Low-spin Electronic Configuration Research Articles

Density Functional Theory Study on How Electron Spin Configuration of Co3+ Ion Affects the Structure and Magnetism of Co-Doped Bismuth Ferrite

In this paper, we use first-principles calculations carried out under the density functional theory for probing ways to enhance the magnetization of materials from BiFeO3 (BFO) family while preserving robust ferroelectricity. In addition to analyzing ferrimagnetic arrangements of B-site magnetic ions with different magnetic moments (Fe3+ and Co3+) in BFO-based supercells, we have also studied the impact of various electronic configurations (high-spin, low-spin, intermediate-spin) of Co3+ ion on crystal structure, ferroelectricity and magnetism of such systems. Our results suggest that tetragonal Co-doped BFO (BFCO) compounds with low-spin and intermediate-spin electronic configuration of Co have robust coupling between the spontaneous polarization and magnetic degrees of freedom. Furthermore, we have found an additional route to enhance the magnetization of tetragonal BFCO compounds with Co3+ in intermediate spin state, while remaining coupled to ferroelectric degrees of freedom, namely by inducing a ferrimagnetic-to-ferromagnetic phase transition.

Magnetic Structures of HoCoO 3 and TbCoO 3

The magnetic behavior of HoCoO3 and TbCoO3 has been studied from magnetic measurements and neutron diffraction experiments performed on polycrystalline samples. The magnetic measurements indicate that the Co3+ ions exhibit a low-spin electronic configuration t2g6eg0 with S = 0 below room temperature. The neutron diffraction data show that HoCoO3 and TbCoO3 order with an antiferromagnetic structure below TN = 3 and 3.6 K, respectively. The magnetic structure for HoCoO3 and TbCoO3 are given by the basis vectors (Ax,0,Cz) and (Cx,0,Az), respectively. Therefore, the magnetic moments are in the (010) plane. At T = 1.5 K, the ordered magnetic moments of Ho3+ and Tb3+ ions are 6.48(4) and 7.86(4) μB, respectively.

Lattice Effects on the Spin‐Crossover Profile of a Mononuclear Manganese(III) Cation

Six solvated salts of a mononuclear manganese(III) complex with a chelating hexadentate Schiff base ligand are reported. One member of the series, [MnL]PF(6)⋅0.5 CH(3)OH (1), shows a rare low-spin (LS) electronic configuration between 10-300 K. The remaining five salts, [MnL]NO(3)⋅C(2)H(5)OH(2), [MnL]BF(4)⋅C(2)H(5)OH(3), [MnL]CF(3)SO(3)⋅C(2)H(5)OH (4), [MnL]ClO(4)⋅C(2)H(5)OH (5) and [MnL]ClO(4)⋅0.5 CH(3)CN (6), all show gradual incomplete spin-crossover (SCO) behaviour. The structures of all were determined at 100 K, and also at 293 K in the case of 3-6. The LS salt [MnL]PF(6)⋅0.5 CH(3)OH is the only member of the series that does not exhibit strong hydrogen bonding. At 100 K two of the four SCO complexes (2 and 4) assemble into 1D hydrogen-bonded chains, which weaken or rupture on warming. The remaining SCO complexes 3, 5 and 6 do not form 1D hydrogen-bonded chains, but instead exhibit discrete hydrogen bonding between cation/counterion, cation/solvent or counterion/solvent and show no significant change on warming.

Electronic Structure Description of a [Co(III)3Co(IV)O4] Cluster: A Model for the Paramagnetic Intermediate in Cobalt-Catalyzed Water Oxidation

