Octahedral Centers Research Articles

Colour centres in neutron irradiated aluminium oxy-nitride

Aluminium oxy-nitride (9Al2O3.5AlN) has been neutron irradiated to fluences varying from 1018-1024 n m-2 (En>1 MeV) at 60 degrees C. The defect structures were studied by X-ray lattice parameter measurements and optical spectroscopy. The lowest neutron doses produced an anomalous volume expansion of 0.17% which was also reflected in a shift of the absorption edge to longer wavelengths. The exponential variation of the absorption edge with photon energy suggests that the 'edge' is the long wavelength side of an exciton band. The identification of two V bands with the intrinsic octahedral aluminium vacancy (2.225 eV) and the tetrahedral aluminium vacancy (3.15 eV), and their relative strengths after low neutron doses, suggested that the volume expansion may be associated with the octahedral aluminium vacancy centre. The oscillator strength of the two V bands was calculated to be 3*10-5. At higher neutron fluences the increase in lattice parameter and anion vacancy concentration showed good agreement with those found for equivalent doses in Al2O3. Comparison with the absorption spectrum of irradiated Al2O3 leads to the following assignment of anion vacancy centres to absorption bands in AlON:F-5.46 eV, F+-5.00 eV, FA-4.6 eV, F2-4.1 eV, F2+-3.68 eV.

Rubidium vanadium pyrophosphate RbV3P4O17+x (x = 0.14): a novel mixed-valence vanadium pyrophosphate

This nonstoichiometric phase crystallizes in the tetragonal space group P4 2 /mnm with a=13.651 (2) A, c=7.289 (2) A, V=1358.3 (4) A 3 , Z=4, R=0.0366, and R w =0.0360 for 835 unique reflections with I>2.5σ(I). The structure may be regarded as built from ReO 3 -type infinite chains along the c axis (in which each VO 6 octahedron shares two opposite vertices) and finite chains parallel to the directions, which are linked by P 2 O 7 groups to form a three-dimensional structure. Each finite chain consists of four VO 6 subunits. The oxygen atom shared between the central and side octahedra in the four-membered chain is about two-thirds occupied. Variable-temperature powder magnetic susceptibility data suggest the presence of two V 4+ and one V 5+ per formula unit and support the formula determined from single-crystal X-ray diffraction data

Preparation, characterization, and catalytic activity of a new solid acid catalyst system

A series of novel, solid, strong acids have been prepared by the reaction of inorganic oxides with Al{sub 2}Cl{sub 6}. The objective was to create a solid acid with tetrahedral aluminum centers, which are expected to be strong Lewis acids than octahedral aluminum centers. Investigations of the acid sites of these solids by the infrared spectroscopy of adsorbed pyridine, calorimetric titrations, and solid-state nuclear magnetic resonance confirm that new strong solid acids have been prepared. The systems exhibit high catalytic activity and selectivity for acid-catalyzed cracking reactions under very mild conditions where present commercial catalysts do not react. 46 refs., 5 figs., 6 tabs.

Metal ion complexes of 2-picolinamineN-oxide

A series of cobalt(II), nickel(II) and copper(II) complexes of 2-picolinamineN-oxide, HA, has been prepared. Solids of formula [M(HA)3](BF4)2 (M=cobalt(II) or nickel(II); [Cu(HA)2]X2 (X=BF4−, NO3−); [Co(HA)2X2] (X=Cl− or Br−); [Ni(HA)2Cl2] and [Cu(HA)X2] (X=Cl− or Br−] have been isolated and characterized by partial elemental analyses, molar conductivities, magnetic susceptibilities, DSC-TGA, and spectral methods. All complexes were found to be monomeric, and their spectral parameters are compared with those of the metal ion complexes ofN-alkyl-2-picolinamineN-oxides, 2-dialkylaminopyridineN-oxides and 2-picolinamine. The cobalt(II) and nickel(II) halide complexes spectrally show a mixture of octahedral and tetrahedral centres.

