Covalency Parameters Research Articles

Comparative absorption spectroscopy involving 4f–4f transitions to explore the kinetics of simultaneous coordination of uracil with Nd(III) and Zn(II) and its associated thermodynamics

The interaction of uracil with Nd(III) has been explored in presence and absence of Zn(II) using the comparative absorption spectroscopy involving the 4f–4f transitions in different solvents. The complexation of uracil with Nd(III) is indicated by the change in intensity of 4f–4f bands expressing in terms of significant change in oscillator strength and Judd–Ofelt parameters. Intensification of this bands became more prominent in presence of Zn(II) suggesting the stimulative effect of Zn(II) towards the complexation of Nd(III) with uracil. Other spectral parameters namely Slator–Condon ( F k 's), nephelauxetic effect ( β), bonding ( b 1/2) and percent covalency ( δ) parameters are computed to correlate their simultaneous binding of metal ions with uracil. The sensitivities of the observed 4f–4f transitions towards the minor coordination changes around Nd(III) has been used to monitor the simultaneous coordination of uracil with Nd(III) and Zn(II). The variation of intensities (oscillator strengths and Judd–Ofelt parameters) of 4f–4f bands during the complexation has helped in following the heterobimetallic complexation of uracil. Rate of complexation with respect to hypersensitive transition was evaluated. Energy of activation and thermodynamic parameters for the complexation reaction were also determined.

Nickel(ii) complexes of tripodal 4N ligands as catalysts for alkane oxidation using m-CPBA as oxidant: ligand stereoelectronic effects on catalysis

Several mononuclear Ni(II) complexes of the type [Ni(L)(CH(3)CN)(2)](BPh(4))(2) 1-7, where L is a tetradentate tripodal 4N ligand such as N,N-dimethyl-N',N'-bis(pyrid-2-ylmethyl)ethane-1,2-diamine (L1), N,N-diethyl-N',N'-bis(pyrid-2-ylmethyl)ethane-1,2-diamine (L2), N,N-dimethyl-N'-(1-methyl-1H-imidazol-2-ylmethyl)-N'-(pyrid-2-ylmethyl)ethane-1,2-diamine (L3), N,N-dimethyl-N',N'-bis(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-diamine (L4), N,N-dimethyl-N',N'-bis(quinolin-2-ylmethyl)ethane-1,2-diamine (L5), tris(benzimidazol-2-ylmethyl)amine (L6) and tris(pyrid-2-ylmethyl)amine (L7), have been isolated and characterized using CHN analysis, UV-Visible spectroscopy and mass spectrometry. The single-crystal X-ray structures of the complexes [Ni(L1)(CH(3)CN)(H(2)O)](ClO(4))(2) 1a, [Ni(L2)(CH(3)CN)(2)](BPh(4))(2) 2, [Ni(L3)(CH(3)CN)(2)](BPh(4))(2) 3 and [Ni(L4)(CH(3)CN)(2)](BPh(4))(2) 4 have been determined. All these complexes possess a distorted octahedral coordination geometry in which Ni(II) is coordinated to four nitrogen atoms of the tetradentate ligands and two CH(3)CN (2, 3, 4) or one H(2)O and one CH(3)CN (1a) are located in cis positions. The Ni-N(py) bond distances (2.054(2)-2.078(3) Å) in 1a, 2 and 3 are shorter than the Ni-N(amine) bonds (2.127(2)-2.196(3) Å) because of sp(2) and sp(3) hybridizations of the pyridyl and tertiary amine nitrogens respectively. In 3 the Ni-N(im) bond (2.040(5) Å) is shorter than the Ni-N(py) bond (2.074(4) Å) due to the stronger coordination of imidazole compared with the pyridine donor. In dichloromethane/acetonitrile solvent mixture, all the Ni(ii) complexes possess an octahedral coordination geometry, as revealed by the characteristic ligand field bands in the visible region. They efficiently catalyze the hydroxylation of alkanes when m-CPBA is used as oxidant with turnover number (TON) in the range of 340-620 and good alcohol selectivity for cyclohexane (A/K, 5-9). By replacing one of the pyridyl donors in TPA by a weakly coordinating -NMe(2) or -NEt(2) donor nitrogen atom the catalytic activity decreases slightly with no change in the selectivity. In contrast, upon replacing the pyridyl nitrogen donor by the strongly σ-bonding imidazolyl or sterically demanding quinolyl/benzimidazolyl nitrogen donor, both the catalytic activity and selectivity decrease, possibly due to destabilization of the intermediate [(4N)(CH(3)CN)Ni-O˙](+) radical species. Adamantane is selectively (3°/2°, 12-17) oxidized to 1-adamantanol, 2-adamantanol and 2-adamantanone while cumene is selectively oxidized to 2-phenyl-2-propanol. In contrast to cyclohexane oxidation, the incorporation of sterically hindering quinolyl/benzimidazolyl donors around Ni(ii) leads to a high 3°/2° bond selectivity for adamantane oxidation. A linear correlation between the metal-ligand covalency parameter (β) and the turnover number has been observed.

