Low-spin Cr Research Articles

Influence of strong π-acceptor ligands on Cr-K-edge X-ray absorption spectral signatures and consequences for the interpretation of surface sites in the Phillips catalyst.

X-ray absorption spectroscopy (XAS) has been central to the study of the Phillips polymerization catalyst (CrO3/SiO2). As Cr K-edge XAS signatures are sensitive to the oxidation state, geometry and types of ligands on surface (active) sites, the superposition of these effects makes their interpretation challenging. Notably, CO has been particularly used as a reductant to generate low valent Cr sites from CrO3/SiO2 and as a structural IR probe for analysing reduced Cr surface sites. Hence, it is essential to establish a solid understanding of the spectroscopic impact of CO on low-valent Cr sites. We thus built a series of fully characterized low-valent Cr molecular compounds bearing isoelectronic isocyanide ligands in place of CO, with the goal of understanding the effect of the coordination of π-acceptor ligands on the XANES signature of Cr sites. Cr K-edge spectra supplemented with DFT calculations elucidate the effect of the coordination of π-acceptor ligands on XAS signatures, giving a sharp resonance at the white line while modifying the fine structure due to short Cr-C distances and stability of low-spin Cr(ii/iii) species. The isocyanide references allow the deconvolution of the XAS spectra of the reduced CrO3/SiO2 catalyst by evaluating the types of surface species and relative amounts of bound CO at different CO pressures and temperatures.

Active Sites in Cr(III)-Based Ethylene Polymerization Catalysts from Machine-Learning-Supported XAS and EPR Spectroscopy.

The ethylene polymerization Phillips catalyst has been employed for decades and is central to the polymer industry. While Cr(III) alkyl species are proposed to be the propagating sites, there is so far no direct experimental evidence for such proposal. In this work, by coupling Surface organometallic chemistry, EPR spectroscopy, and machine learning-supported XAS studies, we have studied the electronic structure of well-defined silica-supported Cr(III) alkyls and identified the presence of several surface species in high and low-spin states, associated with different coordination environments. Notably, low-spin Cr(III) sites are shown to participate in ethylene polymerization, indicating that similar Cr(III) alkyl species could be involved in the related Phillips catalyst.

Insights to the fracture toughness, damage tolerance, electronic structure, and magnetic properties of carbides M2C (M = Fe, Cr)

The fracture toughness, damage tolerance, electronic structure, and magnetic properties of M2C (M = Fe, Cr) carbides were analyzed using first-principles calculations. Calculations of formation energy and modulus of elasticity indicate that a Cr/Fe ratio of 1/3 is a critical threshold which triggers a significant increase in the corresponding stability and related mechanical properties. Cr atomic content enhances the crack resistance, while Cr has a significantly detrimental effect on damage resistance. The electronic properties demonstrated that the Cr atom content enhances the metallic, ionic and covalent bonding. Furthermore, the reduction in the coordination number of Fe atoms is the main reason for the reduction in the local magnetic moment of the low-spin Cr atoms, which is strongly supported by the electronic structure. These studies provide detailed insights into Cr-containing carbides, providing valuable theoretical and technological information for the knowledge-based design and prediction of the mechanical properties of chromium-containing iron-based materials.

Chromium-Catalyzed syn-Selective Ring-Opening Aryl-/Alkylation of 7-Oxabenzonorbornadiene Derivatives with Grignard Reagents

Chromium(III) trichloride in combination with aryl/alkyl Grignard reagents under ligand-free conditions exhibited extremely high catalytic activity (turnover number of up to 2.4 × 104) toward the syn-selective ring-opening aryl/alkylation of 7-oxabenzonorbornadiene derivatives, producing syn-2-aryl/alkyl-1,2-dihydronaphthalen-1-ols in high yields. The superior catalytic activity of the chromium catalyst under the ring-opening arylation conditions enabled us to use unstable aryl Grignard reagents with esters and N-protecting groups synthesized from the corresponding aryl iodides and turbo-Grignard reagent. Control experiments to elucidate the reactivity of arylchromium species such as neutral triarylchromium(III) and anionic tetraarylchromate(III) suggested that in situ formed diarylchromate(I) from tetraarylchromate(III) served as the active species in this catalytic reaction. The magnetic susceptibility of the in situ generated chromium species from CrCl3(thf)3 and excess amounts of p-tolylmagnesium bromide was reasonable for the low-spin Cr(I) species (μ = 1.93 μB).

