Covalency Parameters Research Articles

Metal complexes of phenobarbituric acid. Chelating behavior of the phenobarbiturate ring. Anticonvulsant properties of the K 2[Cu(N-methylphenobarbiturato)] 4 · 8H 2O

Na 2Ni(phenobarbiturato) 4·3H 2O, Na 2Ni 3(phenobarbiturato) 2(OH) 6·4H 2O, and NaZn(phenobarbiturato) 2(OH)·H 2O derivatives were prepared from Ni(II) and Zn(II) and phenobarbital. The Na 2Ni(phenobarbiturato) 4·3H 2O complex is diamagnetic and isostructural with the complex previously reported, Na 2Cu(phenobarbiturato) 4, suggesting a square-planar environment around the Ni(II) ion. The DMF solutions of this complex show the existence of two species. The EPR spectra of the Cu(II) doped complex show the hyperfine and superhyperfine structures. The covalence parameters α 2, β 2, and δ 2 show a strong bonding in the equatorial plane and suggests the formation of a [CuN 4] chromophore. The anticonvulsant properties of the K 2Gu(N-methylphenobarbiturato) 4·8H 2O are reported.

Electron paramagnetic resonance studies on Cu(II) ions doped in Tl 2Ni(SO 4) 2·6H 2O single crystals

The X-band electron paramagnetic resonance (EPR) studies on a single crystal of Cu(II) ions doped in thallium nickel sulphate hexahydrate have been made over an extended temperature range of 120 K to 390 K. Two chemically equivalent, but magnetically inequivalent Cu(II) complexes have been observed. The angular variation spectra reveal that the Cu(II) ion enters the Ni(II) site in the host lattice. From the observed EPR spectrum the spin-Hamiltonian parameters have been evaluated. The orbital reduction factor and covalency parameters are also evaluated. The optical absorption spectrum exhibits a broad band at 11,850 cm −1 which is assigned to the transition 2 B 1 g → 2 B 2 g .

Chemical Bond in AIIBV2 Systems from Magnetic Susceptibility Data

AbstractSome peculiarites of chemical bond in AIIBV2 systems from the data on the magnetic susceptibility of the AIIBV2 compounds and their solid solutions are discussed. With this aim in view the magnetic susceptibility has been divided into the Langevin diamagnetic and van Vleck paramagnetic terms. The diamagnetic susceptibility of valence electrons, affecting the chemical bond in a crystal against the covalency parameter along the A – B, A – A, B –B bonds in AIIBV2 compounds, has been analysed. The covalency parameter of the AIIBV2 compounds has been evaluated.Closeness of the covalency parameter values for monoclinic β‐ZnP2 and tetragonal CdP2, CdAs2, is evidently, one of the reasons for the solid solution formation between the above mentioned compounds.

Absorption and emission spectroscopic parameters of Nd 3+ and Eu 3+ ions in their hexamethylphosphoramide complexes

Syntheses of the following complexes are reported: LnX 3·6L, LnX′ 3·4L, LnX″ 3·3L and Eu(NCS) 3·3L, where Ln = Nd 3+, Eu 3+; L = hexamethylphosphoramide (hmpa); X = ClO 4 −, PF 6 −; X′ = NCS −, NO 3 −, Br −, ClO 4 −; X″=Cl −. Spectra of the complexes of Nd 3+ (absorption) and Eu 3+ (emission) in dichloromethane solutions were measured. The oscillator strengths of the Nd 3+ f-f absorption bands within the 11 000–30 000 cm −1 region were determined and the τ λ intensity parameters were obtained according to the Judd-Ofelt formalism. Covalency parameters were also determined for the Nd 3+ complexes. The intensities relationship η 21 of the 5D 0→ 7F 2 and 5D 0→ 7F 1 transitions of the Eu 3+ was calculated. A good correlation between τ 2 and the oscillator strength of the hypersensitive band of Nd 3+ was found, as well as a correlation between τ 2 and η 21. There are only qualitative relations between τ 2 and the covalency parameter.