Multifrequency electron paramagnetic resonace (EPR) spectroscopy and electronic structure calculations were performed on [Co(4)O(4)(C(5)H(5)N)(4)(CH(3)CO(2))(4)](+) (1(+)), a cobalt tetramer with total electron spin S = 1/2 and formal cobalt oxidation states III, III, III, and IV. The cuboidal arrangement of its cobalt and oxygen atoms is similar to that of proposed structures for the molecular cobaltate clusters of the cobalt-phosphate (Co-Pi) water-oxidizing catalyst. The Davies electron-nuclear double resonance (ENDOR) spectrum is well-modeled using a single class of hyperfine-coupled (59)Co nuclei with a modestly strong interaction (principal elements of the hyperfine tensor are equal to [-20(±2), 77(±1), -5(±15)] MHz). Mims (1)H ENDOR spectra of 1(+) with selectively deuterated pyridine ligands confirm that the amount of unpaired spin on the cobalt-bonding partner is significantly reduced from unity. Multifrequency (14)N ESEEM spectra (acquired at 9.5 and 34.0 GHz) indicate that four nearly equivalent nitrogen nuclei are coupled to the electron spin. Cumulatively, our EPR spectroscopic findings indicate that the unpaired spin is delocalized almost equally across the eight core atoms, a finding corroborated by results from DFT calculations. Each octahedrally coordinated cobalt ion is forced into a low-spin electron configuration by the anionic oxo and carboxylato ligands, and a fractional electron hole is localized on each metal center in a Co 3d(xz,yz)-based molecular orbital for this essentially [Co(+3.125)(4)O(4)] system. Comparing the EPR spectrum of 1(+) with that of the catalyst film allows us to draw conclusions about the electronic structure of this water-oxidation catalyst.

Using a Combination of Experimental and Computational Methods To Explore the Impact of Metal Identity and Ligand Field Strength on the Electronic Structure of Metal Ions

In this exercise, students apply a combination of techniques to investigate the impact of metal identity and ligand field strength on the spin states of three d5 transition-metal complexes: Fe(acac)3, K3[Fe(CN)6], and Ru(acac)3, where acac– is acetylacetonate. Students first use crystal field theory to predict the most likely spin state based on the metal identity (3d versus 4d metal) and the ligand field strength of the acac– and CN– ligands (weak-field versus strong-field ligands). Next, students use density functional theory (DFT) to determine the geometries and the energies of the high- and low-spin electronic configurations of each complex, allowing them to predict the most energetically favorable configuration and to compare their computed geometries with X-ray structural data. Finally, students experimentally determine the magnetic susceptibility and spin state of each complex and compare these results with their predictions. This combined application of basic chemical principles, computational chemistry, and experimental measurement provides students with practical insight into the application of complementary methods to better understand chemical properties.

Mononuclear NiIII−Alkyl Complexes (Alkyl = Me and Et): Relevance to the Acetyl-CoA Synthase and Methyl-CoM Reductase

Mononuclear, distorted trigonal bipyramidal [PPN][Ni(III)(R)(P(C(6)H(3)-3-SiMe(3)-2-S)(3))] (R = Me (1); R = Et (2)) were prepared by reaction of [PPN][Ni(III)Cl(P(C(6)H(3)-3-SiMe(3)-2-S)(3))] and CH(3)MgCl/C(2)H(5)MgCl, individually. EPR, SQUID studies as well as DFT computations reveal that the Ni(III) in 1 has a low-spin d(7) electronic configuration in a distorted trigonal bipyramidal ligand field. The Ni-C bond lengths of 1.994(3) A in 1 and 2.015(3) A in 2 are comparable to that in the Ni(III)-methyl state of MCR (approximately 2.04 A) (Sarangi, R.; Dey, M.; Ragsdale, S. W. Biochemistry 2009, 48, 3146). Under a CO atmosphere, CO triggers homolytic cleavage of the Ni(III)-CH(3) bond in 1 to produce Ni(II)-thiolate carbonyl [PPN][Ni(II)(CO)(P(C(6)H(3)-3-SiMe(3)-2-S)(3))] (3). Additionally, protonation of 1 with phenylthiol generates Ni(III)-thiolate [PPN][Ni(III)(SPh)(P(C(6)H(3)-3-SiMe(3)-2-S)(3))] (4).