New Complexes of Chromium(III) Containing Organic π-Radical Ligands: An Experimental and Density Functional Theory Study

The electronic structures of a series of chromium complexes 1-7 have been experimentally investigated using a combination of X-ray crystallography, magneto- and electrochemistry, and Cr K-edge X-ray absorption and UV-vis spectroscopies. Reaction of the dimer [Cr(II)2(μ-CH3CO2)4](0) with 2,2'-bipyridine (bpy(0)) produced the complex [Cr(III)(bpy(0))(bpy(•))(CH3CO2)2](0) (S = 1) (1), but in the presence of isopropylamine ((i)PrNH2) [Cr(III)(bpy(•))((i)PrNH2)2(CH3CO2)2](0) (S = 1) (2) was obtained. Both 1 and 2 contain a Cr(III) ion and a single (bpy(•))(1-) ligand, so are not low-spin Cr(II) species. One-electron oxidation of 1 and 2 yielded [Cr(III)(bpy(0))2(CH3CO2)2]PF6 (S = 3/2) (3) in both cases. In addition, the new neutral species [Cr(III)(DAD(•))3](0) (S = 0) (4) and [Cr(III)(CF3AP(•))3](0) (S = 0) (5) have been synthesized. Both complexes contain three π-radical anion ligands, which derive from one electron reduction of 1,4-bis(cyclohexyl)-1,4-diaza-1,3-butadiene and one electron oxidation of 2-(2-trifluoromethyl)-anilino-4,6-di-tert-butylphenolate, respectively. Intramolecular antiferromagnetic coupling to d(3) Cr(III) gives the observed singlet ground states. Reaction of [Cr(II)(CH3CN)6](PF6)2 with 2,6-bis[1-(4-methoxyphenylimino)ethyl]pyridine (PDI(0)) under anaerobic conditions affords dark brown microcrystals of [Cr(III)(PDI(0))(PDI(•))](PF6)2 (S = 1) (6). This complex is shown to be a member of the electron transfer series [Cr(III)(PDI)2](3+/2+/1+/0), in which all one-electron transfer processes are ligand-based. By X-ray crystallography, it was shown that 6 possesses a localized electronic structure, such that one ligand is neutral (PDI(0)) and the other is a π-radical monoanion (PDI(•))(1-). Again, it should be highlighted that 6 is not a Cr(II) species. Lastly, the structure of [Cr(III)((Me)bpy(•))3](0) (S = 0) (7, (Me)bpy = 4,4'-dimethyl-2,2'-bipyridine) has been established by high resolution X-ray crystallography and clearly shows that three ((Me)bpy(•))(1-) radical anions are present. To further validate our electronic structure assignments, complexes 1-6 were investigated computationally using density functional theory (DFT) and found in all cases to contain a Cr(III) ion. This oxidation state assignment was experimentally confirmed for complexes 2, 4, 5, and 6 by Cr K-edge X-ray absorption spectroscopy.