An EPR study of some copper(II) coordination compounds of substituted biguanides. Part IV

EPR and optical absorption spectra of some bis- (substituted biguanide)copper(II) bases and their corresponding chloride salts have been measured both in the solid state and in aqueous media in order to obtain information concerning the structure and nature of the bonding between the central metal atom and the ligands. Powder spectra of all copper(II) complexes provided approximate g values. In aqueous media and in nickel(II) complex matrix all the copper(II) complexes exhibited nine nitrogen superfine lines on the high field 3 2 ↔ 3 2 copper hyperfine splitting components, indicating the presence of four equivalent or nearly equivalent nitrogen atoms surrounding the copper(II) ion. The evaluation of covalency parameters indicated that the unpaired electron of the copper(II) ion spends about 35–37% of its time in the nitrogen donor sites of the biguanides. The hyperfine structure of the compounds further revealed the interaction between the unpaired electron of the copper(II) ion with four equivalent nitrogen nuclei of the title ligands. The degree of covalence of the σ-bond calculated by the hyperfine splitting is consistent with that obtained by the hyperfine structure of the copper(II) ion. α ′2 values indicated that, like other strong field ligands (biguanides, β-phthalocyanine and tetraphenylporphine), substituted biguanides and their protonated forms are equally strong field in character and suggest the formation of an [CuN 4] chromophore.

Physicochemical and Pesticidal Studies of Lanthanide Complexes with Schiff Bases Derived from Aldehyde and Dithiocarbamate Derivatives

Abstract Schiff bases from S-benzoyl dithiocarbamate and thiophene 2-aldehyde (A) and 4-N-N-dimethyl aminobenzaldehyde (B) have been synthesized. Their complexes of the type (Ml2Cl)2.4H20 with lanthanides have been characterized on the basis of IR, NMR and UV-visible spectroscopy, elemental analysis and magnetic moment measurements. The nephelauxetic effect (1-β), bonding parameter β b1/2, Sinha covalency parameter δ and the molar conductance measurement in nitrobenzene reflect covalent nature for the complexes. The stability constants and other thermodynamic data have been determined in aqueous solutions at 25, 30 and 35°C. The toxicity of the Schiff bases and their complexes were evaluated against insects and fungi. The LD50 for cockroaches and percent growth inhibition of fungi show greater efficacy for the complexes than for the Schiff bases.

Solvent effects in the reactions of peroxyl radicals with organic reductants: evidence for proton-transfer-mediated electron transfer

Absolute rate constants for the reactions of substituted methylperoxyl radicals with ascorbate, urate, trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), and TMPD (N,N,N{prime},N{prime}-tetramethyl-p-phenylenediamine) have been determined by pulse radiolysis is different solvents. In water-alcohol or water-dioxane solutions, the rate constants for trihalomethylperoxyl radicals generally increase with increasing water content. The rate constant for reaction of CCl{sub 3}O{sub 2}{center dot} radicals with trolox was measured in water, MeOH, i-PrOH, t-BuOH, ethylene glycol, diethyl ether, dioxane, acetone, acetonitrile, formamide, dimethylformamide, pyridine, and CCl{sub 4}. The rate constants were found to correlate well with a two-parameter equation that includes the dielectric constant of the solvent and the coordinate covalency parameter, a measure of the proton-transfer basicity of the solvent. Kinetic isotope effects in H{sub 2}O/D{sub 2}O of about 2 and activation entropies of about -1o eu for reduction of RO{sub 2}{center dot} by the organic reductants indicate that electron transfer to the peroxyl radical is concerted with the transfer of proton from the solvent to the incipient hydroperoxide anion.

Photoluminescence of the SrS:Mn2+Phosphor and Pb2+-Sensitized Luminescence of the SrS:Pb2+, Mn2+Phosphor

Emission and excitation spectra of the Mn 2+ ion in the SrS:Mn 2+ and SrS:Pb 2+ , Mn 2+ phosphor are investigated at various temperatures. The analysis of the excitation spectra of the Mn 2+ emission band leads to the Racah parameters B =712 cm -1 , C =3352 cm -1 and the crystal field parameter D q =576 cm -1 with the covalency parameter e =0.046. By the transient spectroscopy technique, it is found that the emission band due to the single Mn 2+ ion center has a peak at 553 nm and a decay time of ∼5.5 ms, while that due to the paired Mn 2+ center has a peak at 565 nm and a decay time of 0.6 ms. Energy transfer from Pb 2+ ion to Mn 2+ ion is observed in SrS:Pb 2+ , Mn 2+ . By observing the change of band shapes, the intensities and decay characteristics of both the Pb 2+ and Mn +2 lumimescence, it is concluded that the transfer is of resonance type. It is found that the energy of Pb 2+ ion is transferred preferentially to the isolated Mn 2+ ion.

