Co-Co Distances Research Articles

The nano-scale molecule with the longest delocalized metal–metal bonds: linear heptacobalt(ii) metal string complexes [Co7(µ7-L)4X2

A new type of pyrazine-modulated oligo-alpha-pyridylamino ligands, N2-(pyrazin-2-yl)-N6-(6-(pyrazin-2-ylamino)pyridin-2-yl)pyridine-2,6-diamine (H3pzpz) and N2-(pyrazin-2-yl)-N6-(6-(pyridin-2-ylamino)pyridin-2-yl)pyridine-2,6-diamine (H3tpz), were synthesized and characterized by IR, 1H NMR and MS(FAB). Using and , the linear heptacobalt(II) metal string complexes [Co7(micro7-L)4X2] (L=pzpz3-, X=Cl-, NCS-; L=tpz3-, X=Cl-, X=NCS-) were synthesized and structurally characterized. The structures showed the shortest Co-Co distance (2.194 A) and the longest Co chain (13.5 A) obtained to date with direct Co-Co bonds. The Co-Co distances are in the range 2.194-2.309 A. Electrochemical studies showed two reversible oxidations and one reversible reduction, while all the redox reactions of H3pzpz complexes, and , occurred at higher potentials than H3tpz complexes, and . The complexes are fairly stable to oxidation. Temperature-dependent magnetic research on revealed anomalous magnetic behavior with intermediate magnetic moment values between quartet and doublet states, and deviation from the Curie-Weiss law.

New Indides Sc6Co2.18In0.82, Sc10Ni9In19.44 And Sccu4In – Synthesis, Structure, And Crystal Chemistry

New indides Sc6Co2.18In0.82, Sc10Ni9In- and ScCu4In have been synthesized from the elements by arc-melting. Single crystals were grown by special annealing modes. The thee indides were investigated via X-ray powder and single crystal diffraction: Ho6Co2Ga type, Immm, a = 886.7(3), b = 878.0(2), c = 932.1(3) pm, wR2 = 0.0517, 711 F2 values, 35 variables for Sc6Co2.18In0.82, Ho10Ni9In20 type, P4/nmm, a = 1287.5(2), c = 884.7(1) pm, wR2 = 0.0642, 1221 F2 values, 63 variables for Sc10Ni9In19.44, and MgCu4Sn type, F ¯43m, a = 704.03(7) pm, wR2 = 0.0267, 101 F2 values, and 7 variables for ScCu4In. The scandium rich indide Sc6Co2.18In0.82 contains two Co2 dumb-bells at Co-Co distances of 221 and 230 pm. Each cobalt atom within these dumb-bells has a tricapped trigonal prismatic coordination. The In1 site has a distorted cube-like coordination by scandium and shows a mixed occupancy (36%) with cobalt. The In2 atoms have distorted icosahedral scandium coordination. As a consequence of the small size of the scandium atoms, the In4 site in Sc10Ni9In19.44 shows defects and was furthermore refined with a split model leading to a new distorted variant within the family of Ho10Ni9In20 compounds. ScCu4In is an ordered version of the cubic Laves phase with scandium and indium atoms in the CN16 voids of the copper substructure. The Cu-Cu distances within the three-dimensional network of corner-sharing tetrahedra are 248.6 and 249.2 pm. The crystal chemical peculiarities of these three indide structures are briefly discussed

Extended metal atom chains (EMACs) of five chromium or cobalt atoms: symmetrical or unsymmetrical?

Structural studies of pentachromium and pentacobalt extended metal atom chain (EMAC) systems are presented in which the metal chains are helically wrapped by either the pentadentate dianion tripyridyldiamide (tpda) or its diethyl-substituted analogue bis(4-ethyl-2-pyridylamido)pyridine (etpda). The compound Cr(5)(tpda)(4)(NCS)(2), has alternating long and short Cr-Cr distances, contrary to recent reports that describe it as symmetrical with essentially equally spaced chromium atoms. The linear Cr(5)(10+) chain is composed of two Cr(2)(4+) quadruply bonded units and an isolated high spin Cr(II) unit. The new compounds Cr(5)(etpda)(4)Cl(2), [Cr(5)(etpda)(4)FCl]PF(6), and Co(5)(etpda)(4)(NCS)(2)() employ the etpda ligand as "insulation" around a central pentametal-atom "wire." Compound, like all other oxidized pentachromium compounds, has very disparate alternating short-long-short-long Cr-Cr distances, 2.032(3) A, 2.560(6) A, 1.873(5) A, and 2.509(4) A. Compound shows a nearly uniform spacing of the Co atoms, although the outer Co-Co distances (2.279[4] A) are slightly longer than the inner ones (2.239[4] A).