Scrutinizing Low-Spin Cr(II) Complexes

The oxidation state of the chromium center in the following compounds has been probed using a combination of chromium K-edge X-ray absorption spectroscopy and density functional theory: [Cr(phen)(3)][PF(6)](2) (1), [Cr(phen)(3)][PF(6)](3) (2), [CrCl(2)((t)bpy)(2)] (3), [CrCl(2)(bpy)(2)]Cl(0.38)[PF(6)](0.62) (4), [Cr(TPP)(py)(2)] (5), [Cr((t)BuNC)(6)][PF(6)](2) (6), [CrCl(2)(dmpe)(2)] (7), and [Cr(Cp)(2)] (8), where phen is 1,10-phenanthroline, (t)bpy is 4,4'-di-tert-butyl-2,2'-bipyridine, and TPP(2-) is doubly deprotonated 5,10,15,20-tetraphenylporphyrin. The X-ray crystal structures of complexes 1, [Cr(phen)(3)][OTf](2) (1'), and 3 are reported. The X-ray absorption and computational data reveal that complexes 1-5 all contain a central Cr(III) ion (S(Cr) = (3)/(2)), whereas complexes 6-8 contain a central low-spin (S = 1) Cr(II) ion. Therefore, the electronic structures of 1-8 are best described as [Cr(III)(phen(•))(phen(0))(2)][PF(6)](2), [Cr(III)(phen(0))(3)][PF(6)](3), [Cr(III)Cl(2)((t)bpy(•))((t)bpy(0))], [Cr(III)Cl(2)(bpy(0))(2)]Cl(0.38)[PF(6)](0.62), [Cr(III)(TPP(3•-))(py)(2)], [Cr(II)((t)BuNC)(6)][PF(6)](2), [Cr(II)Cl(2)(dmpe)(2)], and [Cr(II)(Cp)(2)], respectively, where (L(0)) and (L(•))(-) (L = phen, (t)bpy, or bpy) are the diamagnetic neutral and one-electron-reduced radical monoanionic forms of L, and TPP(3•-) is the one-electron-reduced doublet form of diamagnetic TPP(2-). Following our previous results that have shown [Cr((t)bpy)(3)](2+) and [Cr(tpy)(2)](2+) (tpy = 2,2':6',2"-terpyridine) to contain a central Cr(III) ion, the current results further refine the scope of compounds that may be described as low-spin Cr(II) and reveal that this is a very rare oxidation state accessible only with ligands in the strong-field extreme of the spectrochemical series.

Experimental Fingerprints for Redox-Active Terpyridine in [Cr(tpy)2](PF6)n (n = 3–0), and the Remarkable Electronic Structure of [Cr(tpy)2]1–

The molecular and electronic structures of the four members, [Cr(tpy)(2)](PF(6))(n) (n = 3-0; complexes 1-4; tpy = 2,2':6',2″-terpyridine), of the electron transfer series [Cr(tpy)(2)](n+) have been determined experimentally by single-crystal X-ray crystallography, by their electro- and magnetochemistry, and by the following spectroscopies: electronic absorption, X-ray absorption (XAS), and electron paramagnetic resonance (EPR). The monoanion of this series, [Cr(tpy)(2)](1-), has been prepared in situ by reduction with KC(8) and its EPR spectrum recorded. The structures of 2, 3, 4, 5, and 6, where the latter two compounds are the Mo and W analogues of neutral 4, have been determined at 100(2) K. The optimized geometries of 1-6 have been obtained from density functional theoretical (DFT) calculations using the B3LYP functional. The XAS and low-energy region of the electronic spectra have also been calculated using time-dependent (TD)-DFT. A consistent picture of the electronic structures of these octahedral complexes has been established. All one-electron transfer processes on going from 1 to 4 are ligand-based: 1 is [Cr(III)(tpy(0))(2)](PF(6))(3) (S = (3)/(2)), 2 is [Cr(III)(tpy(•))(tpy(0))](PF(6))(2) (S = 1), 3 is [Cr(III)(tpy(•))(2)](PF(6)) (S = (1)/(2)), and 4 is [Cr(III)(tpy(••))(tpy(•))](0) (S = 0), where (tpy(0)) is the neutral parent ligand, (tpy(•))(1-) represents its one-electron-reduced π radical monoanion, (tpy(2-))(2-) or (tpy(••))(2-) is the corresponding singlet or triplet dianion, and (tpy(3-))(3-) (S = (1)/(2)) is the trianion. The electronic structure of 2 cannot be described as [Cr(II)(tpy(0))(2)](PF(6))(2) (a low-spin Cr(II) (d(4); S = 1) complex). The geometrical features (C-C and C-N bond lengths) of these coordinated ligands have been elucidated computationally in the following hypothetical species: [Zn(II)Cl(2)(tpy(0))](0) (S = 0) (A), [Zn(II)(tpy(•))Cl(NH(3))](0) (S = (1)/(2)) (B), [Zn(II)(tpy(2-))(NH(3))(2)](0) (S = 0 or 1) (C), and [Al(III)(tpy(3-))(NH(3))(3)](0) (S = (1)/(2) and (3)/(2)) (D). The remarkable electronic structure of the monoanion has been calculated and experimentally verified by EPR spectroscopy to be [Cr(III)(tpy(2-))(tpy(••))](1-) (S = (1)/(2)), a complex in which the two dianionic tpy ligands differ only in the spin state. It has been clearly established that coordinated tpy ligands are redox-active and can exist in at least four oxidation levels.

