Characterization and X-ray absorption spectroscopic studies of bis[quinato(2-)]oxochromate(V)

A new Cr(V) complex, K[CrVO(qaH3)2].H2O (Ia; qaH3 = quinato = (1R,3R,4R,5R)-1,3,4,5-tetrahydroxycyclohexanecarboxylato(2-)), synthesized by the reaction of K2Cr2O7 with excess qaH5 in MeOH (Codd, R.; Lay, P. A. J. Am. Chem. Soc. 1999, 121, 7864-7876), has been characterized by microanalyses, electrospray mass spectra, and UV-visible, CD, IR, EPR, and X-ray absorption spectroscopies. This complex is of interest because of its ability to act as both a structural and a biomimetic model for a range of Cr(V) species believed to be generated in vivo during the intracellular reduction of carcinogenic Cr(VI). The Na+ analogue of Ia (Ib) has also been isolated and characterized by microanalyses and IR and X-ray absorption spectroscopies. The reaction of Cr(VI) with MeOH in the presence of qaH5 that leads to I proceeds via a Cr(IV) intermediate (observed by UV-visible spectroscopy), and a mechanism for the formation of I has been proposed. DMF or DMSO solutions of I are stable for several days at 25 degrees C, while I in aqueous solution (pH = 4) disproportionates to Cr(VI) and Cr(III) in minutes. The likely structures in the solid state for Ia (14 K) and Ib (approximately 293 K) have been determined using both single-scattering (Ia,b) and multiple-scattering (Ia) analyses of XAFS data. These analyses have shown the following: (i) In agreement with the results from the other spectroscopic techniques, the quinato ligands are bound to Cr(V) by 2-hydroxycarboxylato moieties, with Cr-O bond lengths of 1.55, 1.82, and 1.94 A for the oxo, alcoholato, and carboxylato O atoms, respectively. (ii) The position of an oxo O atom is somewhat disordered. This is consistent with molecular mechanics modeling of the likely structures. The XAFS, EPR, and IR spectroscopic evidence points to the existence of hydrogen bonds between the oxo ligand and the 3,4,5-OH groups of the quinato ligands in the solid state of I.     

Seven-coordinate rhenium(III) complexes of 1-(2-pyridylazo)-2-naphtholate: X-ray studies,spectroscopic characterization and DFT calculations

The reactions of [ReX₃(MeCN)(PPh₃)₂] (X = Cl or Br) with 1-(2-pyridylazo)-2-naphthol (HPAN) have been examined and the [ReBr(PAN)₂] · 2CHCl₃ (1) and [ReCl(PAN)₂] (2) complexes have been obtained. The both complexes have been structurally and spectroscopically characterized, and compound 1 has been additionally studied by magnetic measurements. The magnetic behavior is characteristic of mononuclear seven-coordinated Re(III) complex with d⁴ low-spin configuration, which gives diamagnetic ground state.     

Triamidoamine complexes of chromium(III) and chromium(IV)

Treatment of [CrCl3(THF)3] with slightly more than 1 equiv of Li3(N3N) [(N3N)(3-) = ((Me3SiNCH2CH2)3N)(3-)] affords the triamidoamine complex [Cr(N3N)] (1) in 75% yield. 1 is oxidized by PhICl2, CuCl2, or AgCl to give the chromium(IV) complex [Cr(N3N)Cl] (2) in moderate yields. Alternatively, complex 2 is obtained directly from [CrCl3(THF)3] in 50% yield after treatment with 0.5 equiv of Li3(N3N). Both compounds are high-spin complexes bearing three and two unpaired electrons, respectively. Their molecular structures are described revealing a trigonal monopyramidal and trigonal bipyramidal coordination geometry of the chromium center, respectively.     

X-ray absorption spectroscopic studies of high-spin nonheme (alkylperoxo)iron(III) intermediates