Matrix embedded cobalt-carbon nano-cluster magnets: behavior as room temperature single domain magnets

A new type of Co-C nanoparticles is synthesized from CH2Cl2 solution of Co4(CO)12 by heating up to 210 °C in a closed vessel. Transmission electron microscope (TEM) and electron energy loss spectroscopy (EELS) observation show that the particles are embedded in amorphous carbon and their average size is 12 nm. The radial structure function obtained from the extended X-ray absorption fine structure (EXAFS) of the Co K-edge absorption of the Co-C nanoparticles provides a Co-C average distance of 2.08 A and the Co-Co distances of 3.18 A and 3.9 (±0.2) A. The particles exhibit the magnetic hysteresis curve with a coercive force of 200 Oe at 20 K and 260 Oe at 300 K. The temperature dependence of the magnetic susceptibility measured under zero-field cooling and 10 Oe field cooling conditions exhibits the behavior characteristic of a set of single magnetic domain nanomagnets in an amorphous carbon matrix.

Distribution and magnetization of Co impurities in anataseTiO2

Using the first-principles density-functional approaches, impurity distributions and magnetic properties of Co-doped anatase ${\mathrm{TiO}}_{2}$ were investigated. From the total energies calculated for various possible configurations, Co atoms were found to prefer the non-uniform doping mode with short Co-Co distances. This non-uniform distribution furthermore is crucial for the ferromagnetic ordering observed in ${\mathrm{Co}}_{x}{\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{O}}_{2},$ whereas the antiferromagnetic state prevails in uniformly doped cases investigated $(x=0.0625).$ Large exchange parameters appear to explain high ${T}_{c}.$

Magnetic, spectroscopic, and structural studies of dicobalt hydroxamates and model hydrolases.

The cobalt(II) urease model complex [Co(2)(mu-OAc)(3)(urea)(tmen)(2)][OTf] (2) prepared from the cobalt model hydrolase [Co(2)(mu-H(2)O)(mu-OAc)(2)(OAc)(2)(tmen)(2)] (1) undergoes facile reaction with acetohydroxamic acid (AHA) to give the monobridged hydroxamate complex [Co(2)(mu-OAc)(2)(mu-AA)(urea)(tmen)(2)][OTf]( )()(3) while 1 gives the dibridged hydroxamate complex [Co(2)(mu-OAc)(mu-AA)(2)(tmen)(2)][OTf] (4). The structures and Co-Co distances of the hydroxamate derivatives of 1 and 2 are very close to those of their nickel analogues and suggest that hydroxamic acids can also inhibit cobalt-based hydrolases as well as inhibiting urease. 1 also reacts with glutarodihydroxamic acid (gluH(2)A(2)) to eliminate hydroxylamine with formation of [Co(2)(mu-OAc)(2)[mu-O(N) (OC)(2)(CH(2))(3)](tmen)(2)][OTf] (5), the structure of which is very close to that of its nickel analogue. Both 1 and 3 show weak antiferromagnetic coupling. Oxidation of 1 with H(2)O(2) gives three dicobalt(III) hydroxy complexes (7-9), the first of which [Co(2)(mu-OAc)(2)(OAc)(2)(mu-OH)(tmen)(2)][OTf] (7) contains a bridging hydroxyl and the second [Co(2)(mu-OAc)(2)(OAc)(mu-OH)(OH)(tmen)(2)][OTf] (8) containing both a bridging and terminal hydroxyl, while the third [Co(2)(mu-OAc) (OAc)(2)(mu-OH)(2)(tmen)(2)][OTf] (9) contains two bridging OH groups with mixed-valence Co(II)/(Co(III) intermediates.