Electronic and Molecular Structures of the Members of the Electron Transfer Series [Cr(tbpy)3]n (n = 3+, 2+, 1+, 0): An X-ray Absorption Spectroscopic and Density Functional Theoretical Study

The electron transfer series of complexes [Cr((t)bpy)(3)](n)(PF(6))(n) (n = 3+, 2+, 1+, 0 (1-4)) has been synthesized and the molecular structures of 1, 2, and 3 have been determined by single-crystal X-ray crystallography; the structure of 4 has been investigated using extended X-ray absorption fine structure (EXAFS) analysis. Magnetic susceptibility measurements (4-300 K) established an S = 3/2 ground state for 1, an S = 1 ground state for 2, an S = 1/2 ground state for 3, and an S = 0 ground state for 4. The electrochemistry of this series in CH(3)CN solution exhibits three reversible one-electron transfer waves. UV-vis/NIR spectra and Cr K-edge X-ray absorption spectra (XAS) are reported. The same experimental techniques have been applied for [Cr(III)(tacn)(2)]Br(3)·5H(2)O (5) and [Cr(II)(tacn)(2)]Cl(2) (6), which possess an S = 3/2 and an S = 2 ground state, respectively (tacn = 1,4,7-triazacyclononane, a tridentate, pure σ-donor ligand). The Cr K-edge XAS spectra of the corresponding complexes K(4)[Cr(II)(CN)(6)]·10H(2)O (S = 1) (7) and K(3)[Cr(III)(CN)(6)] (S = 3/2) (8) have also been recorded. All complexes have been studied computationally with density functional theory (DFT) using the B3LYP functional. The molecular and electronic structures of the anionic members of the series [Cr(bpy)(3)](1-,2-,3-) have also been calculated. It is unequivocally shown that all members of the electron transfer series 1-4 and [Cr(bpy)(3)](n) (n = 3+, 2+, 1+, 0, 1-, 2, 3-) possess a central Cr(III) ion ((t(2g))(3), S = 3/2). The three N,N'-coordinated neutral (bpy(0)) ligands in the trication 1 and [Cr(III)(bpy)(3)](3+) are one-electron reduced in a stepwise fashion to localized one, two, and three π-radical anions (bpy(•))(1-) in the dicationic, monocationic, and neutral species, respectively. Complexes 2 and [Cr(bpy)(3)](2+) cannot be described as low-spin Cr(II) species; they are in fact best described as [Cr(III)((t)bpy(•))((t)bpy(0))(2)](2+) and [Cr(III)(bpy(•))(bpy(0))(2)](2+) species. Further one-electron reductions yield one, two, and three diamagnetic (bpy(2-))(2-) dianions in the mono-, di-, and trianion. Thus, [Cr(III)(bpy(2-))(3)](3-) is a normal Werner-type Cr(III) (!) species. In all complexes containing (bpy(•))(1-) ligands, the ligand spins are strongly antiferromagnetically coupled to the spins of the central Cr(III) ion (d(3), S(Cr) = 3/2) affording the observed ground states given above. Thus, all redox chemistry of [Cr(bpy)(3)](n) complexes is ligand-based and documents that the ligand 2,2'-bipyridine is a redox noninnocent ligand; it exists in three oxidation levels in these complexes: as N,N'-coordinated neutral (bpy(0)), monoanionic π-radical (bpy(•))(1-), and diamagnetic dianionic (bpy(2-))(2-).

Monitoring Structure and Valence State of Chromium Sites during Catalyst Formation and Ethylene Oligomerization by in Situ EPR Spectroscopy

For the first time, a Cr(acac)3/PNP mixture (PNP = Ph2PN(i-Pr)PPh2) and a [(PNP)CrCl2(μ-Cl)]2 complex were monitored by in situ EPR in cyclohexane and toluene, respectively, during oligomerization of ethylene. For Cr(acac)3/PNP, Cr3+ reduction is faster than low-spin Cr+ formation, suggesting that the major active species is EPR-silent, possibly an antiferromagnetic Cr+ dimer and/or Cr2+. Fast decomposition of [(PNP)CrCl2(μ-Cl)]2 to [Cr(η6-CH3C6H5)2]+ is a probable reason for low activity.

