT) calculations. While the torsion between the molecular halves is caused by hydrogen bonding between the TAML moieties, the bending of the Fe-O-Fe unit is an intrinsic property of the bridge. The values for the (57)Fe isomer shift and quadrupole splitting obtained with DFT are in good agreement with experimental results and indicate that the irons have intermediate spin states (S1 = S2 = 1). The iron spins are coupled by strong antiferromagnetic exchange to yield a ground state with system spin S = 0. The Fe-O distances in the excited S > 0 states are significantly longer than in the ground state. Since the wave function of the ground state, in which the iron spins are antiferromagnetically coupled to give system spin S = 0, is a linear combination of Slater determinants that cannot be treated with existing DFT codes, the Fe-O distance for the S = 0 state has been estimated by extrapolation from the optimized geometries for the ferromagnetic state (S = 2) and the broken symmetry state to be 1.748 A, in good agreement with the crystallographic distance 1.728 A. To accommodate the spin-dependent reorganization energies, the conventional bilinear spin Hamiltonian has been extended with a biquadratic coupling term: H(ex) = c' + j0S1 x S2 + j1(S1 x S2)(2). A computational scheme is presented for estimating the exchange parameters, yielding the values j0 = 199 cm(-1) and j1 = -61 cm(-1) for [(Fe(IV)B*)2O](2-). Two mechanisms for biquadratic exchange are discussed.

Stabilization of Fully Reduced Iron−Sulfur Clusters by Carbene Ligation: The [FenSn]0 Oxidation Levels (n = 4, 8)

The all-ferrous [Fe4S4](0) state has been demonstrated in the fully reduced Fe protein of the Azotobacter vinelandii nitrogenase complex. We seek synthetic analogues of this state more tractable than the recently prepared but highly unstable cluster [Fe4S4(CN)4](4-) (Scott, Berlinguette, Holm, and Zhou, Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 9741). The N-heterocyclic carbene 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (Pr(i)2NHCMe2) has been found to stabilize the fully reduced clusters [Fe8S8(Pr(i)2NHCMe2)6] (4) and [Fe4S4(Pr(i)2NHCMe2)4] (5), which are prepared by cluster assembly or phosphine substitution of FenSn (n = 8, 16) clusters. Cluster 4 is also obtained by reaction of the carbene with all-ferrous [Fe7S6(PEt3)5Cl2] (3) and cluster 5 by carbene cleavage of 4. Detailed structures of 3 (monocapped prismatic), 4, and 5 are described; the latter two are the first iron-sulfur clusters with Fe-C sigma bonds. Cluster 4 possesses the [Fe8(mu3-S) 6(mu4-S)2] edge-bridged double cubane structure and 5 the cubane-type [Fe4(mu3-S)4] stereochemistry. The all-ferrous formulations of the clusters are confirmed by X-ray structure parameters and (57)Fe isomer shifts. Both clusters are stable under conventional aprotic anaerobic conditions, enabling further study of reactivity. The collective properties of 5 indicate that it is a meaningful synthetic analogue of the core of the fully reduced protein-bound cluster.

Mössbauer spectroscopy on Fe impurities in diamond

Radioactive 57 Mn + ions have been implanted into a natural diamond held at 300–1100 K for Mossbauer spectroscopy studies on the 14 keV γ -radiation emitted in the decay of the 57 Fe daughter atoms. The Mn parent isotopes are found to be implanted into defect sites of non-cubic symmetry, which anneal partially with increasing temperature during the 57 Mn lifetime. This is concluded to lead to their incorporation on substitutional lattice sites. This site is inherited by the majority of the 57 Fe daughter atoms, however, a sizeable fraction is expelled into interstitial sites owing to the recoil in the nuclear -decay. Thus the Fe isomer shifts in both lattice sites have been determined.

Density functional study of the electric hyperfine interactions and the redox-structural correlations in the cofactor of nitrogenase. Analysis of general trends in (57)Fe isomer shifts.