X-ray absorption spectroscopy study of Co structure in the epitaxial Co/Pt multilayers on Al2O3(11-20) substrates.

In this study, five epitaxial [Co(t nm)/Pt(1 nm)]30, multilayer samples (t=0.16-1.07 nm) were studied using polarized X-ray absorption spectroscopy method. These samples were prepared on Mo(110)/ Al2O3(11-20) substrates by MBE technique. The results show that the Co layer is more like an fcc pseudomorphic structure for the Co thickness of less than 0.3 nm. For Co layer thickness of 1 nm, the first shell distance is 0.25 nm, which is very close to the Co-Co distance of bulk hcp Co. On the other hand, for Co layer of less than 0.3 nm, the in plane first shell distance is expanded by 4% and most of the neighboring atoms are Pt atoms. The fitting results of the Co/Pt multilayers seem to support a sharp boundary model rather than an interdiffusion model.

Tuning the metal-metal bonds in the linear tricobalt compound Co3(dpa)(4)Cl2: bond-stretch and spin-state isomers.

Sixteen crystal structures have been determined for the Co3(dpa)(4)Cl2 (1) molecule in the following five crystalline solvates: 1.0.85(C2H5)(2)O.0.15CH2Cl2 (at 120, 213, 296 K); 1.C(4)H(8)O (at 120, 295 K); 1.C(6)H(6) (at 170, 213, 260, 316 K); 1.C(6)H(12) (at 120, 213, 295 K); and 1.1.75C(7)H(8).0.5C(6)H(14) (at 90, 110, 170, 298 K). For 1.0.85(C(2)H(5))(2)O.0.15CH2Cl2 the molecule of 1 is almost symmetrical at 120 K (Co-Co distances of 2.3191(3) and 2.3304(3) A) and remains so at 296 K (2.2320(3) and 2.3667(4) A). For 1.C(4)H(8)O the Co(3) chain is precisely symmetric at both 120 and 295 K though the Co-Co distances increase from 2.3111(4) to 2.3484(4) A as the temperature rises. Compound 1.C(6)H(6) is isomorphous with 1.C(4)H(8)O at 213 and 295 K and has rigorously symmetrical molecules at these two temperatures. Between 213 and 120 K the space group changes from Pccn to P2(1)/c, so that a symmetrical arrangement is no longer required and the two Co-Co distances then differ slightly (by 0.013 A). For 1.C(6)H(6) there is a phase change between 316 K (Pca2(1)) and 260 K (Pna2(1)). At all four temperatures, however, the molecule is almost symmetrical, with the two independent Co-Co distances never differing by more than 0.026 A. 1.1.75C(7)H(8).0.5C(6)H(14) contains, at all temperatures between 90 and 298 K, two crystallographically independent molecules, each of which is distinctly unsymmetrical at 298 K (Co-Co distances of 2.312(2) and 2.442(2) A for one and 2.310(2) and 2.471(2) for the other). In the first of these the distances converge to a much smaller separation (0.056 A) at 90 K while in the second the difference decreases to only 0.006 A at 90 K. Magnetic susceptibility measurements from 1.8 to 350 K indicate in each case that a gradual spin crossover, from a doublet to a quartet state, occurs over this temperature range.

Compounds with symmetrical tricobalt chains wrapped by dipyridylamide ligands and cyanide or isothiocyanate ions as terminal ligands.

Three new linear compounds of the type Co(3)(dpa)(4)X(2), where dpa is the anion of di(2-pyridyl)amine and X is NCS(-) (5), CN(-) (6), and N(CN)(2)(-) (7), have been prepared, and their structures and magnetic behavior have been studied. In all of them, including three different solvates of 5, the Co(3) chains are symmetrical with Co-Co distances of ca. 2.31-2.32 A. The appearance of four lines in the (1)H NMR spectra of the three compounds is also consistent with a symmetrical structure in solution. For all compounds, the magnetic behavior is quite similar with mu(eff) of ca. 1.9-2.0 micro(B) at temperatures between 1.8 and 200 K. As the temperature increases, the effective moments increase gradually, but since saturation is not reached, even at 400 K, the high-spin state cannot be assigned.