The reactions of iron(II) complexes [Fe(T(pt-Bu,i-Pr))(OH)] (1a, Tp(t-Bu,i-Pr) = hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate), [Fe(6-Me2BPMCN)(OTf)2] (1b, 6-Me2BPMCN = N,N'-bis((2-methylpyridin-6-yl)methyl)-N,N'-dimethyl-trans-1,2-diaminocyclohexane), and [Fe(L8Py2)(OTf)](OTf) (1c, L8Py2 = 1,5-bis(pyridin-2-ylmethyl)-1,5-diazacyclooctane) with tert-BuOOH give rise to high-spin FeIII-OOR complexes. X-ray absorption spectra (XAS) of these high-spin species show characteristic features, distinct from those of low-spin Fe-OOR complexes (Rohde, J.-U.; et al. J. Am. Chem. Soc. 2004, 126, 16750-16761). These include (1) an intense 1s --> 3d preedge feature, with an area around 20 units, (2) an edge energy, ranging from 7122 to 7126 eV, that is affected by the coordination environment, and (3) a 1.86-1.96 A Fe-OOR bond, compared to the 1.78 A Fe-OOR bond in low-spin complexes. These unique features likely arise from a flexible first coordination sphere in those complexes. The difference in Fe-OOR bond length may rationalize differences in reactivity between low-spin and high-spin FeIII-OOR species.     

Charge distribution in chromium and vanadium catecholato complexes: X-ray absorption spectroscopic and computational studies

Transition-metal complexes with redox-active catecholato ligands are of interest as models of bioinorganic systems and as potential molecular materials. This work expands our recent X-ray absorption spectroscopic (XAS) studies of Cr(V/IV/III) triscatecholato complexes (Levina, A.; Foran, G. J.; Pattison, D. I.; Lay, P. A. Angew. Chem., Int. Ed. 2004, 43, 462-465) to a Cr(III) monocatecholato complex, [Cr(tren)(cat)]+ (tren = tris(2-aminoethyl)amine, cat = catecholato2-), and its oxidized analogue, as well as to a series of V(V/IV/III) triscatecholato complexes ([VL3]n-, where L = cat, 3,5-di-tert-butylcatecholato2-, or tetrachlorocatecholato2-, and n = 1-3). Various oxidation states of these complexes in solutions were generated by bulk electrolysis directly in the XAS cell. Increases in the edge energies and pre-edge absorbance intensities in XANES spectra, as well as decreases in the average M-O bond lengths (M = Cr or V) revealed by XAFS data analyses, are consistent with predominantly metal-based oxidations in both the Cr(V/IV/III) and V(V/IV/III) triscatecholato series, but the degree of electron delocalization between the metal ion and the ligands was higher in the case of Cr complexes. By contrast, oxidation of [Cr(III)(tren)(cat)]+ was mainly ligand-based and led to [Cr(III)(tren)(sq)]2+ (sq = semiquinonato-), as shown by the absence of significant changes in the pre-edge and edge features and by an increase in the average Cr-O bond length. The observed differences in electron-density distribution in various oxidation states of Cr and V mono- and triscatecholato complexes have been discussed on the basis of the results of density functional calculations. A crystal and molecular structure of (Et3NH)2[V(IV)(cat)3] has been determined at 25 K and the same complex with an acetonitrile of crystallization at 150 K.     

Chlorodiphenyltin(IV) and triphenyltin(IV) complexes of N-alkylated 2-amino-1-cyclopentene-1-carbodithioic acids: synthesis, spectroscopic characterization and X-ray studies

Four new complexes, [Ph₃Sn(isopropylACDA)] (1), [Ph₂SnCl(isopropylACDA)] (2), [Ph₃Sn(secbutylACDA)] (3), and [Ph₂SnCl(secbutylACDA)] (4), have been prepared from reaction between N-alkylated 2-amino-1-cyclopentene-1-carbodithioic acids (ACDA) with Ph₂SnCl₂ and Ph₃SnCl in 1:1 ratio. All complexes are characterized by FTIR, multinuclear NMR (¹H, ¹³C, and ¹¹⁹Sn) and mass spectrometry. In all complexes, the S–H proton has been removed and coordination takes place through the carbodithioate moiety. The ¹¹⁹Sn NMR data are consistent with five coordination of tin atom in solution. Complexes 2, 3, and 4 have also been confirmed by single X-ray crystallography. All three crystals are triclinic with space group P − 1. In complexes 2 and 4, the geometry around tin atom is distorted trigonal bipyramidal while in 3 the geometry is in between distorted tetrahedral and trigonal bipyramid. In all three structures, ligands are asymmetrically coordinated to tin atom. In addition, crystal structures are further stabilized by N–H···S hydrogen bonding.     