Absorption spectroscopic probe to investigate the interaction between Nd(III) and calf-thymus DNA

The interaction between Nd(III) and Calf Thymus DNA (CT-DNA) in physiological buffer (pH 7.4) has been studied using absorption spectroscopy involving 4f–4f transition spectra in different aquated organic solvents. Complexation with CT-DNA is indicated by the changes in absorption intensity following the subsequent changes in the oscillator strengths of different 4f–4f bands and Judd–Ofelt intensity ( T λ ) parameters. The other spectral parameters namely Slator–Condon ( F k 's), nephelauxetic effect ( β), bonding ( b 1/2) and percent covalency ( δ) parameters are computed to correlate with the binding of Nd(III) with DNA. The absorption spectra of Nd(III) exhibited hyperchromism and red shift in the presence of DNA. The binding constant, K b has been determined by absorption measurement. The relative viscosity of DNA decreased with the addition of Nd(III). Thermodynamic parameters have been calculated according to relevant absorption data and Van’t Hoff equation. The characterisation of bonding mode has been studied in detail. The results suggested that the major interaction mode between Nd(III) and DNA was external electrostatic binding.

Studies on oxovanadium(IV), Cr(III), Co(II), Ni(II), and Cu(II) chelates of some bisketimino ligands

The oxovanadium(IV), Cr(III), Ni(II), Co(II), and Cu(II) chelates of some bisketimino Schiff-base ligands (H2L) obtained by condensation of 4-butyryl-3-methyl-1-phenyl-(or-1,3-diphenyl)-2-pyrazolin-5-ones with thiourea were synthesized and characterized by elemental analyses and thermogravimetric analyses, molar conductivities, magnetic susceptibility measurements, mass, infrared, and electronic spectroscopies. Ligand field parameters, such as splitting energy, Racah parameter, spin–orbit coupling constant, and covalency parameter of the Cr(III), Ni(II), and Co(II) chelates were calculated by band-fitting methods. Based on these studies, tetragonally distorted octahedral environment around OV(IV) and Cu(II) and octahedral Cr(III), Ni(II), and Co(II) have been proposed.

Influence of configuration interaction on splitting of multiplets of TmF 6 3− and TmCl 6 3− molecular complexes

The Stark structure of elpasolites activated by Tm 3+ ions is analyzed with the help of standard and modified crystal field theories. Application of the modified approach reduces considerably the root-mean-square deviation between theoretical and experimental energy values. Crystal field and covalence parameters are determined, the latter being in good agreement with the covalence parameters calculated using microscopic models.

An EPR study of Cu 2+ doped [Na(Hmal)(H 2O) 5] single crystals

The magnetic environments of Cu 2+ doped Na + complex have been identified by electron paramagnetic resonance (EPR) technique. The angular variation of the EPR spectra has shown that two different Cu 2+ complexes are located in different chemical environments, and each environment contains one magnetic Cu 2+ site occupying substantial positions in the lattice and showing very high angular dependence. The principal g, and the hyperfine structure parameter ( A) values of two sets of Cu 2+ complex groups are determined. The covalency parameter, mixing coefficients and Fermi-contact term of the complex are obtained, and the ground state wave function of the Cu 2+ ion in the lattice has been constructed.