New linear tricobalt complex of di(2-pyridyl)amide (dpa), [Co3(dpa)4(CH3CN)2][PF6]2.

Reaction of the linear tricobalt compound Co3(dpa)4Cl2 (1) (dpa = di(2-pyridyl)amide) with silver hexafluorophosphate in acetonitrile yields [Co3(dpa)4(CH3CN)2][PF6]2 (2). Two crystalline forms are obtained from the same solution, namely, a monoclinic (P2(1)) form 2xCH3CNx2Et2O and a triclinic (P1) form, 2x3CH3CN. The tricobalt units in both crystals are essentially symmetrical, though this is not required by crystal symmetry, with Co-Co distances in the range 2298-2304 A. Each of the two terminal Co atoms is coordinated to an acetonitrile molecule with Co-N distances in the range 2068-2111 A at 213 K. The spiral arrangement of ligands gives an overall idealized D4 point group symmetry for the cation [Co3(dpa)4(CH3CN)2]2+ . Chiral crystals of both delta and lambda configurations in the P2(1) form have been isolated. The absolute configurations were determined by X-ray crystallography and their mirror-image circular dichroism spectra measured. The D4 symmetry of the cation appears to be preserved in solution as judged by the presence of only five proton resonance signals in the 1H NMR spectrum. Magnetic susceptibility measurements in the solid state indicates that 2 has a doublet ground state and exhibits an increase of the effective moment at high temperature (approximately 160 K) due to a spin crossover process.

Relation between Hydrogen Storage Properties and Structure of Nd–Co–B Amorphous Alloys

Hydrogen storage properties of Nd x Co 85-x B 15 (x=5-30) amorphous alloys produced by melt-spinning, such as hydrogen absorbing rates, storage capacities and desorption spectra, have been measured and compared with their structural features such as radial distribution functions and coordination numbers obtained by X-ray diffraction analyses. The storage capacity under I MPa H 2 at 423 K increases nearly quadratically with Nd concentration as H/M≅0.001x 2 (M=Nd+Co+B). The absorbed hydrogen is desorbed upon heating in stages around 350-600 K and at higher temperatures. The first stage is desorption from the amorphous phase, and most of the hydrogen is desorbed at this stage in the alloys with x ≤ 20. Hydrogen atoms are accommodated in tetrahedral sites of Nd 2 Co 2 and Nd 3 Co 1 configurations separated from trigonal prisms composed of Co and B atoms, and extend the Nd-Nd pair distance by 9% and Nd-Co by less than 3% but not influencing the Co-Co distance at all. The coordination numbers about Nd and Co atoms are preserved before and after hydrogenation, indicating that the medium- as well as long-range atomic distribution is not modified by hydrogen absorption in these amorphous alloys.

Synthesis, Structure, and Magnetic Properties of Low-Valent Triangulo Cobalt-Hydride Clusters [XCo(3)(&mgr;-CO)(3)(PMe(3))(6)

Syntheses and properties of low-valent clusters [X{Co(&mgr;-CO)(PMe(3))(2)}(3)], X = none (1), H (2), and H(3) (3), are reported. All solids are isostructural as explicitly shown by single-crystal structure data. The molecular structures of C(21)H(54)Co(3)O(3)P(6) (1) and C(21)H(57)Co(3)O(3)P(6) (3) contain central Co(3) units which form perfect equilateral triangles with Co-Co distances of 2.4055(5) and 2.432(1) Å, respectively. Both compounds crystallize in trigonal space group R&thremacr;c, with Z = 6: 1, a = 10.678(1) Å, c = 52.298(11) Å; 3, a = 10.679(2) Å, c = 52.729(12) Å. Compounds 1 and 2 form a continuous range of solid solutions. Powder samples exhibit molecular paramagnetism of different extent: At 298 K the effective magnetic moments for 1-3 are &mgr;(eff)/&mgr;(B) = 1.9, 3.1, and 2.7, respectively. The magnetic behavior could be rationalized in terms of the Curie-Weiss law for a one electron system (1) and two electron system (2). The two electron approximation has been discussed as well for 3 above 6.2 K. A description of the scope of exchange coupling is presented for 1.