Effects of Covalency on the Phonon Dispersion Relations in the NaCl-Type Alkali Halide Crystals. III.

A lattice dynamical model for the NaCl-type crystals are constructed which is a composite of the covalency model and the deformation dipole model. Using this model, the covalency parameter n c , the number of electrons occupying the outer most s orbitals of an alkali metal atom, is determined from the measured dispersion curves of 13 NaCl-type alkali halides by the least-squares analyses subject to the constraint that the model parameters reproduce the dielectric and elastic constants. The obtained n c values are contrary to the expectation of the traditional idea ( n c =0): about 0.08 for the fluorides, 0.10 the chlorides, and 0.15 the bromides and iodides.

ESR study of some copper(II) coordination compounds with substituted biguanidines—I

Copper(II) complexes of substituted biguanides have been studied by ESR spectroscopy in water, in their polycrystalline form and in the corresponding nickel(II) complex matrix. In aqueous solution, nine nitrogen superfine lines on the higher field 3/2 á3 3/2 copper hyperfine splitting component is indicative of four equivalent or nearly equivalent nitrogen atoms surrounding the copper(II) ion. α′ 2 values indicate that like biguanide, 1-amidino-O-alkylureas, β-phthalocyanine and tetraphenylporphine, substituted biguanides and their protonated forms are equally strong-field ligands. The evaluation of the covalency parameters indicate that the unpaired electron of copper(II) spends about 36% of its time in the nitrogen donor sites of the ligands.

Dependence of the covalency of the (MnF6)4- complex ion upon the Mn2+-F- distance.

The covalency of the (${\mathrm{MnF}}_{6}$${)}^{4\mathrm{\ensuremath{-}}}$ cluster has been examined by means of cluster-in-vacuo and cluster-in-the-lattice Hartree-Fock-Roothaan (HFR) calculations. The lattice potentials of ${\mathrm{RbMnF}}_{3}$ and ${\mathrm{KMgF}}_{3}$ have been considered. We have found that the covalency parameters ${f}_{s}$ and ${f}_{p}$=${f}_{\ensuremath{\sigma}}$-${f}_{\ensuremath{\pi}}$ change as the ${\mathrm{Mn}}^{2+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ distance R changes from 1.90 to 2.32 A\r{}, as ${R}^{\mathrm{\ensuremath{-}}8.2}$ and ${R}^{\mathrm{\ensuremath{-}}3.2}$, respectively. This result might be helpful in future determinations of these parameters under high pressure. Several contributions to the covalency of the ${\mathrm{Mn}}^{2+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ bond, including the 3d-orbital deformation, the action of the empty 4s and 4p metallic orbitals, and the effects of the crystal lattice, have been analyzed from the HFR wave function. Calculations show that, these three contributions being small, the (${\mathrm{MnF}}_{6}$${)}^{4\mathrm{\ensuremath{-}}}$ complex is highly ionic and its ${A}_{s}$ and ${A}_{p}$ superhyperfine constants behave as local observables, i.e., changes in their observed values from crystal to crystal are mainly determined by changes in the ${\mathrm{Mn}}^{2+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ distance.The traditional analysis of the superhyperfine tensor directed to obtain empirical covalency parameters has also been reexamined. Quantitative evaluation of the often-neglected metal-ligand terms and fluoride relaxation accompanying impurity substitution has shown that these two factors play a key role in determining the anisotropic covalency parameters from magnetic-resonance data. From electron-nuclear double-resonance measurements in cubic fluoroperovskites doped with ${\mathrm{Mn}}^{2+}$, we have found ${f}_{p}$=0.60\ifmmode\pm\else\textpm\fi{}0.20 %. The new empirical values of ${f}_{p}$ derived in this work for ${\mathrm{Mn}}^{2+}$:${\mathrm{KZnF}}_{3}$, ${\mathrm{Mn}}^{2+}$:${\mathrm{RbCdF}}_{3}$, and ${\mathrm{Mn}}^{2+}$:${\mathrm{CsCaF}}_{3}$ do not show a definite trend when R changes. More experimental work is needed to determine the nature of this variation.

