Covalency Parameters Research Articles

Electron paramagnetic resonance studies of Cu 2+ ion in Tetraaqua-di(nicotinamide)Ni(II)-saccharinates single crystals

X-band (∼9.8 GHz) electron paramagnetic resonance (EPR) measurement at ambient temperature in three mutually perpendicular planes have been carried out on a single crystal of Cu 2+ doped mixed ligand complex of Ni(II) with saccharin and nicotinamide [Ni(Nic) 2(H 2O) 4](sac) 2. The angular dependent spectra showed that the Cu 2+ ion enters Ni 2+ sites in the lattice and distorted local environment of Ni 2+ site. The principal g and A values, covalency parameter ( α′ 2), mixing coefficients ( α and β) and Fermi contact term ( K) have been evaluated from the EPR analysis. The ground-state wave function of the Cu 2+ ion has been constructed using the α′ 2, α and β values. The nature of the distortion present in the lattice is obtained from the values of the mixing coefficients.

Transition metal quinone–thiosemicarbazone complexes 2: Magnetism, ESR and redox behavior of iron (II), iron (III), cobalt (II) and copper (II) complexes of 2-thiosemicarbazido-1,4-naphthoquinone

Reactions of 2-thiosemicarbazido-1,4-naphthoquinone (NQTSC) with iron (II), iron (III), cobalt (II) and copper (II) chloride yield different complexes depending on the synthetic conditions. Under acidic medium, thionato coordination is favored with the formation of cationic complexes (A-1 to A-4) while neutral (B-1, B-3 and B-4) and cationic (B-2) complexes are isolated with thiolato binding under basic conditions. IR spectral data indicates that NQTSC acts as both a neutral and a mono-anionic tridentate ligand, bonding through thione, thiol or both as observed in the case of iron (III) and cobalt (III) complexes. The magnetic and Mössbauer measurements of iron (II) complexes reveal the presence of an unusual ( S = 1) spin state possessing small zero-field splitting (ZFS). Reaction of NQTSC with iron (III) chloride results in the stabilization of the spin-triplet state ( S = 3/2) with negligible contribution from ZFS components while cobalt complexes exhibit different magnetic properties displaying diamagnetic (A-3) cobalt (III) and low-spin cobalt (II) states (B-3). Evaluation of ESR covalency parameters for A-4 and B-4 indicates considerable delocalization of the unpaired electron present in the d x 2 - y 2 ground state of the tetragonally distorted octahedral geometry over the entire chelate rings formed by the NQTSC ligands. Electrochemical behavior of the complexes exhibit mostly metal-centered redox changes with the iron (III) complexes having a reversible Fe(III)/Fe(II) couple at relatively positive potentials while the copper (II) compounds display quasi-reversible Cu(II)/Cu(I) as well as Cu(III)/Cu(II) redox processes. Thiolato binding probably exerts a pronounced effect on the oxidation processes while thionato coordination seems to influence reduction steps in these complexes.

Transition metal quinone–thiosemicarbazone complexes 1: Evaluation of EPR covalency parameters and redox properties of pseudo-square-planar copper(II)–naphthoquinone thiosemicarbazones

EPR, optical and redox studies have been carried out on six copper(II) complexes of naphthoquinone–thiosemicarbazones having [CuLCl] stoichiometry to elucidate the electronic structure, nature of metal–ligand bonding and electrochemical features. EPR spectra, simulated with an axial spin-Hamiltonian, exhibit a four-line pattern with nitrogen super-hyperfine couplings originating from imine/hydrazinic nitrogen atoms. These planar complexes possess a significant amount of tetrahedral distortion leading to a pseudo-square-planar geometry as evidenced from EPR and optical properties. The evaluation of covalency parameters suggest that the unpaired electron, present in the d x 2 - y 2 orbital, spends about 42–45% of its time on the nitrogen donor site of the coordinated thiosemicarbazones, reflecting the π-acceptor property of the sulfur center as well as the charge accumulating character of the quinone molecules. The presence of a strong π-interaction leads to extensive delocalization thorough the entire chelate rings formed by the tridentate ligands. Moderate covalency is observed in σ-bonding while in plane π-bonding possess appreciable covalent character. IR spectral data indicate a tridentate ONS donor set for these ligands. A quasi-reversible Cu(II)/Cu(I) redox couple is observed at relatively higher potential (Δ E p = −0.45 to −0.66 V) as a consequence of structural reorganizations, while these complexes exhibit lower redox potentials for the Cu(II)/Cu(III) oxidation process. There exists a linear relationship between the degree of tetrahedral distortion f( α) with spectroscopic and redox parameters.