Metal Atoms in the Synthesis of Metal Clusters, VIII.On the Reaction of Sterically Demanding Cyclopentadiene Ligands with Cobalt Atoms: Synthesis, Crystal Structure, Spectroscopic Behavior, and Reactivity of Di-, Tri- and Tetranuclear Hydrido Clusters of Cobalt

The metal-vapor reactions of Co atoms with 1,3-tBu2CpH (1a), 1,3-tBu2CpH (1a), 1,2,4-tBu3CpH (1b) and EtMe4CpH (1c) are described. With 1a the two mononuclear complexes [(η5-tBu2Cp) (η4-tBu2-1,3-cyclopentadiene)Co] (2a) and [(η5-tBu2Cp)2Co] (3a) were isolated, together with the dinuclear cobalt cluster [{(η5-tBu2Cp)Co}2H3] (4a) and trace amounts of the tetranuclear cluster [{(η5-tBu2Cp)CoH}4] (5a). The molecular structures of 3a and 4a were determined by X-ray diffraction. Reaction of 1b with Co atoms afforded a single product, the dinuclear cluster [{(η5-tBu3Cp)Co}2H3] (4b), whose molecular structure was determined by single-crystal X-ray diffraction. Both, 4a and 4b exhibit extremely short Co-Co distances [2.244(1) (4a) and 2.242(1) A (4b)], as found for the Me5Cp analog [{(η5-Me5Cp)Co}2H3] (4c). Reaction of an isomeric mixture of Me4EtCpH (1c) with Co atoms furnished the mononuclear sandwich [ (η5-Me4EtCp)(η4-Me4Et-1,3- cyclopentadiene)Co] (2b), the trinuclear hydridocobalt cluster [{(η5-Me4EtCp)Co}3H4] (6a) and the tetranuclear hydridocobalt cluster [{(η5-Me4EtCp)Co}4H4] (5c). The molecular structure of 5c was determined by X-ray crystallography and revealed a tetrahedral arrangement of the cobalt atoms. The electrochemical behavior of the dinuclear complexes 4a-4c was studied by cyclic voltammetry. Reversible redox couples were found for all three compounds, with a correlation between the degree of alkyl substitution and their respective cathodic shifts. Compounds 4a and 4b react with CO to yield the mononuclear and dinuclear complexes [ (η5- CpR)Co(CO)2] (R1,3-tBu2, 1,2,4-tBu3) (7a and 7b) as well as [{(η5-CpR)CO}2(CO)2] (8a and 8b). Reaction of the trinuclear hydridocobalt cluster 6a and its Me5Cp analog 6b with AgBF4 in the presence of PEt3 yielded the heteronuclear clusters [{η5-Me4CPR)Co}3AgP(Et)3H4]+[BF4]− (REt, Me) (9a and 9b). 9a was structurally characterized by X-ray crystallography.

Refinements of the crystal structures of 1,4-diphenylbutadiyne and its complex with two moles of Co2(CO)6

1,4-diphenylbutadiyne, C16H10, crystallizes in monoclinic space group P21/n witha=6.6152(6),b=6.0768(7),c=14.634(1) A, β=100.981(8)0,V=577.5(2) A3,Z=2. The structure was refined toR=0.077 andR w=0.069 for 1007 observed reflections with CuKα radiation. The molecule lies on an inversion center. Our determination represents and increase in precision of a factor of approximately three over the original determination. The 2∶1 cobalt complex, C16H10[Co2(CO)6]2, also crystallizes in monoclinic space group P21/n witha=8.9568(12),b=17.7057(10),c=18.5671(12) A, β=90.606(12)o,V=2944.3(5) A3,Z=4. The structure was refined toR=0.032 andR w=0.031 for 4312 observed reflections. The diphenylbutadiyne triple bonds elongate to 1.346(3) A and the angles at the diyne C atoms decrease to 134.1(3)–144.9(3)o upon complexation. The Co-Co distance is 2.4580(4)A. Our data represent an increase in precision by a factor of 2–3 over the original determination.