The influence of the interstitial atom, X, discovered in a recent crystallographic study of the MoFe protein of nitrogenase, on the electric hyperfine interactions of (57)Fe has been investigated with density functional theory. A semiempirical theory for the isomer shift, delta, is formulated and applied to the cofactor. The values of delta for the relevant redox states of the cofactor are predicted to be higher in the presence of X than in its absence. The analysis strongly suggests a [Mo(4+)4Fe(2+)3Fe(3+)] oxidation state for the S = 3/2 state M(N). Among C(4-), N(3-), and O(2-), oxide is found to be the least likely candidate for X. The analysis suggests that X should be present in the cofactor states M(OX) and M(R) as well as in the alternative nitrogenases. The calculations of the electric field gradients (EFGs) indicate that the small values for DeltaE(Q) in M(N) result from an extensive cancellation between valence and ligand contributions. X emerges from the analysis of the hyperfine interactions as an ionically bonded species. Its major effect is on the asymmetry parameters for the EFGs at the six equatorial sites, Fe(Eq). A spin-coupling scheme is proposed for the state [Mo(4+)4Fe(2+)3Fe(3+)] that is consistent with the measured (57)Fe A-tensors and DeltaE(Q) values for M(N) and identifies the unique site exhibiting the small A value with the terminal Fe site, Fe(T). The optimized structure of a cofactor model has been calculated for several oxidation states. The study reveals a contraction in the average Fe-Fe distance upon increasing the number of electrons stored in the cluster, in accord with extended X-ray absorption fine structure studies. The reliability of the adopted methodology for predicting redox-structural correlations is tested for cuboidal [4Fe-4S] clusters. The calculations reveal a systematic increase in the S...S sulfide distances, in quantitative agreement with the available data. These trends are rationalized by a simple electrostatic model.

Vanadium-iron-sulfur clusters containing the cubane-type [VFe3S4] core unit: synthesis of a cluster with the topology of the PN cluster of nitrogenase.

A synthetic method affording a topological analogue of the electron-transfer P-cluster of nitrogenase (Fe(8)S(7)(mu(2)-S(Cys))(2)) in the P(N) state has been devised, based in part on our previous development of cubane-type VFe(3)S(4) clusters (Hauser, C.; Bill, E.; Holm, R. H. Inorg. Chem. 2002, 41, 1615-1624). The cluster [(Tp)VFe(3)S(4)Cl(3)](2-) (1) is converted to [(Tp)VFe(3)S(4)(PR(3))(3)](1+) (R = Et (2), Bu (3)) by reaction with R(3)P. The phosphine ligands are readily substituted, leading to [(Tp)VFe(3)S(4)(SR)(3)](2-) (R = Ph (4), H (5)). Reduction of 2 or 3 with cobaltocene produces the edge-bridged double cubanes [(Tp)(2)V(2)Fe(6)S(8)(PR(3))(4)] (R = Et (6), Bu (7)), which are readily converted to [(Tp)(2)V(2)Fe(6)S(8)(SPh)(4)](4-) (8). The structures of clusters 3-5 and 8 were proven crystallographically. Cluster 8 has the double-cubane structure previously shown for 6, in which two cubane units are bridged by two Fe-(mu(4)-S) bonds. (57)Fe isomer shifts are consistent with the formulation [VFe(2.33+)(3)S(4)](2+) for the single cubanes and the all-ferrous description 2[VFe(2+)(3)S(4)](1+) for the double cubanes. Reaction of 6 with 4 equiv of (Et(4)N)(HS) in acetonitrile results in the insertion of sulfide with concomitant structural rearrangement and the formation of [(Tp)(2)V(2)Fe(6)S(9)(SH)(2)](4-) (10), obtained in ca. 50% yield as the Et(4)N(+) salt. The cluster has C(2) symmetry, with two all-ferrous VFe(3)S(4) fragments bridged by a common mu(6)-S atom and two mu(2)-S atoms that simulate the bridging atoms in the two Fe-(mu(2)-S(Cys))-Fe bridges of the P(N) cluster. The bridge pattern V(2)Fe(6)(mu(2)-S)(2)(mu(3)-S)(6)(mu(6)-S) and cluster shape match those of the native cluster. A best-fit superposition of the cores of 10 and the P(N) cluster affords a weighted rms deviation in atom positions of 0.33 A. Cluster 10 and [(Tp)(2)Mo(2)Fe(6)S(9)(SH)(2)](3-), prepared by a related route (Zhang, Y.; Holm, R. H. J. Am. Chem. Soc. 2003, 125, 3910-3920), demonstrate that the topology of the P(N) cluster can be achieved in molecular form in the absence of protein structure (Tp = tris(pyrazolyl)hydroborate).