Electronic structure analysis of electron-transfer matrix elements for transition-metal redox pairs

Calculated electron-transfer matrix elements (H/sub if/) for redox processes of the type ML/sub 6//sup 2 +/ + ML/sub 6//sup 3 +/ reversible ML/sub 6//sup 3 +/ + ML/sub 6//sup 2 +/ (M = Fe, Co, or Ru; L = H/sub 2/O or NH/sub 9/ have been analyzed in terms of various orbital concepts. The matrix elements are based on ab initio wave functions for model supermolecule clusters of the type (ML/sub n/ ...L/sub n/M)/sup 5 +/, with n = 1 or 3. The many-electron H/sub if/ quantities are analyzed as effective one-electron expressions of the type H/sub if/ proportional to (lambda')/sup 2/h/sub LlLr//sup c/, where lambda' is the metal-ligand covalency parameter and h/sub LlLr//sup c/ is a local one-electron matrix element for ligand orbitals in contact in the transition state. The data for apex-to-apex approach of reactants yield an excellent linear least-squares fit (r/sup 2/ = 0.996) and imply a value of approx. 5000 cm/sup -1/ for h/sub LlLr//sup c/, thus showing that significant coupling can occur in the absence of formal bonding between reactants.

Superexchange in helical Zn 1−xGa [formula omitted]Cr 2Se 4

The exchange interactions in insulating, helical spinels have been explained in the basis of the Anderson-Kanamori theory of superexchange. The value of the exchange constants J BB in the first three spheres of cordinatios between the Cr n + ions via Se 2- ligands have been estimated after taking into account the experimental data of the saturation field, the paramagnetic Curie-Weiss temperature and the assumption that the spin spiral angle increases linearly with increasing of Ga concentration. From the observed values of the exchange constants the trnsfer and direct integrals have been obtained which on further analysis yield the values of the covalency parameters. These parameters are found to be consistent with the chemical theory of covalency.

The contribution of the inward displacement of all ligands surrounding the Cr3+ions in MgO to the fine-structure splittings

Using the same method as that given by Du and Zhao (see ibid., vol.19, p.2935, 1986) Zheng shows that their assumption that the contribution of the inward displacement of all ligands surrounding the Cr3+ ions in MgO to the fine-structure splittings can be omitted is not always correct, because the research in this comment shows that for Cr3+ ions in tetragonal sites in MgO the fine-structure splitting of first excited state (2E) is very sensitive to the slight inward displacement of ligands although that of ground state (4A2) is not. Therefore for the structure parameters of the Cr3+-Vc centre the method of Du and Zhao can only determine the difference Delta R between the ligand's coordinates R0 and R, but the exact values of R0 and R cannot be determined by it. An improved method is perhaps to determine accurately the average covalency parameter N and the dipole moment u by other means.

Magnetisation density and covalency in ferrous fluoride

Polarised neutron scattering measurements have been made on single crystals of FeF2. The observation of a polarisation-dependent cross section for mixed magnetic and nuclear reflections showed that the two types of antiferromagnetic domains were unequally populated. Integrated intensity measurements were fitted to an extinction model which enabled the domain population to be established quantitatively. Moduli of the magnetic interaction vectors were deduced from polarisation ratio measurements of both mixed magnetic and nuclear and 'forbidden' magnetic reflections. The former were compared with those calculated on an ionic model using the wavefunction deduced from resonance studies and the parameters of this wavefunction were fitted to the higher-angle measurements. Deviation of the lower-angle reflections from this model were interpreted as evidence of covalency and gave a value of 0.049(10) for the sum of the covalency parameters. By comparing the experimental structure factors for the forbidden reflections with those calculated for a covalent wavefunction, individual covalent mixing parameters could be estimated.