Determination of intensity parameters from the fine details of the Stark structure in the energy spectrum of Tm3+ ions in Y3Al5O12

Analysis of the structure of the energy spectrum of Tm3+ ions in Y3Al5O12 is performed in terms of the approximation of intermediate-force configuration interaction. The parameters of the even and odd crystal fields and the covalence parameters are determined. When the interconfiguration interaction is taken into account, the description of the energy spectrum becomes more adequate and the root-mean-square deviation decreases by 25%. The intensity parameters calculated using the odd crystal field and covalence parameters are in satisfactory agreement with experimental values. It is concluded that, on the basis of the analysis of the Stark structure of an energy spectrum, the intensity characteristics can be quantitatively estimated.

Electron paramagnetic resonance studies on copper(II)-cyclodextrin systems.

Three copper(II) complexes containing beta-cyclodextrin (betaCD) derivatives as ligands [mono-6-methylamino-6-deoxy-betaCD dithiocarbamate (CDTC), mono-6-histamino-6-deoxy-betaCD (CDHM) and mono-6-Nalpha-arginyl-6-deoxy-betaCD (CDARG)] have been studied by electron paramagnetic resonance. The spectra have been recorded at X- and S-bands and different temperatures and simulated to obtain the best set of magnetic parameters. In particular, the 300 K spectra are typical of the slow motion regime, as expected considering the high molecular weight of the ligands. Some structural characteristics of the complexes are proposed on the basis of dynamic and covalency parameters obtained from simulation.

Variable temperature EPR study for confirming dynamic Jahn–Teller distortion in Cu(II) doped zinc ammonium phosphate hexahydrate

Single crystal EPR rotations done about the three mutually orthogonal axes at 300 and 143 K for copper(II) doped zinc ammonium phosphate hexahydrate indicate that the spin Hamiltonian parameters ( g and A) are highly temperature dependent, whereas their direction cosines are not to that extent. Powder spectra of the sample recorded from 300 to 77 K confirm dynamic Jahn–Teller distortion for Cu(II) ion. Two out of three g/ A values are temperature dependent, whereas the third one almost remains constant in the temperature range. Various Jahn–Teller and covalence parameters are calculated.

Elaboration and characterization of glassy cobalt phosphomolybdates

Selecting two series of P 2O 5-based oxide glasses containing at the same time the MoO 3 and CoO oxides, an investigation of structural properties has been undertaken by infrared (IR) and electron paramagnetic resonance (EPR) spectroscopies. The structure of glasses inside the CoO–MoO 3–P 2O 5 system may be considered as made of several types of molybdenum and phosphorous structural groups, the ratios of which depend on composition. The EPR spectra of Mo(V) ions are characterized by the large anisotropy of g- and A-tensors along the local z axis. The values of the covalency parameters k ‖ and k ⊥ determined for some glasses are correlated to the axial distortion of (Mo VO 6) octahedron along the z axis. EPR spectra carried out at lower temperatures show additional broad line near g≈4 and a narrow one at g≈2. The former is attributed to the HS form of cobalt(II) in octahedral site. The latter is associated with LS electron configuration of Co 2+ ions and/or Mo 5+ centers. The existence of Co 2+ in octahedral environment in both HS and LS forms is interpreted in term of a spin-crossover for Co(II) in these glasses.

4 f–4 f hypersensitivity in the absorption spectra and NMR studies on paramagnetic lanthanide chloride complexes with 1,10-phenanthroline in non-aqueous solutions

The optical absorption and NMR studies of trivalent lanthanide chloride complexes with 1,10-phenanthroline (phen) are presented and discussed. The 1H NMR spectra of the complexes of La, Pr, Nd, Eu and Yb have been studied in methanol- d 4. The resonances of phen in the NMR spectra of the paramagnetic complexes have been shifted to lower as well as higher fields, which is a manifestation of dipolar interaction. The H (2) protons of the heterocyclic amine display broad resonances. The degree of broadening in Pr, Nd, and Yb complexes follows the order Pr<Nd<Yb. The inter- and intra-molecular shift ratios show that the paramagnetic shift is predominantly due to dipolar interaction. The electronic spectra of Pr, Nd, Ho and Er complexes have been investigated in methanol, pyridine, DMSO and DMF, which reveal that the hypersensitive transitions exhibit larger variation in oscillator strength values and band shapes. The change in the coordination geometry of the complexes and relative basicity of ligand are found responsible for oscillator strength and band shape variation. The interelectronic repulsion and covalency parameters show covalent nature of bonding between the metal and the ligand.