Cyrilovite from Italy: Structure and crystal chemistry

The crystal structure of the cyrilovite from Bosa, Sardinia, Italy, a new locality for this rare mineral, has been refined in the P 41212 space group to aR value of 0.053 for 907 observed reflections. The structure consists of sheets of Fe3+-centered octahedra sharing vertices to form a square net, in the center of which the 8-fold coordinated Na+ ion is located. The Na-Fe-O sheets are linked together by P04 distorted tetrahedra and by hydrogen bonds. The unit-cell parameters (a = 7.313 (2) A,c = 19.315 (3) A, V = 1033.0 (4) A3,c/a 2.641) are substantially greater than those of wardite, the other endmember of the solid solution series. The chemistry is close to the end-member, but shows a limited solid solution of Al for Fe, and of Ca for Na; the latter may imply a yet poorly known possibility of vacancies in the 8-fold coordinated sites. The IR spectra show the presence of AI substituting for Fe3+ in the form of additional weak bands.[▭

Preparation and characterisation of mononuclear organoimido complexes of tungsten: X-ray crystal structures of [WCl4(NC6H4Me-p)(thf)], [P(CH2Ph)Ph3][WCl5(NC6H4Me-p)], and [WCl2(NPh)2(bipy)

Some mononuclear organoimido complexes of WVI and WV have been prepared and the X-ray structures of [WCl4(NC6H4Me-p)(thf)](thf = tetrahydrofuran)(1) and [P(CH2Ph) Ph3][WCl5(NC6H4Me-p)](2) determined. In both species the octahedral WVI centre is strongly bound to the tolylimido group [W[graphic omitted]N 1.711(7) for (1) and 1.709(11)A for (2)] causing a bending down of the four equatorial W–Cl bonds [W–Cl 2.313(4)–2.349(6)A] and a lengthening of the trans bond [W–O 2.237(6), W–Cl 2.464(5)A]. The 14N and 15N n.m.r. spectra of the ethylimido compounds [WCl4(NEt)(thf)] and [P(CH2Ph)Ph3][WCl5(NEt)], and the t-butylimido compound [W(NBut)2(NHBut)2] have been obtained. From reactions of the binuclear bis(organoimido) complexes [{WCl2(NR)(NR′)(NH2R′)}2] with neutral ligands [PMe3 or 2,2′-bipyridyl (bipy)] the mononuclear bis(organoimido) complexes [WCl2(NR)(NR′)(PMe3)2] and [WCl2(NR)(NR′)(bipy)] were obtained. The X-ray crystal structure of [WCl2(NPh)2(bipy)] has been determined and the distorted octahedral structure comprises cis-phenylimido groups and trans-dichloro ligands with the bipy nitrogens trans to the imido nitrogens. The phenylimido ligands are equivalent with mean W[graphic omitted]N bond lengths of 1.775(14) and 1.782(14)A and mean W–N–C angles of 165.3(12) and 166.3(12)°.

Analysis of the electronic structure of incompletely coordinated tetra- and octahedral surface centers on oxide compounds of molybdenum and their complexes with propylene

The model of orbitally stoichiometric clusters and the semiempirical MO-LCAO calculation scheme in the self-consistent charge and configuration approximation have been applied to the investigation of the electronic structure and energy spectrum of incompletely coordinated octahedral (I) and tetrahedral (II) surface centers on molybdenum oxides. It has been shown that centers of type II containing Mo/sup n+/ ions (n = 6, 5, 4) have greater electron-acceptor ability in comparison to Mo/sup 6 +/ centers of type I. The interaction of propylene with centers of type II results in considerable weakening of all the carbon-carbon bonds and some of the carbon-hydrogen bonds, and the interaction with centers of type I results in more moderate weakening of the double bond and one of the C/sup ..cap alpha../-H/sup ..cap alpha../ bonds (owing to the appearance of a hydrogen bond between H/sup ..cap alpha../ and a surface oxygen ion). The nature of the surface Mo centers of oxides in various catalytic conversion of olefins has been discussed.