Disproportionation and nuclease activity of bis[2-ethyl-2-hydroxybutanoato(2-)]oxochromate(V) in neutral aqueous solutions

Complex 1, [Cr(V)O(ehba)2]- (ehba = 2-ethyl-2-hydroxybutanoate(2-)) is the most studied model compound of relevance to the biological activity of Cr(V) with regard to Cr-induced cancers. The first detailed kinetic study of disproportionation of 1 under neutral pH conditions (pH 6.0-8.0, [NaClO4] = 1.0 M, 37 degrees C) is reported. Kinetic data were collected by stopped-flow and conventional UV-vis spectroscopies and processed by the global analysis method. The disproportionation, which follows the stoichiometry 3Cr(V) --> 2Cr(VI) + Cr(III) (1), leads to release of 5 mol of H+/3 mol of Cr(V). Reaction 1 is accelerated by phosphate, but is not affected by acetate, HEPES, or Tris buffers. Initial rates of Cr(V) decay are directly proportional to [Cr(V)]0 (0.020-1.0 mM); they increase with an increase in the pH values and decrease in the presence of a large excess of ehba ligand. The first direct evidence for the formation of Cr(IV) intermediates in reaction 1 has been obtained; however, their UV-vis spectral properties were different from those of the well-characterized Cr(IV)-ehba complexes. The Cr(III) products of reaction I in phosphate buffers differ from those in the other buffers. A mechanism is proposed for reaction 1 on the basis of kinetic modeling. Influences of the reaction time and conditions on the extent of plasmid DNA cleavage induced by 1 have been studied under conditions corresponding to those of the kinetic studies. A comparison of the kinetic and DNA cleavage results has shown that direct interaction of 1 with the phosphate backbone of DNA is the most likely first step in the mechanism of DNA cleavage in neutral media. Small additions of Mn(II) ((0.01-0.1)[Cr(V)]0) did not affect the rate and stoichiometry of reaction 1, but suppressed the formation of Cr(IV) intermediates (presumably due to the catalysis of Cr(IV) disproportionation). However, much higher concentrations of Mn(II) ((0.1-1.0)[Cr(V)]0) were required to inhibit DNA cleavage induced by 1. Thus, contrary to previous reports (Sugden, K. D.; Wetterhahn, K. E. J. Am. Chem. Soc. 1996, 118, 10811-10818), inhibition by Mn(II) does not indicate a key role of Cr(IV) in Cr(V)-induced DNA cleavage.     

Reactivity of chromium(III) nutritional supplements in biological media: an X-ray absorption spectroscopic study

Chromium(III) nutritional supplements are widely used due to their purported ability to enhance glucose metabolism, despite growing evidence on low activity and the potential genotoxicity of these compounds. Reactivities of Cr(III) complexes used in nutritional formulations, including [Cr3O(OCOEt)6(OH2)3](+) (A), [Cr(pic)3] (pic=2-pyridinecarboxylato(-) (B), and trans-[CrCl2(OH2)4](+) (CrCl3.6H2O; C), in a range of natural and simulated biological media (artificial digestion systems, blood and its components, cell culture media, and intact L6 rat skeletal muscle cells) were studied by X-ray absorption near-edge structure (XANES) spectroscopy. The XANES spectroscopic data were processed by multiple linear-regression analyses with the use of a library of model Cr(III) compounds, and the results were corroborated by the results of X-ray absorption fine structure spectroscopy and electrospray mass spectrometry. Complexes A and B underwent extensive ligand-exchange reactions under conditions of combined gastric and intestinal digestion (in the presence of a semisynthetic meal, 3 h at 310 K), as well as in blood serum and in a cell culture medium (1-24 h at 310 K), with the formation of Cr(III) complexes with hydroxo and amino acid/protein ligands. Reactions of compounds A-C with cultured muscle cells led to similar ligand-exchange products, with at least part of Cr(III) bound to the surface of the cells. The reactions of B with serum greatly enhanced its propensity to be converted to Cr(VI) by biological oxidants (H2O2 or glucose oxidase system), which is proposed to be a major cause of both the insulin-enhancing activity and toxicity of Cr(III) compounds (Mulyani, I.; Levina, A.; Lay, P. A. Angew. Chem. Int. Ed. 2004, 43, 4504-4507). This finding enhances the current concern over the safety of consumption of large doses of Cr(III) supplements, particularly [Cr(pic)3].     