REACTIONS OF DIPYRIDYLKETONE WITH GROUP 8 METAL CARBONYLS IN DIFFERENT MEDIA

Reaction of di-(2-pyridyl)ketone (dpk) with [M3(CO)12], M =Ru or Os in dioxan under reduced pressure gave the tricarbonyl derivative [Ru(CO)3(dpk)] (1) and [Os(CO)3(dpk)] (2). When these reactions were carried out in benzene/ethanol 1:1 (v:v) under reflux and reduced pressure gave the binuclear ruthenium complex [Ru2(CO)4(m-CO)(h3-dpkO,Oeth)2] (3),where h3-dpkO,Oeth is N,O,N-ethoxybis(2-pyridyl)-methanolato and the mononuclear [Os(CO)3(h3-dpkO,Oeth) (4). All complexes were characterized by elemental analysis, infrared, mass and 1H NMR spectroscopy. The spectroscopic measurements indicated that in dioxane as solvent the dpk act as bidentate ligand while in benzene/ethanol solution the dpk act as tridentate ligand. The magnetic measurements revealed that the complexes (3) and (4) are paramagnetic with change in the formal oxidation state of the ruthenium and osmium atoms from zero to +1 via oxidative addition of the OH group to ruthenium or osmium with a proton displacement to give a low-spin d7 electronic configuration The UV–Vis studies of the complexes showed blue shift compared to the ligand confirming complex formation.

Ligand-Field Dependence of the Excited State Dynamics of Hangman Bisporphyrin Dyad Complexes

A new Hangman porphyrin architecture has been developed to interrogate the ligand-field dependence of photoinduced PCET versus excitation energy transfer and intersystem crossing in PZn(II)-PFe(III)-OH dyads (P = porphyrin). In this design, a hanging carboxylic acid group establishes a hydrogen-bonding network to anchor the weak-field OH- ligand in the distal site of the PFe(III)-OH acceptor, whereas the proximal site is left available to accept strong-field imidazole ligands. Thus, controlling the tertiary coordination environment gives access to the first synthetic example of a porphyrin dyad with a biologically relevant weak-field/strong-field configuration of axial ligands at the heme. Transient absorption spectroscopy has been employed to probe the fate of the initial PZn(II)-based S1 excited state, revealing rapid S1 quenching for all dyads in the presence and absence of strong-field imidazole ligands (tau = 6-50 ps). The absence of a (P*+)Zn(II) signal that would complement photoinduced PCET at the PFe(III)-OH subunit (i.e., PFe(III)-OH --> PFe(II)-OH2) shows that excitation energy transfer and intersystem crossing channels dominate the quenching, regardless of whether proximal strong field ligands are present. Moreover, this photophysical assignment is independent of the solvent dielectric constant and whether a phenylene or biphenylene spacer is used to span the two porphyrin subunits. Electronic structure calculations suggest that the structural reorganization attendant to reductive PCET at the high-spin Fe(III)-OH center imposes a severe kinetic cost that can only be alleviated by inducing a low-spin electronic configuration with two strong-field axial ligands.

Cluster study of surface radicals of Si(111)-7 × 7 reconstructed surface

The Si(111)- 7 × 7 surface was theoretically investigated using four different QM/MM surface models. Although low-spin states turned out to be the ground states, there also exist low-lying high-spin states. The energy gap between the low-spin and the high-spin states per dangling bond is nearly constant (~4 kcal/mol) regardless of the number of dangling bonds in the model clusters, showing that no particular pattern of electron pairing among surface dangling bonds exists. Natural orbital occupation numbers showed that surface radicals due to dangling bonds have ideal open shell configurations. In addition to this, optimized geometries of the low-spin and the high-spin states are nearly identical with each other, suggesting the possibilities of intersystem crossings. So both low spin and high spin electronic configurations are important in the descriptions of the Si(111)- 7 × 7 reconstructed surface. In contrast, geometric differences between pure QM and QM/MM models are significant especially near the adatom region, indicating that the QM/MM embedded model is necessary to adequately represent the Si(111)- 7 × 7 surface especially for small QM region models.