Microwave assisted synthesis, spectroscopy and biochemical aspects of lanthanum(III) and praseodymium(III) complexes with oxadiazole functionalised dithiocarbazinates

The new lanthanum (III) and praseodymium (III) complexes of the general formula [Ln(L) 3] (Ln=La(III) or Pr(III); LK=potassium salt of dithiocarbazinates) were prepared by both, conventional thermal and by the use of microwave technology. Elemental analyses, electrical conductance, magnetic moment and electronic, infrared, far-infrared, 1H and 13C NMR spectral studies were used to characterize the complexes. The molecular weights of few complexes were determined by FAB-mass spectra. Nephelauxetic ratio, covalency parameter and bonding parameter for these complexes were also calculated. The probable structures of the complexes were proposed. The antifungal and antibacterial activities of the complexes were evaluated. The activities were correlated with the structures of the compounds.

Spectroscopic and antimicrobial studies of La 3+, Pr 3+, Nd 3+ and Gd 3+ complexes of a dipodal [N,N,N] chelating ligand

The lanthanide complexes of bis(benzimidazole-2′-yl-methyl)amine (BImz) having molecular formula [M(BImz)X 3H 2O]· nH 2O (M = La, Pr, Nd, or Gd; X = Cl or ClO 4 and n = 1, 2 or 3) were prepared and characterized spectroscopically through IR, 1H and 13C NMR, FAB-mass, UV–visible and luminescence spectroscopy. TGA data suggested presence of coordinated and the lattice water. The oscillator strengths of the f–f transitions and the covalency parameters ( β, b 1/2 and δ) have been evaluated from the electronic spectral data. The proposed hepta-coordinate geometry for the complexes has been ascertained from the molecular model computations. CV studies indicate formation of a stable quasi-reversible redox couple Gd III/IV in the solution. The in vivo antimicrobial activities of the complexes have been evaluated against gram +ve and gram −ve bacteria and fungi.

Bridging Kohn - Sham DFT and the Angular Overlap Model. Ligand-Field Parameters and Bond Covalencies in Tetrahedral Complexes

Kohn–Sham density functional theory (DFT), constrained by the average-of-configuration computations, allows the valence shell of regular tetrahedral chlorido complexes of a complete series of 3d transition metal ions to be orbitally compared. The concept of classificational parentage provides a handle on the discussion of the energetic ordering of all the valence orbitals and illuminates an almost identical ordering for all the systems. Only the participation of the metal 4s orbital in bonding causes a few minor fluctuations. The partially filled ‘3d’ molecular orbitals sit in an energy window framed by completely filled ‘ligand orbitals’ on the low-energy side and an empty metal ‘4s’ orbital on the high-energy side. Regular tetrahedral symmetry requires the halides to be linearly ligating and this property is stable within the ‘experimental’ uncertainty for small distortions. By lowering the symmetry towards the planar configuration, keeping the equivalence of the ligands stable, the information content of the computations was doubled and the angular overlap energy parameters referring to the individual ligands obtained. The orbital energies of the partially filled shell depend linearly on the Angular Overlap Model (AOM) parameters eλ, the slope being the sum of the squares of the single-ligand λ angular overlaps (λ = σ and π). Mulliken population analysis shows the contents of the appropriate ligand orbitals in the ‘d’ orbitals to vary in parallel with the molecular orbital AOM energies and to increase pronouncedly with the oxidation number z. Results for tetraoxidoferrate(vi) show a remarkable resemblance with the chloride complexes of even the divalent metal ions. However, although the bonding orbitals are more π-bonding, the totally symmetrical bonding orbitals use M_4s less in the oxido complex. The sensitivity of covalency and spectroscopic energy parameters towards radial distortions are examined and show Werner-type complexes and the high-valent FeO42– to behave somewhat differently.

Self-consistent description of the Stark structure of multiplets and intensities of absorption transitions in Pr3+ ions in Y3Al5O12

The Stark structure of multiplets and the oscillator strengths of absorption transitions in Pr3+ ions in a Y3Al5O12 crystal have been described with allowance for the effect of the excited opposite-parity configuration and the charge-transfer configuration. To take into account this effect, a modified crystal-field Hamiltonian and an effective line-strength operator for intermultiplet electric-dipole transitions are proposed. Their use in description of experimental data makes it possible to decrease the standard deviation by 37 and 20%, respectively, in comparison with the standard theories. The even-and odd-crystal-field parameters and the covalence parameters have been obtained from the description of Stark levels. The intensity parameters calculated on the basis of the covalence parameters and odd-crystal-field parameters are in satisfactory agreement with the parameters obtained from the description of the experimental oscillator strengths.