Reactions of 2-(1-carboxyl-2-hydroxyphenyl)thiazolidine with chromium(III) salts leading to newer synthesis of chromium(III) complexes

Reactions of 2-(L-carboxyl-2-hydroxyphenyl)thiazolidine with different chromium(III) salts [CrCl₃?·?6H₂O, K₃[Cr(SCN)₆], NH₄[Cr(NH₃)₂(SCN)₄]?·?H₂O, [Cr(urea)₆]Cl₃?·?3H₂O and [Cr(CH₃COO)₂H₂O]₂] under varied reaction conditions afforded many new mixed-ligand chromium(III) complexes. The ligand is a tridentate dibasic NSO donor except for complexes 1 and 4 where two moles of the ligand are present for each molecule of complex, one functioning as a dibasic tridentate (NSO) and the other as a monobasic bidentate (NS) (phenolic OH and carboxylic COOH groups remaining uncoordinated). The complexes have been characterized by elemental analyses, magnetic susceptibilities, molar conductances, molecular weights and spectroscopic (IR, Uv-vis) data. The ligand field parameters and NSH Hamiltonian parameters suggest tetragonal geometries of the complexes.     

Synthesis,characterization, and spectroscopic studies of tetradentate Schiff base chromium(III) complexes

Eight chromium(III) complexes of tetradentate Schiff bases have been prepared in situ by condensing of a substituted salicylaldehyde compound with ethylenediamine. These were characterized by elemental analysis, m.p., IR, molar conductivity, magnetic moment measurements, and electronic spectra. The free ligands were also characterized by ¹H and ¹³C NMR spectra. The ¹³C NMR spectra are discussed in terms of possible substituent effects. The IR and electronic spectra of the free ligand and the complexes are compared and discussed. The electrospray ionization (ESI) mass spectra of four free ligands and their complexes were measured. The deconvolution of the visible spectra of the complexes, C_2v symmetry, in DMSO yields three peaks at ca. 15 600–17 600, 18 400–20 400 and 20 000–23 100, and are assigned to the three d–d transitions, ⁴B_1g → ⁴E_g(⁴T_2g); ⁴B_1g → ⁴B_2g(⁴T_2g); ⁴B_1g → ⁴E_g(⁴T_1g), respectively. The complexes showed magnetic moment in the range of 3.5–4.2 BM which corresponds to three unpaired electrons.     

Hard and soft X-ray absorption spectroscopic investigation of aqueous Fe(III)-hydroxamate siderophore complexes

Microorganisms release organic macromolecules, such as siderophores, to obtain Fe(III) from natural systems. While the relative stabilities of Fe(III)-siderophore complexes are well-studied, the structural environments of Fe(III) and ligands in the complex are not well-understood. Using the X-ray absorption spectroscopy (XAS) at the Fe- and N-K absorption edges, we characterized the nature of Fe(III) interactions with a hydroxamate siderophore, desferrioxamine B (desB), and its small structural analogue, acetohydroxamic acid (aHa), as a function of pH (1.4-11.4). These experimental studies are complemented with DFT calculations. The Fe-XAS studies suggest that Fe(aHa)3 is the dominant species in aqueous solutions in the pH range of 2.8-10.1, consistent with thermochemical information. However, the N-XAS and resonance Raman studies show that the chemical state of the ligand in the Fe(aHa)3 complex changes significantly with pH, and these variations are correlated with further deprotonation of the Fe(aHa)3 complex. The N-XAS studies also indicate that the overlap of Fe 3d orbitals with the molecular orbitals of the hydroxamate group is significant. The Fe- and N-XAS studies of Fe(III)-desB complexes indicated that Fe(desB)+ is the dominant species between pH values of 1.4 and 11.4, consistent with predicted stability constants. This information is useful in understanding the role of iron in bacterial transport, siderosis treatment, and actinide sequestration at contaminated sites. This is the first N-XAS study of aqueous metal ligand complexes, which demonstrates the applications of soft-XAS in studying the electronic structure of metal complexes of organic macromolecules in aqueous solutions.     

X-ray absorption fine structure spectroscopic studies of Octakis(DMSO)lanthanoid(III) complexes in solution and in the solid iodides