Characterization and toxicity of lanthanide complexes with nitrogen- and sulphur-containing Schiff bases

La(III), Ce(III), Pr(III), Nd(III), Gd(III), Dy(III), Ho(III) and Er(III) complexes of Schiff bases derived from sulphamethoxazole and salicylaldehyde ( I), and thiophene-2-aldehyde ( II) have been characterized on the basis of IR, NMR, UV-Vis spectroscopy, elemental analysis, conductance measurements and molecular weight determinations. The Schiff bases act as monobasic bidentate ligands, rendering the metal eight-coordinated. The equilibrium constants have been studied in solution at 25, 30 and 35 °C. The nephelauxetic effect (1 - β), bonding parameter (β), b 1/2 and Sinha covalency parameter (δ) have been calculated. The positive values indicate the covalent nature of the metal-ligand bond. The covalent nature of the complexes is also supported by molar conductance measurements. The toxicities of the Schiff bases and their complexes were evaluated against insects. The LD 50 value for cockroaches and percent growth inhibition of the fungi show greater efficacy for the complexes than the Schiff bases.

Hypersensitivity in the 4f–4f absorption spectra of lanthanide(III) complexes

Praseodymium, neodymium, holmium and erbium complexes of bis(acetylacetone)ethylenediamine (ac 2en) and bis(acetylacetone)benzidine (ac 2bz) have been spectrophotometrically investigated in DMSO solution and in the solid state. The observed spectra in the visible region have been analysed to calculate oscillator strength and covalency parameters ( β , b 1 2 and δ). The covalency parameters reflect that ac 2bz complexes are more covalent and ac 2bz is a stronger ligand in a nephelauxetic sense. It is found that ac 2bz is especially effective in promoting 4f–4f spectral intensity. The larger values of the oscillator strengths for ac 2bz complexes are correlated to the low molecular symmetry of these complexes compared to ac 2en complexes.

Effects of Covalency on the Phonon Dispersion Relations in the NaCl Type Alkali Halide Crystals.II

A lattice dynamical model for the NaCl-type crystals are constructed which is a composite of the covalency model and the deformation dipole model. Using this model, the covalency parameter n c , the number of electrons occupying the outer most s orbitals of an alkali metal atom, is determined from the measured dispersion curves of 13 NaCl-type alkali halides by the least-squares analyses subject to the constraint that the model parameters reproduce the dielectric and elastic constants. The obtained n c values are contrary to the expectation of the traditional idea ( n c =0): about 0.08 for the fluorides, 0.10 the chlorides, and 0.15 the bromides and iodides.

Electronic structure of Cr+, Fe+, and Ni+ in octahedral fluoride lattices

The electronic structure of three octahedral MF6 (5-) (M = Cr, Fe and Ni) clusters has been computed. Ground state nuclear potentials are presented and metal-fluoride equilibrium distances are given. Although these clusters are classically repulsive, the nuclear potentials predict shallow minima at reasonable values of the metal-ligand coordinate. The bonding of these unstable compounds is discussed in terms of different charge-transfer mechanisms. Covalency parameters deduced from the antibonding, mainly metal MO’s correlate with the metal-ligand overlap in the manner assumed by Moreno, Aramburu and Barriuso (Phys. Letters, 87A, 307 (1982)). Metal-fluoride distances deduced from the isotropic superhyperfine parameter As , in the way proposed by these authors, are given. They compare well with the information deduced from the ground state nuclear potentials.

EPR studies of Cu 2+ ion in CdK 2 (SO 4) 2·6H 2O single crystals

The EPR spectra of Cu 2+ in CdK 2(SO 4) 2·6H 2O crystals have been studied at 77 K and 300 K in three ( bc, a ∗c and a ∗b ) planes. The angular variation spectra showed that the Cu 2+ ion enters the Cd 2+ site in the lattice. 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, and the signs for the hyperfine coupling constants are also determined. The covalency value indicates the percentage of unpaired spin density present at the metal (Cu 2+) d orbital. The nature of the distortion present in the lattice is obtained from the values of the mixing coefficients.