YTTRIUM AND LANTHANIDE NITRATE COMPLEXES OF 2,3-DIMETHYL-4-FORMYL(BENZ HYDRAZIDE)-1-PHENYL-3-PYRAZOLINE-5-ONE

ABSTRACT Complexes of yttrium and lanthanide nitrates with the Schiff base 2,3-dimethyl-4-formyl(benzhydrazide)-1-phenyl-3-pyrazoline-5-one (L) have been synthesized and characterized by elemental analyses, electrical conductance in non-aqueous solvents and electronic as well as infrared spectra. The complexes have the general molecular formulae [Ln(L)2(NO3)]-(NO3)2 (Ln=Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho or Er). The ligand chelates with the metal ion in a neutral tridentate fashion through both carbonyl oxygens and the azomethine nitrogen in all of these complexes. One of the nitrate ions is monodentately coordinated and the other two remain as counter ions. A coordination number of seven is assigned to the metal in all of these complexes. The covalency parameters evaluated from solid and solution phase electronic spectra suggest weak covalent character of the metal-ligand bond.

Spectroscopic studies of novel porphyrin-copper(II) and zinc(II) complexes that share the pinch-porphyrin family structure of iron(III) complex models of peroxidases

Six novel pinch-porphyrin complexes [(picdien)(protoporphyrinate dimethyl ester)]copper(II) (7), [(picdien)(mesoporphyrinate dimethyl ester)]copper(II) (8) and [(picdien)(deuteroporphyrinate dimethyl ester)]copper(II) (9), [(picdien)(protoporphyrinate dimethyl ester)]zinc(II) (13), [(picdien)(mesoporphyrinate dimethyl ester)]zinc(II) (14) and [(picdien)(deuteroporphyrinate dimethyl ester)]zinc(II) (15), were prepared from the corresponding free copper(II)-porphyrins (4–6), and zinc(II)-porphyrins (10–12) and picdien (N-(3H-imidazol-4-ylmethyl)-N′-{2-[(3H-imidazol-4-ylmethyl)-amino]-ethyl}-ethane-2,3-diamine). Spectroscopic studies show that complexes (7–9) and (13–15) have the pinch-porphyrin type structure previously found in iron(III) complex models of peroxidases. Complexes (7–9), were characterized by u.v.–vis., m.c.d., and e.s.r. spectroscopy. E.s.r. spectra of the copper parent compounds (4–6) at ca. 10−2–10−4 M concentrations were typical of copper(II)-dimers. The addition of the picdien ligand broke up the dimers as detected by e.s.r. Compounds (7–9) are predominantly monomeric at ca. 10−3 M concentration. The presence of picdien in (7–9) distorts the porphyrin internal portion of the plane so as to make these four internal nitrogen atoms, coordinated to copper(II), e.s.r.-distinguishable. MO and ligand field theories were used to characterize and to evaluate the directional covalence parameters of compounds (7–9). A non-fully axial, out-of-the-porphyrin-plane bonding was found for (7–9), similar to the bonding of the pinch-porphyrins-iron(III). However the in-plane distortion produced by the presence of the picdien ligand on copper(II) is significantly larger than in pinch-porphyrin-iron(III). The n.m.r. data show that the porphyrin-zinc(II) is the less strained and has the weakest bonded structure. The coordination number of the pinch-porphyrin with iron(III), copper(II) and zinc(II), is in all cases six.

Synthesis and physiochemical studies on binuclear Cu(II) complexes derived from 2,6-[(N-phenylpiperazin-1-yl)methyl]-4-substituted phenols

Preparation of the ligands HL1 = 2,6-[(N-phenylpiperazin-1-yl)methyl]-p-ethylphenol; HL2 = 2,6-[(N-phenylpiperazin-1-yl)methyl]-p-methoxyphenol and HL3 = 2,6-[(N-phenylpiperazin-1-yl)methyl]-p-nitrophenol are described together with their Cu(II) complexes with different bridging units. The exogenous bridges incorporated into the complexes are: hydroxo [Cu2L(OH)(H2O)2](ClO4)2.H2O (L1=1a, L2 =1b, L3 =1c), acetato [Cu2L(OAc)2]ClO4.H2O (L1 =2a, L2 =2b, L3 =2c) and nitrito [Cu2L1(NO2)2(H2O)2]ClO4.H2O (L1=3a, L2 =3b, L3 =3c). Complexes1a,1b,1c and2a,2b,2c contain bridging exogenous groups, while3a,3b,3c possess only open μ-phenolate structures. Both the ligands and complexes were characterized by spectral studies. Cyclic voltammetric investigation of these complexes revealed that the reaction process involves two successive quasireversible one-electron steps at different potentials. The first reduction potential is sensitive to electronic effects of the substituents at the aromatic ring of the ligand system, shifting to positive potentials when the substituents are replaced by more electrophilic groups. EPR studies indicate very weak interaction between the two copper atoms. Various covalency parameters have been calculated.