Octakis(DMSO)lanthanoid(III) iodides (DMSO = dimethylsulfoxide), [Ln(OS(CH3)2)8]I3, of most lanthanoid(III) ions in the series from La to Lu have been studied in the solid state and in DMSO solution by extended X-ray absorption fine structure (EXAFS) spectroscopy. L3-edge and also some K-edge spectra were recorded, which provided mean Ln-O bond distances for the octakis(DMSO)lanthanoid(III) complexes. The agreement with the average of the Ln-O bond distances obtained in a separate study by X-ray crystallography was quite satisfactory. The crystalline octakis(DMSO)lanthanoid(III) iodide salts have a fairly broad distribution of Ln-O bond distances, ca. 0.1 A, with a few disordered DMSO ligands. Their EXAFS spectra are in excellent agreement with those obtained for the solvated lanthanoid(III) ions in DMSO solution, both of which show slightly asymmetric distributions of the Ln-O bond distances. Hence, all lanthanoid(III) ions are present as octakis(DMSO)lanthanoid(III) complexes in DMSO solution, with the mean Ln-O distances centered at 2.50 (La), 2.45 (Pr), 2.43 (Nd), 2.41 (Sm), 2.40 (Eu), 2.39 (Gd), 2.37 (Tb), 2.36 (Dy), 2.34 (Ho), 2.33 (Er), 2.31 (Tm), and 2.29 A (Lu). This decrease in the Ln-O bond distances is larger than expected from the previously established ionic radii for octa-coordination. This indicates increasing polarization of the LnIII-O(DMSO) bonds with increasing atomic number. However, the S(1s) electron transition energies in the sulfur K-edge X-ray absorption near-edge structure (XANES) spectra, probing the unoccupied molecular orbitals of lowest energy of the DMSO ligands for the [Ln(OS(CH3)2)8](3+) complexes, change only insignificantly from Ln = La to Lu. This indicates that there is no appreciable change in the sigma-contribution to the S-O bond, probably due to a corresponding increase in the contribution from the sulfur lone pair to the bonding.     

Synthesis and spectroscopic studies of homo- and heteroleptic N-arylsalicylaldiminates of titanium(IV), zirconium(IV) and chromium(III)

Homo- and heteroleptic N-arylsalicylaldiminate derivatives of Ti^IV and Zr^IV of the type, MX_4–x (OC₆H₄CH=NAr) _x (X = OPrⁱ, x = 2,3; X = Cl, x = 1,2,3,4; Ar = C₆H₃Me₂-2,6, C₆H₃Et₂-2,6) have been prepared by reactions in the desired molar ratios of: (i) Ti(OPrⁱ)₄/Zr(OPrⁱ)₄·PrⁱOH with N-arylsalicylaldimines in benzene, and (ii) MCl₄ (M = Ti, Zr) with Me₃SiOC₆H₄CH=NAr or HOC₆H₄CH=NAr in the presence of Et₃N as a base or the potassium salt of N-arylsalicylaldimines in benzene. The three homoleptic derivatives of Cr^III, Cr(OC₆H₄CH=NAr)₃ (Ar = C₆H₂Me₃-2,4,6, C₆H₃Et₂-2,6, C₆H₃Prⁱ ₂-2,6) have also been prepared by salt-elimination. All of these new derivatives have been characterized by elemental analyses, spectroscopic [i.r., ¹H and ¹³C-n.m.r. (Ti and Zr complexes), and electronic (for Cr complexes)] studies, as well as molecular weight measurements.     

Kinetics of cerium(IV) oxidation of macrocyclic complexes of chromium(III) and fate of chromium(IV) intermediates

Kinetics and mechanism of the cerium(IV) oxidation of Cr(III) complexes of a series of macrocyclic (or pseudomacrocyclic) ligands with [14]-membered intraligand ring-sizes have now been investigated at I = 1.0 M (LiClO₄) Temp. 30°C. The complexes of the formulation Cr(macrocycle)(X)(H₂O) where X = CHCl₂ and H₂O, n = 0 or 1 undergo oxidation to Cr(VI) with the formation of chromium(IV) intermediates. The observed kinetic parameters for the Ce(IV) oxidation of Cr(III) macrocyclic complexes have been discussed in terms of changes brought about by the macrocyclic ligands on the Cr(III)—Cr(IV) redox potentials and in specific rates for Cr(IV)—Cr(V) conversion. On the basis of this study, it has been suggested that the trapping of Cr(IV) is easier when a macrocyclic ligand having a symmetrical intra-ligand ring size and unsaturation in the cyclic structure is coordinated equatorially. Cyclic voltammetric studies indicate the formation of Cr(IV) transient in the case of electrochemical oxidation of trans-Cr(Me₄[14]tetraene)(H₂O).     