YcdB from Escherichia coli Reveals a Novel Class of Tat-dependently Translocated Hemoproteins

The Tat (twin-arginine translocation) system of Escherichia coli serves to translocate folded proteins across the cytoplasmic membrane. The reasons established so far for the Tat dependence are cytoplasmic cofactor assembly and/or heterodimerization of the respective proteins. We were interested in the reasons for the Tat dependence of novel Tat substrates and focused on two uncharacterized proteins, YcdO and YcdB. Both proteins contain predicted Tat signal sequences. However, we found that only YcdB was indeed Tat-dependently translocated, whereas YcdO was equally well translocated in a Tat-deficient strain. YcdB is a dimeric protein and contains a heme cofactor that was identified to be a high-spin Fe(III)-protoporphyrin IX complex. In contrast to all other periplasmic hemoproteins analyzed so far, heme was assembled into YcdB in the cytoplasm, suggesting that heme assembly could take place prior to translocation. The function of YcdB in the periplasm may be related to a detoxification reaction under specific conditions because YcdB had peroxidase activity at acidic pH, which coincides well with the known acid-induced expression of the gene. The data demonstrate the existence of a class of heme-containing Tat substrates, the first member of which is YcdB.

Is low-spin Fe2+ present in Earth's mantle?

Abstract Recently, X-ray spectroscopy indicated that the low-spin (LS) electronic configuration of Fe 2+ and Fe 3+ ions substituting in silicate perovskite of various compositions, and also in magnesiowustite is stable at room temperature above pressures of ∼49 to 120 GPa [J. Badro, G. Fiquet, F. Guyot, J.-P. Rueff, V.V. Struzhkin, G. Vanko, G. Monaco, Iron partitioning in Earth's mantle: toward a deep lower mantle discontinuity, Science 300 (2003), 789–791.; J. Badro, J.-P. Rueff, G. Vanko, G. Monaco, G. Figuet, F. Guyot, Electronic transition in perovskite: possible non-convecting layers in the lower mantle, Science 305 (2004) 383–386.; J. Li, V.V. Struzhkin, H.K. Mao, J. Shu, R.J. Hemley, Y. Fei, B. Mysen, P. Dera, V. Prakapenka, G. Shen, Electronic spin state of iron in lower mantle perovskite, Proc. Natl. Acad. Sci. 101 (2004) 14027–14030.; J.M. Jackson, W. Sturhahn, G. Shen, J. Zhao, M.Y. Hu, D. Errandonea, J.D. Bass, Y. Fei, A synchrotron Mossbauer study of (Mg,Fe)SiO 3 perovskite up to 120 GPa, Am. Mineral. 90 (2005) 199–205.; J.F. Lin, V.V. Struzhkin, S.D. Jacobsen, M.Y. Hu, P. Chow, J. Kung, H. Liu, H.K. Mao, R.J. Hemley, Spin transition of iron in magnesiowustite in the Earth's lower mantle, Nature 436 (2005) 377–380.]. Simple thermodynamic relationships combined with crystal field theory provide a minimum Clausius–Clapeyron slope of ∼0.23 to 0.4 GPa/K for the high-spin (HS) to low-spin transition of Fe ions in perovskite, consistent with lower mantle temperatures stabilizing the disordered HS state. This computation of ∂ P / ∂ T utilizes experimentally determined parameters only ( P and T at the transition), and is supported by microstructural analysis of the change in volume and entropy considerations. Similarly, (∂ P / ∂ T ) min ∼0.18 to 0.31 GPa/K for magnesiowustite. High spin Fe 2+ should be stable in silicates throughout Earth's mantle for compositions of Fe / (Mg + Fe) ∼0.1, consistent with disorder prevailing at high temperature, although partial conversion of octahedral Fe 3+ to LS may occur. Similarly, partial conversion of octahedral Fe 2+ in magnesiowustite to the LS is possible for Fe-rich compositions, which transform at relatively low pressures.