Single- and double-cubane clusters in the multiple oxidation states [VFe(3)S(4)](3+,2+,1+).

A new series of cubane-type [VFe(3)S(4)](z)() clusters (z = 1+, 2+, 3+) has been prepared as possible precursor species for clusters related to those present in vanadium-containing nitrogenase. Treatment of [(HBpz(3))VFe(3)S(4)Cl(3)](2)(-) (2, z = 2+), protected from further reaction at the vanadium site by the tris(pyrazolyl)hydroborate ligand, with ferrocenium ion affords the oxidized cluster [(HBpz(3))VFe(3)S(4)Cl(3)](1)(-) (3, z = 3+). Reaction of 2 with Et(3)P results in chloride substitution to give [(HBpz(3))VFe(3)S(4)(PEt(3))(3)](1+) (4, z = 2+). Reaction of 4 with cobaltocene reduced the cluster with formation of the edge-bridged double-cubane [(HBpz(3))(2)V(2)Fe(6)S(8)(PEt(3))(4)] (5, z = 1+, 1+), which with excess chloride underwent ligand substitution to afford [(HBpz(3))(2)V(2)Fe(6)S(8)Cl(4)](4)(-) (6, z = 1+, 1+). X-ray structures of (Me(4)N)[3], [4](PF(6)), 5, and (Et(4)N)(4)[6] x 2MeCN are described. Cluster 5 is isostructural with previously reported [(Cl(4)cat)(2)(Et(3)P)(2)Mo(2)Fe(6)S(8)(PEt(3))(4)] and contains two VFe(3)S(4) cubanes connected across edges by a Fe(2)S(2) rhomb in which the bridging Fe-S distances are shorter than intracubane Fe-S distances. Mössbauer (2-5), magnetic (2-5), and EPR (2, 4) data are reported and demonstrate an S = 3/2 ground state for 2 and 4 and a diamagnetic ground state for 3. Analysis of (57)Fe isomer shifts based on an empirical correlation between shift and oxidation state and appropriate reference shifts results in two conclusions. (i) The oxidation 2 --> 3 + e(-) results in a change in electron density localized largely or completely on the Fe(3) subcluster and associated sulfur atoms. (ii) The most appropriate charge distributions are [V(3+)Fe(3+)Fe(2+)(2)S(4)](2+) (Fe(2.33+)) for 1, 2, and 4 and [V(3+)Fe(3+)(2)Fe(2+)S(4)](3+) (Fe(2.67+)) for 3 and [V(2)Fe(6)S(8)(SEt)(9)](3+). Conclusion i applies to every MFe(3)S(4) cubane-type cluster thus far examined in different redox states at parity of cluster ligation. The formalistic charge distributions are regarded as the best current approximations to electron distributions in these delocalized species. The isomer shifts require that iron atoms are mixed-valence in each cluster.

Heterometal cubane-type MFe(3)S(4) clusters (M = Mo, V) trigonally symmetrized with hydrotris(pyrazolyl)borate(1-) and tris(pyrazolyl)methanesulfonate(1-) capping ligands.