Metal complexes of dicamba: the crystal and molecular structures of dicamba(3,6-Dichloro-2-methoxybenzoic acid) and catena-μ-Aqua-diaquabis(3,6-dichloro-2-methoxybenzoato)zinc(II) dihydrate

The crystal structures of the herbicide, dicamba (3,6-dichloro-2-methoxybenzoic acid) and the zinc(II) complex of this acid, {[Zn(dicamba)2(H2O)3].2H2O}}n (1), have been determined from X-ray diffraction data and refined by least squares to final residuals of 0.034 and 0.043 respectively. Dicamba is triclinic, space group P1, Z 2 with a cell a 7.232(1), b 7.971(1), c 9.050(3) �, α 102.76(3), β 91.33(3), γ 110.38(1)�, while (1) is monoclinic, space group P21/n, Z 4 with a cell a 10.467(1), b 8.135(3), c 28.079(2) �, β 96.497(7)�. Dicamba forms hydrogen-bonded cyclic dimers [O---O, 2.655(7) �] with the carboxyl and the methoxy groups synclinal to the benzene ring. The dicamba ligands retain their conformation in the ZnII complex and are cis-related and unidentate [Zn-0,2.083,2.095(4) �]. Three of the waters are coordinated [Zn-0, 2.036, 2.055, 2.099(4) �] with one of these also acting as a bridging ligand between the octahedral zinc centres [Zn-0, 2.326(4) �], giving a 'linear' polymer structure [Zn-0-Zn bridge angle, 133.6(3)�]. The MnII and CoII complexes of dicamba [(2) and (3)] have been confirmed as isomorphous and isostructural with the ZnII analogue.

Angular variation of magnetic linear dichroism in Ru-doped YIG(+)

Ru distribution in Y <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</inf> (YIG) is investigated by studying the symmetry of the magnetooptical properties above 1μm wavelength. The method utilized is the recently proposed angular variation of induced anisotropy (AVIA) and Consists in measuring, as the crystal is rotated, the angular variation of an induced anisotropy-property, thus obtaining a pattern from which the site symmetry may be derived. Linear dichroism induced by a saturating magnetic field, MLD, is the anisotropy property utilized to determine the symmetry of the ruthenium centers. The near-infrared absorption spectrum exhibits two distinct regions, one at about λ = 1.3μm and the other one at about λ =2.0μn. The AVIA experiment at the higher-energy region shows a marked octahedral character. The behavior under reducing treatment in H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> atmosphere allows identification with Ru <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> so that this absorption is associated with a Ti→ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> E, E transition of octahedral Ru <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> . Furthermore the exitence of a symmetry-lowering distortion of the octahedral center is determined. The Seebeck coefficient is measured and the conduction is found to be n-type. The possibility is discussed that charge compensation of Ru <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> is provided by Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> to be associated with the 2μm absorption region.

Synthesis and structure of an infinite-chain form of ZrI 2 (α)

The synthesis of a second polymorph of ZrI 2 has been achieved by a transport reaction between ZrI 4 and zirconium metal under a 750 850 ° C gradient in a sealed tantalum tube. The black lath-like crystals produced in the 775°C region occur in space group P2 1 m with a = 6.821(2) Å, b = 3.741(1) Å, c = 14.937(3) Å, β = 95.66(3)°, Z = 4. A total of 669 independent reflections with 2 θ ≤ 50° and I > 3 σ( I) were measured at room temperature on a four-circle automated diffractometer with monochromatized Mo Kα radiation and were corrected for absorption (μ = 190 cm −1). The structure was solved by direct methods and full-matrix least-squares refinement of all atoms with anisotropic thermal parameters to give final residuals R = 0.064 and R w = 0.079. This phase is isoelectronic and isostructural with β-MoTe 2, a distorted CdI 2-type structure in which the zirconium atoms are displaced 0.440 Å from the octahedral centers along a to form infinite zigzag metal chains ( d ZrZr = 3.182(3) Å) parallel to b. The phase is a diamagnetic semiconductor at room temperature ( E g ∼ 0.1 eV).