Structural and magnetic phase transitions in simple oxides using hybrid functionals

We present the structural as well as elastic properties of the alkaline earth oxides and FeO, calculated using hybrid exchange functionals within DFT. We show that by empirically fitting the amount of Fock-exchange in the hybrid functionals, we can accurately reproduce the pressure-induced phase transitions for MgO, CaO, SrO and BaO. For FeO the hybrid functionals predict an insulator↔metal transition at ca. 150 GPa, associated with an i-B8↔B8 structural phase transition. The structural phase transition is accompanied by a spin transition from a high- to low-spin electron configuration on the Fe2+ ions. Hence, FeO undergoes a magnetic phase transition from an anti-ferromagnetic to non-magnetic structure. We also find that as the ionicity of the polymorphs increases a higher fraction of Fock-exchange is required to reproduce the structural volumes reported from experiments.

Reduced and oxidized cytochrome c4 exhibit differences in dynamics

The temperature-dependent dynamics of the fully reduced and fully oxidized forms of Pseudomonas stutzeri cytochrome c4 have been studied by Mössbauer spectroscopy. Prior to the dynamic analysis, an efficient labelling strategy has been developed for the expression of highly enriched (57)Fe recombinant cytochrome c4. Subsequently, the protein has been purified to apparent homogeneity. Mössbauer measurements were recorded in the temperature range 77-240 K for both protein forms. A detailed analysis of the high quality spectra is presented. Based on the information obtained from Mössbauer spectroscopy, similarities and differences between cytochrome c4, cytochrome c and HiPIP are discussed. The obtained results reveal that (a) cytochrome c4 exists in pure low spin electronic configuration in both oxidation states in the temperature range 77-240 K, (b) the heme pocket is more relaxed in cytochrome c4 than in cytochrome c, (c) the reduced cytochrome c4 is the most flexible at low temperatures, and (d) protein specific dynamics are most distinct in the oxidized protein.

Measurement of Quadrupole Interactions in La1 − x Sr x CoO3 Perovskites Using TDPAC Technique

The Perturbed Angular Correlation (PAC) technique was used to study the quadrupole interactions in the La1−x SrxCoO3 (0 ≤ x ≤ 0.08) perovskites using 111Cd and 181Ta probes. The radioactive parent nuclei 111In and 181Hf were introduced in the oxide lattice through chemical process during sample preparation and found to occupy only Co sites. The measurements cover a temperature range from 10 to 1150 K except the pure LaCoO3 for which an additional measurement was made at 4.2 K. The measured quadrupole frequencies were found to decrease linearly with increasing temperature as well as with increasing Sr concentration. Temperature dependence of quadrupole frequency in the pure LaCoO3 shows small discontinuities around 70–90 K and 500–600 K which have been atributed to thermally activated spin state transitions, from the low-spin (LS) ground state electronic configuration of Co+3 ion to the intermediate-spin (IS) state and from intermediate-spin (IS) state to high-spin (HS) state respectively, observed in some recent studies.

Heme Distortion Modulated by Ligand-Protein Interactions in Inducible Nitric-oxide Synthase

The catalytic center of nitric-oxide synthase (NOS) consists of a thiolate-coordinated heme macrocycle, a tetrahydrobiopterin (H4B) cofactor, and an l-arginine (l-Arg)/N-hydroxyarginine substrate binding site. To determine how the interplay between the cofactor, the substrates, and the protein matrix housing the heme regulates the enzymatic activity of NOS, the CO-, NO-, and CN(-)-bound adducts of the oxygenase domain of the inducible isoform of NOS (iNOS(oxy)) were examined with resonance Raman spectroscopy. The Raman data of the CO-bound ferrous protein demonstrated that the presence of l-Arg causes the Fe-C-O moiety to adopt a bent structure because of an H-bonding interaction whereas H4B binding exerts no effect. Similar behavior was found in the CN(-)-bound ferric protein and in the nitric oxide (NO)-bound ferrous protein. In contrast, in the NO-bound ferric complexes, the addition of l-Arg alone does not affect the structural properties of the Fe-N-O moiety, but H4B binding forces it to adopt a bent structure, which is further enhanced by the subsequent addition of l-Arg. The differential interactions between the various heme ligands and the protein matrix in response to l-Arg and/or H4B binding is coupled to heme distortions, as reflected by the development of a variety of out-of-plane heme modes in the low frequency Raman spectra. The extent and symmetry of heme deformation modulated by ligand, substrate, and cofactor binding may provide important control over the catalytic and autoinhibitory properties of the enzyme.