Determination of chromium valence over the range Cr(0)–Cr(VI) by electron energy loss spectroscopy

Chromium is a redox active 3d transition metal with a wide range of valences (−2 to +6) that control the geochemistry and toxicity of the element. Therefore, techniques that measure Cr valence are important bio/geochemical tools. Until now, all established methods to determine Cr valence were bulk techniques with many specific to a single, or at best, only a few oxidation state(s). We report an electron energy loss spectroscopy (EELS) technique along with an extensive suite of affined reference spectra that together, unlike other methods, can determine Cr valence (or at least constrain the possible valences) at high-spatial resolution (tens-of-nanometer scale) across a wide valence range, Cr(0)–Cr(VI). Fine structure of Cr-L 2,3 edges was parametrized by measurement of the chemical shift of the L 3 edge and the ratio of integrated intensity under the L 3 and L 2 edges. These two parameterizations were correlated to Cr valence and also the d n orbital configuration which has a large influence on L-edge fine structure. We demonstrate that it is not possible to unambiguously determine Cr valence from only one fine-structure parameterization which is the method employed to determine metal valence by nearly all previous EELS studies. Rather, multiple fine-structure parameterizations must be used together if the full range of possible Cr valences is considered. However even with two parameterizations, there are limitations. For example, distinguishing Cr(IV) from Cr(III) is problematic and it may be difficult to distinguish low-spin Cr(II) from Cr(III). Nevertheless, when Cr is known to be divalent, low- and high-spin d n orbital configurations can be readily distinguished.

Rational Synthesis and Magnetic Properties of a Family of Low-Dimensional Heterometallic Cr−Mn Complexes Based on the Versatile Building Block [Cr(2,2‘-bipyridine)(CN)4]-

Six heterometallic compounds based on the building block [Cr(bpy)(CN)4]- (bpy = 2,2'-bipyridine) with secondary and/or tertiary coligands as modulators, {Mn(H2O)2[Cr(bpy)(CN)4]2}n (1), {Mn(bpy)(H2O)[Cr(bpy)(CN)4]2 x H2O}n (2), [Mn(bpy)2][Cr(bpy)(CN)4]2 x 5H2O (3), {[Mn(dca)(bpy)(H2O)][Cr(bpy)(CN)4] x H2O}n (4) (dca = N(CN)2(-)), {Mn(N3)(CH3OH)[Cr(bpy)(CN)4] x 2H2O}n (5), and {Mn(bpy)(N3)(H2O)[Cr(bpy)(CN)4] x H2O}2 (6), have been prepared and characterized structurally and magnetically. X-ray crystallography reveals that the compounds 1, 2, 4, and 5 consist of one-dimensional (1D) chains with different structures: a 4,2-ribbon-like chain, a branched zigzag chain, a 2,2-CC zigzag chain, and a 3,3-ladder-like chain, respectively. It also reveals that compound 3 has a trinuclear [MnCr2] structure, and compound 6 has a tetranuclear [Mn2Cr2] square structure. Magnetic studies show antiferromagnetic interaction between Cr(III) and Mn(II) ions in all compounds. All of the chain compounds exhibit metamagnetic behaviors with different critical temperatures (Tc) and critical fields (Hc; at 1.8 K): 3.2 K and 3.0 kOe for 1; 2.3 K and 4.0 kOe for 2; 2.1 K and 1.0 kOe for 4; and 4.7 K and 5.0 kOe for 5, respectively. The noncentrosymmetric compound 2 is also a weak ferromagnet at low temperature because of spin canting. The magnetic analyses reveal Cr-Mn intermetallic magnetic exchange constants, J, of -4.7 to -9.4 cm(-1) (H = -JS(Cr) x S(Mn)). It is observed that the antiferromagnetic interaction through the Mn-N-C-Cr bridge increases as the Mn-N-C angle (theta) decreases to the range of 155-180 degrees, obeying an empirical relationship: J = -40 + 0.2theta. This result suggests that the best overlap between t(2g) (high-spin Mn(II)) and t(2g) (low-spin Cr(III)) occurs at an angle of approximately 155 degrees.