Modified theory of f-f transition intensities and crystal field for systems with anomalously strong configuration interaction

Abstract A new theory explaining the intensity of f-f transitions and the crystal field using an approximation of a strong configuration interaction is proposed. The theory enables the anomalous influence of excited configurations with charge transfer on some multiplets of the f shell to be taken into account. With the help of this theory, a satisfactory description of the absorption transitions and luminescence branching ratios from 1 D 2 and 3 P 0 multiplets for the Pr3+ ion in double molybdates has been achieved for the first time. For further validation the theory, was used to provide a description of Stark splitting of Pr3+ — multiplets in elpasolites and determine the covalence parameters; these parameters were found to be in good agreement with values obtained by the other methods.

Strontium Tetraborate Glasses Doped with Transition Metal Ions: EPR and Optical Absorption Study

Electron paramagnetic resonance (EPR) and optical studies have been carried out on Cu(II)-, VO(II)- and Cr(III)-doped strontium tetraborate glasses to understand the distortion and substitution of these ions. The EPR results of Cu(II) glass indicate that g∥ > g⊥, typical for the tetragonally elongated octahedral site of the Cu(II) impurity. The evaluated covalency parameter 0.788 suggests a moderate covalency for the bonding. By correlating EPR and optical results, the in-plane π-bonding β12 is evaluated as 0.715. In the vanadium-doped glasses, the distortion must be a tetragonally elongated octahedron, similar to Cu(II). However, the EPR studies show that g⊥ > g∥ indicating the tetragonally compressed octahedral site for the ion. The site symmetry is C4V. Supported by the optical absorption, evaluated parameters propose a moderate covalency. The EPR and optical results for Cr(III) glass indicate the distorted octahedral site symmetry in the host lattice. These results further suggest that the bonding between Cr(III) and the ligands is covalent.

Simulation of U4+ ion optical properties in crystalline ZrSiO4

The applicability of crystal field models to describing the spectral properties of uranium is studied. Effective Hamiltonians and operators for adequate simulation are suggested. As a result of description of the energy spectrum of uranium, the parameters of crystal fields of even and odd symmetry, as well as covalence parameters, are obtained. From the covalence parameters and the parameters of the odd crystal field, the intensity parameters for the U4+ ion in ZrSiO4 are calculated.

Lanthanum(III) and prasedymium(III) complexes with piperazine dithiosemicarbazones

Lanthanum(III) and praseodymium(III) complexes of the type [Ln(L)Cl(H2O)]2 (Ln = La(III) or Pr(III); LH2 = dithiosemicarbazone ligands derived from piperazine dithiosemicarbazide and benzaldehyde, 4-nitrobenzaldehyde, and 2-methoxybenzaldehyde) have been synthesized in methanol in the presence of sodium hydroxide. The complexes have been characterized by elemental analyses, molecular weight, molar conductance, electronic absorption, IR, and 1H and 13C NMR spectral studies. Nephelauxetic ratio, covalency parameter, and bonding parameter for these complexes have also been calculated. Thermal studies of the complexes have been carried out using TG, DTG, and DSC techniques. Kinetic parameters, such as apparent activation energy and order of reaction, were determined by the Coats-Redfern graphical method. The heats of reaction for different reaction steps were calculated from DSC curves.