Synthesis, Structures, Characterization, Dynamic Behavior, and Reactions of Novel Late Transition Metal(II) 1-Azaallyls

A series of homoleptic late transition metal(II) 1-azaallyl compounds ([M(L)2] {L = η3-N(SiMe3)C(tBu)CH(C6H4Me-4), M = Fe (5), Co (6), or Ni (7)}, [M(L1)2] {L1 = η3-N(SiMe3)C(Ph)CH(SiMe3), M = Ni (8) or Pd (9)}, [M(L2)2] {L2 = η3-N(SiMe3)C(tBu)CH(SiMe3), M = Fe (10) or Co (11)}, or [M(L3)2] {L3 = η1-N(SiMe3)C(tBu)CH(C10H7-1), M = Fe (12)} have been prepared by reactions of anhydrous late transition metal halides {MX2 = FeBr2, CoCl2, [NiBr2(dme)], or [PdCl2(cod)]} with the appropriate lithium or potassium 1-azallyl KL (1), [LiL1(thf)]2 (2), [LiL2]2 (3), or KL3 (4) in a 1:2 molar ratio. The heteroleptic 1-azallylnickel(II) complex Ni(η3-C3H5)(L3) (13) was prepared from [{Ni(η3-C3H5)(μ-Br)}2] with a stoichiometric amount of the 1-azaallylpotassium reagent KL3 (4). Single-crystal X-ray analysis revealed the 1-azaallyl to be bonded to the metal center in an η3-NCC mode in the mononuclear compounds 5−8 and 10, whereas in 12 the 1-azallyl is N-bound in the κ1-enamido fashion. The iron(II) (5, 10, and 12) and cobalt(II) (6 and 11) compounds are paramagnetic and have magnetic moments in the range 5.01−5.61 μB and 2.73−3.01 μB, respectively, characteristic of a high-spin d6 and low-spin d7 electronic configuration, respectively. The diamagnetic group 10 metal 1-azaallyl compounds (7−9 and 13) were fully characterized by multinuclear NMR spectroscopy and were found to be a mixture of isomers. Several NOE, two-dimensional, and saturation transfer NMR spectroscopic experiments were used to elucidate the nature of the three isomers. Variable-temperature and variable-temperature saturation transfer NMR spectroscopic experiments showed the isomers of each of compounds 7−9 to be involved in a dynamic process. Electrochemical and oxidation studies on compounds 5 and 6 are reported, as are α-olefin oligomerization reactions catalyzed by 13 with MAO or B(C6F5)3.

Octahedral (cis-cyclam)iron(III) complexes with O,N-coordinated o-iminosemiquinonate(1-) pi radicals and o-imidophenolate(2-) anions.

Three octahedral complexes containing a (cis-cyclam)iron(III) moiety and an O,N-coordinated o-iminobenzosemiquinonate pi radical anion have been synthesized and characterized by X-ray crystallography at 100 K: [Fe(cis-cyclam)(L(1-3)(ISQ))](PF(6))(2) (1-3), where (L(1-3)(ISQ)) represents the monoanionic pi radicals derived from one-electron oxidations of the respective dianion of o-imidophenolate(2-), L(1), 2-imido-4,6-di-tert-butylphenolate(2-), L(2), and N-phenyl-2-imido-4,6-di-tert-butylphenolate(2-), L(3). Compounds 1-3 possess an S(t) = 0 ground state, which is attained via strong intramolecular antiferromagnetic exchange coupling between a low-spin central ferric ion (S(Fe) = 1/2) and an o-imino-benzosemiquinonate(1-) pi radical (S(rad) = 1/2). Zero-field Mössbauer spectra of 1-3 at 80 K confirm the low-spin ferric electron configuration: isomer shift delta = 0.26 mm s(-1) and quadrupole splitting DeltaE(Q) = 1.96 mm s(-1) for 1, 0.28 and 1.93 for 2, and 0.33 and 1.88 for 3. All three complexes undergo a reversible, one-electron reduction of the coordinated o-imino-benzosemiquinonate ligand, yielding an [Fe(III)(cis-cyclam)(L(1-3)(IP))](+) monocation. The monocations of 1 and 2 display very similar rhombic signals in the X-band EPR spectra (g = 2.15, 2.12, and 1.97), indicative of low-spin ferric species. In contast, the monocation of 3 contains a high-spin ferric center (S(Fe) = 5/2) as is deduced from its Mössbauer and EPR spectra.