Chiral lanthanide(III) complexes of sulphur–nitrogen–oxygen ligand derived from aminothiourea and sodium D-camphor-β-sulfonate

Six new chiral lanthanide complexes derived from sodium D-camphor-β-sulfonate and aminothiourea with Ln=La 3+,Pr 3+,Nd 3+,Sm 3+,Eu 3+ and Gd 3+ of the type LnL 3 have been synthesized and characterized on the bases of their elemental analysis, molar conductance, IR, XPS, CD, electronic spectra, magnetic moments and thermal analysis methods. It was found that NaL acts as an SNO tridentate ligand. The visible spectra of Pr III and Sm III show characteristic f–f transitions and the nephelauxetic parameter ( β), the bonding parameter ( b 1/2) and covalency parameter ( δ) of these transitions has been evaluated. These data indicate the weak involvement of the 4f orbitals in complex formation. A nine-coordinated model has been proposed for these complexes.

Explanations of the absorption spectra and their pressure coefficients for ZnSe : Ni 2+ crystal

The optical absorption spectra and their pressure coefficients for ZnSe : Ni 2+ crystal have been calculated by diagonalizing the energy matrices based on a modified crystal-field theory. In the matrices, the covalency parameters N t, N e and the spin–orbit coupling coefficients ζ tt, ζ te are obtained from a cluster approach including only one adjustable parameter. The calculated optical spectra are in agreement with the observed values. The calculated pressure coefficients for T 2– 3T 2( 3F) and T 2– 1T 2( 1G) bands under low and high pressures are also consistent with the observed values. The differences of pressure coefficients under low and high pressures for T 2– 3A 2( 3F) and T 2– 3T 1( 3P) bands are analyzed and discussed.

Electronic States of Perovskite-Type Oxides and Ferroelectricity

A relation between B-site transition metal and ferroelectricity is investigated in ABO3 perovskite-type oxides. We focus on intensity of the covalent bonds between the B-site atom and the oxygen octahedron and evaluate Harrison's covalency parameter α. The density functional theory within the local density approximation is adopted to obtain the valence electron energy levels of free atoms from which the covalency α is calculated. Strong correlation between the covalency α and the Curie temperature (TC) is found. As the covalency α increases, TC of ferroelectricity arises. It is also noted that the hybridization between Pb 6p and O 2p is crucial for the high ferroelectricity of PbTiO3.

Rare Earth Iodide Complexes of 4-Formyl-2, 3-Dimethyl-l-Phenyl-3-Pyrazolin-5-One

Rare earth complexes of 4-lbrmyl-2, 3-dimethyl-l-phenyl-3-pyrazolin-5-onc (FDPP) having the general formula [Ln(FDPP)4I2]I, where Ln = Y. La, Pi, Nd. Sm, Eu, Gd, Dy, Ho and Er, have been synthesised and characterised by elemental analyses, molar conductance in non-aqueous solvents, electronic, infrared and proton NMR spectra as well as thermogravimetric analyses. FDPP acts as a neutral monodentate ligand coordinating through the ring carbonyl oxygen. Two of the iodide ions are coordinated. A coordination number of six may be assigned to the metal ion in these complexes. The covalency parameters evaluated from the solid state electronic spectra suggest weak covalent character of the metal-ligand bond. The TG data of the lanthanum complex indicate that the complex is stable up to about 140°C and undergoes decomposition in three stages forming lanthanum oxide as the final product.

Yttrium and Lanthanide Nitrate Complexes of 4-Formyl-2, 3-Dimethyl-1-Phenyl-3-Pyrazolin-5-One

Complexes of yttrium and lanthanide nitrates with 4-formyl-2, 3-dimethyl-1-phenyl-3-pyrazolin-5-one (FDPP) having the general formula [Ln(FDPP)2(NO3)2]NO3, where Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho and Er have been synthesised and characterised by elemental analyses, molar conductance in non-aqueous solvents, electronic, infrared and proton NMR spectra as well as thermogravimetric analyses. FDPP acts as a neutral monodentate ligand, coordinating through the ring carbonyl oxygen and two of the nitrate ions are coordinated in a bidentate manner. A. coordination number of six may be assigned to the metal ion in these complexes. The covalency parameters evaluated from the solid state electronic spectra suggest weak covalent character of the metal-ligand bond. The TG data of the lanthanide complexes indicate that the complexes are stable up to about 170°C. and undergo decomposition in two stages forming the respective anhydrous lanthanide oxide as the final product.