A series of heterometal cubane-type clusters containing [VFe(3)S(4)](2+) and [MoFe(3)S(4)](3+,2+) cores has been prepared. Ligand substitution of [(DMF)(3)VFe(3)S(4)Cl(3)](-) affords [(Tpms)VFe(3)S(4)L(3)](2)(-) (L = Cl(-) (8), EtS(-) (9), p-MeC(6)H(4)S(-), p-MeC(6)H(4)O(-)). A new procedure for the preparation of molybdenum single cubanes is introduced by the reaction of recently reported [(Tp)MoS(S(4))](-) with FeCl(2)/NaSEt to afford [(Tp)MoFe(3)S(4)Cl(3)](-) (1, 75% yield). This procedure is more efficient that the existing multistep synthesis of single cubanes, which generally affords clusters of mirror symmetry. Also prepared were [(Tp)MoFe(3)S(4)L(3)](-) (L = EtS(-) (2), p-MeC(6)H(4)S(-)). Reduction of 1 with borohydride gives [(Tp)MoFe(3)S(4)Cl(3)](2-) (5, 67%). Owing to the nature of the heterometal ligand, all clusters have idealized trigonal symmetry, reflected in their (1)H NMR spectra. Trigonal structures are demonstrated by crystallography of (Bu(4)N)[1,2], (Bu(4)N)(2)[5] x MeCN, and (Me(4)N)(2)[8,9]. The availability of 1 and 5 allows the first comparison of structures and (57)Fe isomer shifts of [MoFe(3)S(4)](3+,2+) in a constant ligand environment. Small increases in most bond distances indicate that an antibonding electron is added in the reduction of 1. Collective synthetic and electrochemical results from this and other studies demonstrate the existence of the series of oxidation states [VFe(3)S(4)](3+,2+,1+) and [MoFe(3)S(4)](4+,3+,2+) whose relative stabilities within a given series are strongly ligand dependent. Isomer shifts indicate that the reduction of 1 largely affects the Fe(3) subcluster and are consistent with the formal descriptions [MoFe(3+)(2)Fe(2+)S(4)](3+) (1) and [MoFe(3+)Fe(2+)(2)S(4)](2+) (5). Reaction of 1 with excess Li(2)S in acetonitrile affords the double cubane [[(Tp)MoFe(3)S(4)Cl(2)](2)(mu(2)(-)S)](2)(-), whose sulfide-bridged structure is supported by two sequential reductions separated by 290 mV, in analogy with previously reported double cubanes of higher charge. Trigonally symmetric single cubanes eliminate isomers in the formation of double cubanes and other cluster structures, and may be of considerable value in the preparation of new types of M-Fe-S clusters. (Tpms = tris(pyrazolyl)methanesulfonate(1-); Tp = hydrotris(pyrazolyl)borate(1-).)

Subsite-differentiated analogs of native iron sulfide [4Fe-4S]2+ clusters: preparation of clusters with five- and six-coordinate subsites and modulation of redox potentials and charge distributions

The subsite-differentiated cluster [Fe 4 S 4 (LS 3 )Cl] 2− (LS 3 =1,3,5-tris((4,6-dimethyl-3-mercaptophenyl) thio)-2,4,6-tris(p-tolylthio) benzene(3-)) in Me 2 SO solution reacts with a variety of bidentate and tridentate ligands to afford the substituted clusters [Fe 4 S 4 (LS 3 )L'] z− . Some ten clusters of this type were prepared in order to examine the effects of cluster charge and coordination number at the unique subsite on relative stabilities of oxidation states and charge distributions as sensed by 57 Fe isomer shifts. This is the first comprehensive study of such effects with Fe 4 S 4 clusters

Search for 2γ states in FCC iron alloys by investigation of the57Fe isomer shift in the paramagnetic state

The authors report on 57Fe Mossbauer isomer shift measurements of Fe0.64Ni0.36, Fe0.68Ni0.32 and Fe0.60Ni0.20 FCC Fe alloys in the temperature range from 13 to 1010K. The Weiss 2 gamma -states model and a simple 'volume model' have been fitted to the experimental data in order to explain the observed temperature dependence of the isomer shift. To separate the isomer shift from the measured centre-of-gravity shifts of the spectra, the authors used the Debye approximation of the second-order Doppler shift. The Debye temperatures were assumed to be dependent on temperature.