Ruthenium, osmium and rhodium-2,3-bis(2'-pyridyl)quinoxaline complexes

Ru3(CO)12 reacts with 2,3-bis (2′-pyridyl)quinoxaline (dpq) in benzene in the presence of either 2,2′(dipyridine (dpy) or pyridine (py) to give the mononuclear complexes Ru(CO)3(dpq) (1) and Ru(CO)2(dpq)(py) (2), respectively. On the other hand, reactions of Os3(CO)12 with dpq alone, or in the presence of dipyridine, yield only Os(CO)3(dpq) (3). Spectroscopic studies of the three complexes were consistent with the proposed structures. Reactions of RuCl3 with dpq, under reduced pressure in PhH/EtOH gave bis-[Ru(dpq)2Cl2]Cl (4). Magnetic measurements showed RuIII, d5, to be in a low-spin electronic configuration. Three complexes, Rh2(dpq)2Cl6(5), [Rh(dpq)2Cl2]Cl (6) and Rh(dpq)(py)Cl3(7), were isolated from reactions of RhCl3 with dpq. The type of product was dependent on the reactants and conditions. The complexes exhibit either irreversible or quasi-reversible ligand-based reductions. In addition, the zerovalent complexes (1)–(3) displayed one metal(based oxidation in their cyclic voltammograms, due to the formation of M+ species.

Spin State Behavior in Some Cobaltites (III) and (IV) with Perovskite or Related Structure

The peculiar behaviors of trivalent (3d6) and tetravalent (3d5) cobalt ions coordinated to six oxygen atoms forming a regular or distorted octahedral site are emphasized. Low spin (LS), intermediate spin (IS), and high spin (HS) electronic configurations can be considered for Co3+ (S=0, S=1, and S=2) and for Co4+ (S=1/2, S=3/2, and S=5/2) depending on the relative magnitude of electronic correlations and crystal field parameters. A simple model including these effects accounts for spin crossover and has led to the derivation of phase diagrams showing the respective stability range of LS, IS, and HS states. Extended Hückel tight-binding and density functional theory–full-potential linearized augmented plane wave calculations were carried out for TlSr2CoO5 and SrCoO3. Results from both techniques are compared and used to interpret the specific properties of these oxides, which are attributed to the peculiar character of cobalt (III) and (IV). Unlike in molecular compounds where only magnetic and optical properties can be modified, spin transitions (intraatomic electron transfer) can be strongly coupled with metal–insulator transitions in ionocovalent oxides. Spin equilibrium can lead to partly occupied energy bands as nonintegral oxidation state does for mixed valence oxides, and, furthermore, spin state ordering can occur upon cooling in a way similar to charge ordering.

(eta 5-Cyclopentadienyl)(kappa 3-hydrotris(pyrazolyl)borate)cobalt(II)--the first high-spin cobalt organometallic complex.

(eta 5-Cyclopentadienyl)(kappa 3-hydrotris(pyrazolyl)borate)-cobalt(II) has been synthesised; magnetic and crystallographic data indicate this to be the first example of a high-spin cobalt organometallic complex whilst its pentamethylcyclopentadienyl analogue is found to have a low-spin electronic configuration.