Electrochemical oxidation of [Cr(CO) 4(tmp)] to the low-spin Cr(I) species [Cr(CO) 4(tmp)] + (tmp=3,4,7,8-tetramethyl-1,10-phenanthroline): an IR, UV–Vis, and EPR spectroelectrochemical and DFT computational study of the accompanying changes in molecular and electronic structure

The complexes [Cr(CO) 4(phen)] and [Cr(CO) 4(tmp)] (phen=1,10-phenanthroline, tmp=3,4,7,8-Me 4-phen) are electrochemically oxidized at −0.01 and −0.05 V versus Fc/Fc + to the corresponding cations. The oxidation is electrochemically reversible and chemically nearly reversible at room temperature and a scan rate of 1000 mV s −1. The cation [Cr(CO) 4(tmp)] + is chemically stable at 193 K but undergoes a complete loss of CO ligands at higher temperatures. It was characterized by EPR, IR, and UV–Vis spectroelectrochemistry at 193 K and its structure and bonding have been investigated by density functional theory (DFT) calculations. It follows that [Cr(CO) 4(tmp)] + can be described as a Cr(I) complex with a low-spin d 5 electron configuration. The EPR spectrum shows a g-anisotropy ( g ‖– g ⊥) of 0.1, which is characteristic of a metal-centered radical. Oxidation is accompanied by a large shift of electron density toward the Cr atom, manifested by nearly a 100 cm −1 shift of the ν(CO) IR bands to higher frequencies on oxidation. Moreover, the IR spectrum of [Cr(CO) 4(tmp)] + suggests that the stretching force constants of the axial and equatorial CO ligands become nearly equal upon oxidation and their interaction strengthens. DFT calculations have revealed that [Cr(CO) 4(tmp)] + has three low-lying electronic states, which are very close in energy. Each of these states can be the electronic ground state. They differ in the symmetry of the singly occupied MO, which, in each case, has a predominant Cr 3d character. While the spin density is localized mostly on the Cr atom, the charge on Cr is nearly the same in the neutral and in the cationic complexes. The charges on the CO ligands and, to a lesser extent, on tmp become more positive upon oxidation, due to electron-density redistribution. Lengthening of the CrC bonds and shortening of CO and CrN bonds are the main calculated structural changes caused by the oxidation of [Cr(CO) 4(tmp)]. Oxidation of the analogous W complexes is chemically irreversible on the cyclic voltammetry time scale at room temperature, reflecting the high reactivity of [W(CO) 4(tmp)] + and [W(CO) 4(phen)] +.

Entropic and enthalpic contributions to the solvent dependence of the thermodynamics of transition-metal redox couples: Part I. Couples containing aromatic ligands

Measurements of the formal potentials E f for Cr(bpy) 3 3+/2+, Fe(bpy) 3 3+/2+, Co(bpy) 3 3+/2+ and Co(phen) 3 3+/2+ (where bpy=2,2′-bipyridine, phen=1,10-phenanthroline) were evaluated as a function of temperature using cyclic voltammetry in eight solvents (water, dimethylsulfoxide, N-N-dimethylformamide, N-methylformamide, formamide, propylene carbonate, acetonitrile and nitromethane) in order to unravel the various structural influences of the solvent upon the redox thermodynamics of such simple redox couples where the oxidized and reduced forms have identical structures and the composition of the coordination shell remains unchanged when the solvent is varied. The reaction entropies Δ S rc 0 for each redox couple in each solvent were evaluated from the temperature dependence of E f using a nonisothermal cell arrangement. Estimates of the free energy of transfer for each redox couple Δ(Δ G rc 0) s−w from water to each non-aqueous solvent were obtained from the difference in E f relative to the corresponding differences for the ferricinium-ferrocene couple. The corresponding enthalpies of transfer Δ(Δ H rc 0) s−w were obtained by combining these values of Δ S rc 0 and Δ(Δ G rc 0) s−w. Substantially larger values of Δ S rc 0 were found for all four redox couples in non-aqueous solvents compared to water, particularly in aprotic media where for a given couple Δ(Δ S rc 0) s−w ∼20–40 cal K −1 mol −1. Markedly larger values of Δ S rc 0 in each solvent were found for the Co(III)/(II) couples compared with the low spin Cr(III)/(II) and Fe(III)/(II) couples. These entropic variations appear to reflect the ease by which solvent molecules are able to be oriented by the additional field around the oxidized compared to the reduced form of the redox couple. Small negative values of Δ(Δ G rc 0) s−w were typically obtained as a result of partial compensation of the entropic terms by the corresponding enthalpic components. The utility of reaction entropy measurements for interpreting the dependence of electron-transfer kinetics upon the nature of the solvent is pointed out.