Absorption spectral studies of 4f–4f transitions for the complexation of Pr(III) and Nd(III) with glutathione reduced (GSH) in presence of Zn(II) in different aquated organic solvents and kinetics for the complexation of Pr(III):GSH with Zn(II)

Abstract Comparative absorption and spectral analysis of 4f–4f transitions of the heterobimetallic complexation of glutathione reduced (GSH) with Pr(III) and Nd(III) in presence of Zn(II) has been explored in different aquated organic solvents like methanol (CH 3 OH), acetonitrile (CH 3 CN), dioxane, DMF and their equimolar mixtures. The variation in the energy parameters like Slater Condon ( F K ), Racah ( E K ) and Lande ( ξ 4f ), Nephelauxetic parameter ( β ) bonding parameter ( b 1/2 ) and percent covalency parameter ( δ ) are computed to explain the nature of complexation. The values of experimentally calculated oscillator strengths ( P ) and computed values of Judd–Ofelt electric dipole intensity parameters, T λ ( λ = 2, 4, 6) are studied for different 4f–4f transitions in different experimental conditions. The changes in the values of oscillator strengths ( P ) and T λ parameters also suggest the specific correlation between relative intensities, ligand structures and nature of interaction of Pr(III) and Nd(III) with GSH in presence of Zn(II). Kinetics for the formation of the complex, Pr(III):GSH:Zn(II) have also been studied at different temperatures in DMF medium and activation energy ( E a ) of the complexation is calculated.

Formation constants for complexes of transition-metal cations with O- and N-donor ligands in aqueous solutions

A method has been developed for the theoretical calculation of stability constants for transitionmetal complexes proceeding from molecular properties. An analytical form of the cation-ligand interaction potential including the partial bond covalence has been developed. The potential is expressed through molecular parameters and a covalence parameter; the latter may be determined from experimentally measured or theoretically calculated gas-phase data. The stability constants have been calculated for 1: 1 complexes of divalent metal cations with hydroxide anion, acetic, glycolic, and lactic acid anions, and ammonia in aqueous solutions. The covalence parameters have been calculated for the bonds of metal cations with O-and N-donor ligands.

Stark structure of the energy spectrum of the Am3+ ion in LaCl3

We have analyzed the structure of the energy spectrum of the Am3+ ion in LaCl3 in the weak and intermediate configuration interaction approximation. We have established that in the intermediate configuration interaction approximation, the mean-square deviation of the calculated values of the Stark level energies from the experimental values is 53% smaller than in the weak configuration interaction approximation. We have determined the even and odd crystal field parameters and the covalency parameters. We conclude that based on analysis of the Stark structure of the energy spectrum, we can make quantitative estimates of the intensity characteristics.

Ligand hyperfine coupling in Mo(V) oxyhalide complexes

For a series of Mo(V) oxyhalide complexes MoOL52− (L = F, Cl, Br, I), interpretation of the ligand hyperfine coupling parameters (AL tensor components) is considered. The role of spin polarization is elucidated and the effect of charge transfer states is taken into account. The AL tensor components were calculated using the covalency parameters derived from experimental EPR and optical spectroscopy data for compounds under study (inverse problem of EPR spectroscopy).

Correlation between the intensities of intermultiplet electric dipole transitions and the fine features of the stark structure of multiplets of Pr3+ ions in the LaCl3 compound

The Stark structure of the multiplets of Pr3+ ions in the LaCl3 compound is described and the even and odd parameters of the crystal field, as well as the covalency parameters, are determined in the approximation of an intermediate interconfiguration interaction. The covalency parameters thus obtained agree well with the parameters calculated in the framework of microscopic models. The intensity parameters of intermultiplet electric dipole transitions are calculated using the odd parameters of the crystal field and the covalency parameters. The results of the calculations are in satisfactory agreement with the experimental data. This allows the conclusion that there is a correlation between the fine features of the Stark structure of the multiplets and the intensities of the intermultiplet electric dipole transitions.

Photoluminescence of Europium(III) Dithiocarbamate Complexes: Electronic Structure, Charge Transfer and Energy Transfer

For the first time, we observed photoluminescence in Eu(III) dithiocarbamate complexes at room temperature -- more specifically in [Eu(Et(2)NCS(2))(3)phen], [Eu(Et(2)NCS(2))(3)bpy] and the novel [Eu(Ph(2)NCS(2))(3)phen], where phen stands for 1,10-phenanthroline and bpy for 2,2'-bipyridine. Correlations between the electronic structure of the dithiocarbamate ligands on one hand, and covalency, intensity, and ligand field spectroscopic parameters on the other, could be established. Moreover, the relative values of the emission quantum efficiencies obtained for these complexes, as well as their dependence with temperature, could be satisfactorily described by a theoretical methodology recently developed.