Magnetic linear dichroism in Sn doped YIG

Comparing the results of Magnetic Circular Dichroism (MCD) measurements on Sn and Zr doped Yttrium Iron Garnets with those of Si doped YIG (1), some interesting differences can be observed. In fact, here, transitions at 1.65, 2.00 and 2.15 μm of wavelength are present other than the one at 1.20 μm characteristic of YIG(Si). We try to explain this difference on the basis of a computation of the electrostatic energy between the dopant and the Fe2+ ions and by considering the electron band scheme of pure YIG. So it is possible to hypothesize the presence, in our samples, of Fe2+ ions localized in both octahedral and tetrahedral sites, in contrast to what is observed in YIG(Si). To check this hypothesis, measurements of Magnetic Linear Dichroism (MLD) at room temperature are performed on a sample of YIG(Sn) (110). The results of the MLD versus the angle between the inducing magnetic field and the [100] crystal direction are presented; the measurements are made at different wavelengths in the interval 1–2.7 μm. The angular behavior of the MLD is in general similar to that expected by the theory for the octahedral centers, but strong contributions by centers with different symmetry appear to be superimposed, especially at low wavelength.

The characterization of Ba 3Re 2O 9 and Sr 3Re 2O 9

The compounds Ba 3Re 2O 9 and Sr 3Re 2O 9 were prepared by the solid state reaction of the corresponding alkaline-earth oxide with ReO 3 at 750 to 900°C in sealed, evacuated, fused silica tubes. The two compounds are isostructural, having the nine-layer ABO 3 structure with vacant central octahedra. The unit cell parameters are given. The magnetic susceptibility for Ba 3Re 2O 9 indicates Curie-Weiss behavior with a Re 6+ moment having localized electrons. The magnetic data for Sr 3Re 2O 9 suggest delocalized electron behavior from its temperature-independent susceptibility. Both compounds appear to have semiconducting properties, but the strontium analog is a better conductor. Both compounds are unstable when heated in air above 400°C. They are readily decomposed by chemical oxidizing agents.

Optical and EPR Studies of Redox Interactions of Polyvalent Ions (Cr, Cu) in Glass

Chromium centers in isolated Cr ions and Cr(III)‐Cu(II) redox interactions were investigated by optical and EPR spectra. Line shapes and intensities of high‐field Cr(III)‐Cu(II) superimposed EPR absorption lines were sensitive to alkali content of host glasses. High‐field chromium g values (1.94 to 1.99) and intensities (190 to 3600) were calculated. The increase in g values from 1.96 to 1.98 is related to an increase in Cr(V) and to a conversion from Cu(I) to Cu(II) with increased alkali content. Two components of Cr powder spectra near g= 2.0 and g= 5.0 correspond to isolated octahedral Cr centers described by Landry et al. Cr‐Cr pair structure was not observed for low Cr2O3 concentrations. Three components of optical absorption bands near 340 nm, 430 to 460 nm, and 600 nm were measured in Cr(III)‐Cu(II) redox systems. Electronic interactions of Cr(III)‐Cu(II) redox systems and the structure of Cr centers are discussed.

Electron-nuclear double-resonance study of NaF:Fe+in an octahedral site

Results of electron-nuclear double-resonance (ENDOR) measurements and an analysis of a paramagnetic ${\mathrm{Fe}}^{+}$ ion in an octahedral site in NaF are presented. The hyperfine constants of the octahedral center have been determined for the fluorine-lattice nuclei in the first five shells (shells I, III, V, IX, and IX') and for the sodium-lattice nuclei in the first two shells (shells II and IV). Both positive and negative Fermi contact interactions are observed. A good example of multiplet structure of ENDOR spectra due to the equivalent fluorine nuclei in the first shell has been observed and analyzed by numerical diagonalization of the spin Hamiltonian of this system.