Crystal field effects in chromium and its partitioning in the mantle

Cr 3+ ions have a strong preference for octahedral sites in mineral structures and discriminate against tetrahedral sites, whereas Cr 2+ ions favor distorted environments, according to crystal field theory. All structure-types proposed for the transition zone and lower mantle contain six-coordinated sites, so that Cr 3+ ions may be distributed over several phases in the Earth's interior. In the upper mantle, however, stabilization energies derived from crystal field spectra suggest that the enrichment of Cr 3+ follows the sequence: spinel > garnet > pyroxene or olivine. High-spin Cr 2+ ions, which may occur at very high pressures and low oxygen fugacities, are predicted to be stabilized in deformed sites of the olivine and Sr 2PbO 4 structure-types. Low-spin Cr 2+ ions may also occur in the CaFe 2O 4 structure-type in the lower mantle. It is proposed that some chromite inclusions in forsterites from the upper mantle are formed by pressure-released oxidation of Cr 2+ ions originally present in olivine or spinel modifications of Mg 2SiO 4.

Magnetic and electrical properties of ternary pyrite-type Cr xCo 1−xS 2 phases prepared at high pressure

New pyrite-type phases Cr xCo 1−xS 2 (0 < x < 0.4) have been prepared at 65 kbars pressure in a tetrahedral anvil apparatus. Magnetic measurements indicate that the saturation magnetization decreases, while the Curie temperature increases, with increasing x. T c reaches a maximum above room temperature at x ⋍ 0.3 . The magnetic data favor an arrangement in which low-spin Cr +2 is aligned antiferromagnetically to Co +2. Electrical measurements show essentially temperature-independent resistivity with ϱ ⋍ 10 −4 ohm-cm.

Descriptions of outer d electrons in thiospinels

The constraints [1] of symmetry imposed by the spinel structure and[2] of internal consistency within the periodic table are used to construct phenomenological energy diagrams for the transition-metal thiospinels. It is found that band narrowing via intra-atomic exchange is sufficient to localize the d electrons to a cation only if there are at least three unpaired spins at the cation. A-site Co 2+ ions may have collective d electrons, but exhibit a spontaneous atomic moment in Co [Cr 2]S 4. The compounds A[ Cr 2] S 4, where A = Zn, Cd, Mn and Fe, contain localized d electrons, although the antiparalled-spin electron per Fe 2+ ion is just localized. It is argued that the compounds Cu[ B 2]S 4, B = V, Cr, Rh and 1 2(Rh+Cr) , have one hole per molecule in primarily d-like, collective orbitais that are primarily Cu-S in character. Where the B cations have no spontaneous atomic moment ( B ≠ Cr), the compounds are superconducting at lowest temperatures. Cu[Ti 2]S 4 and Cu[Co 2]S 4, though metallic and without spontaneous magnetism, exhibit no superconducting transition above 0.05°K. Cu[Ti 2]S 4 and probably Cu[Co 2]S 4 are distinguished by a lack of (or less than one per molecule) holes in the d-like, collective Cu-S states. Cu 2+ + Cr 3+ is argued to be more stable than Cu + + Cr 4+ in the chalcogenide spinels, as it is known to be in Cu[Cr 2]O 4. The low Curie constant of CuCrRhSe 4, as compared with CuCrTiS 4, is attributed to a temperature-dependent molecular field, which is primarily due to mobile charge carriers in the selenide, rather than to Cr 4+ excited states at high temperatures. The decrease in optical energy gap below T c in CdCr 2Se 4, in contrast to its increase in CdCr 2S 4, is attributed to high-spin and low-spin Cr 2+ ions of approximately the same energy.