Novel p-tolylimido rhenium(V) complexes of imidazole-derived ligand - Synthesis, X-ray studies, spectroscopic characterization and DFT calculations

Three novel imido rhenium complexes of biologically relevant ligand 2-hydroxymethylbenzimidazole: [Re(p-NC₆H₄CH₃)Cl₂(hmbzim)(PPh₃)]·CHCl₃ (1), [Re(p-NC₆H₄CH₃)Br₂(hmbzim)(PPh₃)] (2) and [Re(p-NC₆H₄CH₃)(hmbzim)₂(PPh₃)]ReO₄·MeOH (3) have been synthesized and characterized spectroscopically and structurally (by single-crystal X-ray diffraction). The electronic spectra of 1 and 3 were investigated at the TDDFT level employing B3LYP functional in combination with LANL2DZ. Additional information about binding in the complexes 1 and 3 was obtained by NBO analysis, which confirms a linear coordination mode of the p-NC₆H₄CH₃ ligand and triple bond between the rhenium and imido ligand.     

Coordination chemistry of di-2-pyridylketone. Synthesis, spectroscopic investigations, X-ray studies and DFT calculations of Re(III) and Re(V) complexes

The paper presents a combined experimental and computational study of Re(III) and Re(V) complexes containing di-2-pyridylketone and its gem-diol form – [ReCl₃(dpk-N,O)(PPh₃)] (1), [ReCl₃(dpk-N,N′)(OPPh₃)] (2) and [ReOBr₃(dpk-OH)]·2(dpkH⁺Br⁻) (3). All the complexes have been characterized spectroscopically and structurally (by single-crystal X-ray diffraction). The complex 2 has been additionally studied by magnetic measurement. The magnetic behavior of 2 is characteristic of mononuclear octahedral Re(III) complex with d⁴ low-spin (³T_1g ground state) and arise because of the large spin–orbit coupling (ζ = 2500 cm⁻¹), which gives diamagnetic ground state. DFT and time-dependent (TD)DFT calculations have been carried out for [ReCl₃(dpk-N,N′)(OPPh₃)] and [ReOBr₃(dpk-OH), and their UV–vis spectra have been discussed on this basis.     

Synthesis and spectroscopic studies on iron(III) complexes of 1-benzotriazol-1-yl-1-[(p-X-phenyl)hydrazono]propan-2-one

A new series of iron(III) complexes are synthesized from the reaction of the polyfunctional ligands 1-benzotriazol-1-yl-1-[p-X-phenyl]hydrazono]propan-2-one (X=H, Cl, NO(2), CH(3) or OCH(3) corresponding to HL(1),HL(2), HL(3), HL(4) or HL(5), respectively, with iron(III) chloride in the presence of LiOH by the conventional and microwave induced energy methods. The conventional method led to the formation of [FeL(3)].nH(2)O but the microwave induced energy gave [FeLCl(2)], n=1-3 and L is the anion of HL(1)-HL(5). The complexes are characterized by the elemental analysis, molar conductivity, magnetic and spectral (FT-IR, UV-vis and ESR) studies. The magnetic and spectral studies showed that [FeLCl(2)] are polymeric octahedral, [Fe(L(1))(3)].H(2)O is a low spin octahedral and (d(xz),d(yz))(4) (d(xy))(1) ground state, [FeL(3)].nH(2)O, L=anion of HL(4) or HL(5) and are octahedral with intermediate spin (S=32) with ground state (d(xy))(2)(d(xz),d(yz))(3) electronic configuration while for the anions of HL(2) and HL(3), they have (t(2g))(3)(e(g))(5) admixed with (d(xy))(2)(d(xz),d(yz))(3) configurations. From the ESR data, the contribution of the high spin (S=52) and low spin (S=32) to the quantum mechanical spin intermediate (QMS), and the crystal field parameters Delta and V are calculated and related to the electronic and steric effects of the ligands. The electronic spectral data confirm that obtained from the ESR, and the different ligand field parameters as well as the pi-->t(2g), t(2g)-->e(g), e(g)-->pi*, pi-->pi* transitions are estimated and compared with that experimentally obtained.     

Electrochemical and spectroscopic studies of neptunium(VI), -(V) and -(IV) in carbonate-bicarbonate buffers

The behavior of neptunium(VI), -(V) and -(IV) in carbonate-bicarbonate buffers (1 M) has been examined electrochemically and spectroscopically. Np(VI) undergoes a one-electron quasi-reversible reduction to Np(V) at +0.22 V vs SSCE Np(V) is reduced irreversibly to Np(IV) at ca. −1.8 V. Although Np(V) is readily oxidized to Np(VI), no oxidation of Np(IV) was observed. Spectroscopic results indicate that the Np(VI)-(V) reaction involves one reactant and one product. The spectrum of Np(IV) in 1 M carbonate-bicarbonate has been quantitatively analyzed. Comparisons to results for uranium under similar conditions are presented.