Yttrium and lanthanide iodide complexes of N,N′-bis(4-antipyrylmethylidene)ethylenediamine

Complexes of yttrium and lanthanide iodides with N,N′-bis(4-antipyrylmethylidene)ethylenediamine (BAME) having the general formula [Ln(BAME) 3I]I 2 (where Ln=Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho and Er) have been synthesised and characterised by elemental analyses, molar conductance in non-aqueous solvents, electronic, infrared and proton NMR spectra as well as thermogravimetric analyses. BAME acts as a neutral bidentate ligand coordinating through both azomethine nitrogen atoms. One of the iodide ions is coordinated. A coordination number of 7 may be assigned to the metal ion in these complexes. The covalency parameters evaluated from the solid state electronic spectra suggest weak covalent character of the metal-ligand bond. The TG data of the lanthanum complex indicate that the complex is stable up to about 145°C and undergoes decomposition in two stages forming lanthanum oxide as the final product.

Optical Spectroscopy of Mn2+ Ions in CaF2

Excitation and emission spectra of Mn2+ ions in calcium fluoride have been studied using the photoluminescence technique. An analysis of the positions of the Mn2+ crystal-field transitions with aid of the model developed by Curie et al. was performed to determine the relevant crystal-field parameters and the covalency parameters f_s and fp of the Mn2+ ions entering substitutionally for Ca2+ ions in an Oh symmetry cubic environment. The As isotropic and Ap anisotropic superhyperfine coupling constants of Mn2+ in CaF2 are also calculated and compared with those previously measured through the use of EPR technique.

Bonding behavior of metal atoms on Si surfaces

In this paper we suggest a quantitative approach for description of the bonding behavior of the individual metal atoms on Si surfaces. It is proposed to use the relative contribution of electronic and ionic components of the effective polarizability of the metal atom to characterize the type of bond. Individual As, Sb, Na atoms adsorbed on $\mathrm{Si}(001)2\ifmmode\times\else\texttimes\fi{}1$ surface and K atoms adsorbed on both $\mathrm{Si}(001)2\ifmmode\times\else\texttimes\fi{}1$ and $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ surfaces were studied. It was found that the bonding behavior of the potassium on these two surfaces is completely different. The covalency parameters calculated according to this approach allow one to define the As-Si and Sb-Si bonds as almost pure covalent, Na-Si bond as polarized covalent, K-Si bond in the $\mathrm{Si}(001)2\ifmmode\times\else\texttimes\fi{}1$ surface as largely ionic, while in the $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ surface it is predominantly covalent.

Covalency engineering through alloying with beryllium chalcogenides in wide band-gap II–VI crystals

Defects are at the origin of a device problem in “classical” IIB–VI wide-bandgap compounds, e.g., the elusive “longevity” of [(Zn,Cd)-VI]-based blue lasers. Based on a new review of literature data, this work follows our first proposals of novel Be-chalcogenides potentialities (1994, 1995) to obviate it. We identify the crystal shear modulus Cs=(C11–C12)/2 as a structural signature of the epitaxial layer lattice and perform an improved semi-empirical calculation of Cs vs a definite Harrison’s covalency parameter deduced from his LCAO model. As a result, this leads to three observations: (i) As noticed in our previous papers, from the Phillips-Van Vechten ionicity scale the Be-VI compounds have a unique strong covalency degree in the II–VIs, even higher than in GaAs; (ii) the Phillips and Harrison covalency parameters are shown to compare reasonably well if proper scaling is used; (iii) (Zn,Cd)-chalcogenides are characterized by a strong ionicity which induces low Cs-values, leading to a number of defects. As we proposed before, this work confirms the possibility of achieving an elastic rigidity reinforcement of the lattice in (Zn,Cd)-VI materials through alloy Be-substitution, paving the way to what we call a “covalency engineering” in the II–VI family. We discovered previously the possibility of Be-based alloys that are Si lattice-matched. We briefly describe the Si substrate innovative utilizations for heteroepitaxy of electronic and blue or UV devices stressing salient potential applications of the covalency engineering in II–VI blue lasers for acceptable longevities.