Optically active co-ordination compounds. Part XXIII. Inversion at an octahedral centre

One optically pure isomer of the carbonato-bis-L-valinatocobaltate(III) ion, isolated as its potassium salt, has been shown by electronic, circular dichroism, and n.m.r. spectra to be D(+)-cis(N)-cis(O)[Co(L-val)2CO3]–, and this has been confirmed by an X-ray study. The complex reacts rapidly and initially reversibly with acid to give what the circular dichroism spectrum suggests is an inverted product. From the reaction mixture crystals of (–)[Co(L-val)2(H2O)2]+ have been isolated as the perchlorate salt.

Spectroscopic Behavior and Coordination of Nickel(II) in Liquid Mixtures of Zinc and Cesium Chlorides

Optical absorption spectra of dilute solutions of NiCl2 in molten ZnCl2–CsCl mixtures were measured over the complete range of solvent compositions. The temperature extremes investigated were 260°C for melts containing 38 mole % CsCl and 1000°C for 75 mole % CsCl. At temperatures within 100–300°C of the liquidus (depending on melt composition) the spectra were highly composition and temperature dependent and consisted of relatively narrow absorption bands. Analysis of these spectra showed the presence under different conditions of six kinds of nickel(II) centers with well-defined coordination geometries. An equilibrium between octahedral and tetrahedral centers occurred in 0–8 mole % CsCl such that the octahedral form was favored with increasing CsCl content. A different octahedral–tetrahedral pair occurred in 20–50 mole % CsCl with the tetrahedral form favored by increasing CsCl content and temperature. In CsCl-rich melts tetrahedrally coordinated Cl3NiClZnCl33− and NiCl42− were found. In ZnCl2-rich melts at high temperatures the absorptions became broad and only mildly dependent on solvent composition. Suggestions are made regarding the relationships between these coordination effects and structure in the solvent.

Substitution reactions at octahedral centers. IV. Acid and base hydrolysis of chloropentakis(alkylamine)chromium(III) ions

The compounds [Cr(RNH2)5Cl](ClO4)2, where R = H, CH3, C2H5, n-C3H7, and n-C4H9, have been prepared and the replacement of chloride by either water or hydroxide ion subjected to kinetic examination. Below about pH 7 the reaction was acid independent, and ΔHA* was 22.4 kcal mole−1 when ammonia was the inert ligand compared with about 26.4 kcal mole−1 when the inert ligand was a primary amine. The corresponding ΔSA* values increased continuously with R from H to n-C4H9. At higher pH the rate became inversely dependent upon the acidity of the solution, and ΔHB* decreased from 26.7 kcal mole−1 for ammonia as the inert ligand to 25.4 kcal mole−1 for an amine as the inert ligand. For the pH dependent reaction the values obtained for ΔSB* were much larger than for the pH independent reaction but the pattern of variation as the inert ligands were changed was quite similar in the two cases. The great similarity in the trends of ΔS* values for the acid and base hydrolysis reactions has been interpreted in terms of a rate-determining step that is primarily dissociative in both cases.

Isomer counts for polynuclear coordination compounds

Abstract : Application of symmetry concepts to a complete enumeration of all the possible permutations for a particular formulation is the basis for determining the expected number of isomers for polynuclear coordination compounds. Isomer counts are reported for several binuclear coordination compounds based on octahedral centers. Although compounds containing bidentate ligands cannot be handled in the symbolic system employed, isomer counts can be made for them by systematic elimination from an enumeration of all possible permutations. (Author)

Electron paramagnetic resonance of NaF:Fe +

Epr spectra have been observed at 4.2°K from Fe + in substitutional sites with orthorhombic as well as octahedral symmetry in NaF crystals containing trace amounts of iron. Parameters for a spin Hamiltonian containing g tensor and fluorine superhyperfine interaction terms have been obtained. The orthorhombic center alone is produced by X irradiation at 4.2°K and can be converted completely to octahedral centers by annealing to room temperature. The octahedral sites can be obtained by room temperature X irradiation. A model for the orthorhombic center is suggested.