A Hexacobalt Sulfide Cluster Compound, [Co6(μ-S)8(PPh3)6].2PPh3.2C2H4Cl2.H2O

The title compound, octa-μ 3 -thio-hexakis(triphenylphosphine)-octahedro-hexacobalt-triphenylphosphine-1,2-dichloroethane-water (1/2/2/1), [Co 6 S 8 (C 18 H 15 P) 6 ].2C 18 H 15 P.2C 2 H 4 Cl 2 .H 2 O, was prepared by the reaction of CoCl(PPh 3 ) 3 with Na(mpo) (mpo = 2-mercaptopyridine N-oxide) or NaS t Bu. The molecular structure consists of discrete cluster units interspersed with molecules of free triphenylphosphine and solvent. The neutral cluster contains a slightly distorted Co 6 octahedron with each triangular face symmetrically capped by a μ 3 -S ligand. The molecular structure possesses a crystallographic inversion centre. The average Co-Co, Co-S and Co-P bond distances are 2.87 (6), 2.24(1) and 2.162(7)A, respectively. The hexacobalt sulfide cluster compound was formed via the capture of S atoms from thiolate reactants.

Evolution of the Nanostructure of Giant Magnetoresistance Heterogeneous Alloys CoxAg1-x Upon Annealing

AbstractHeterogeneous alloys CoxAg1-x with atomic concentrations x=0.20 and 0.35, presenting giant magnetoresistance properties, have been studied by Total Electron Yield X-ray absorption spectroscopy at liquid nitrogen temperature at the Co K edge after annealings at various temperatures. For both sample concentrations, the Co atoms are mainly in small filament agglomerates for the as-deposited and low temperature annealed samples (T<150°C). In this temperature range, the local Co atomic environment is very stable and disordered. A structural change has been detected by XANES and EXAFS analysis between the 250 and 400°C annealings. After a 400°C annealing during 10 min, the nanostructures evolve towards larger, more compact and well ordered granules. The Co-Co distance varies from 0.246 to 0.248 nm as the annealing temperature increases indicating that the small filament-shaped agglomerates are under strain in the Ag matrix. The Debye-Waller factor of the Co0.35Ag0.65 samples decreases continously as the annealing temperature increases, as this factor drops suddenly after a 250°C annealing for the Co0.20Ag0.80 sample. The Co agglomeration is steadily larger for the Co0.35Ag0.65 sample than for the Co0.20Ag0.80 one. The Co-Ag contribution is negligible for the Co0.35Ag0.65 sample, as it represents approximately one third of the nearest neighbors for the Co0.20Ag0.80 sample from the as-deposit up to a 250°C annealing stage.

Local order in YBa2Cu3−yCoyO6+2x studied by anomalous diffuse x-ray scattering

The local order in a single crystal of YBa2Cu2.53Co0.47O7.13 has been studied with anomalous diffuse x-ray scattering. (For such a Co concentration the compound is nonsuperconducting.) Intensity measurements were carried out at two energies below the Co edge. The difference data could then be expressed in terms of the local structure around a Co atom. The short-range order parameters (α's) indicate that the Co and Cu atoms are nearly randomly distributed on the Cu1 sites. The estimated size of the Co-free “domains” is 5-7 Å. The first neighbor in-plane Co-Co distances are significantly shortened, indicating that the Co atoms are displaced from their average positions. The data also show a significant decrease of the Co-O1 distance, leading to an increase of the Cu2-O1 distance. The lengthening of the Cu2-O1 distance implies a lowering of the Cu2 formal valence. The Co substitution also affects the in-plane Cu2 positions. The present study shows that the Cu-O2 structural coherence is altered on a scale smaller than the superconducting coherence length. As far as the superconductivity is concerned, the Cu2 valence remains one of the most important parameters in determining the superconducting properties of the Co-doped 123 compounds. On the other hand, there is some evidence that in order for superconductivity to occur in this and other doped cuprate compounds, the size of dopant-free regions in the basal plane may have to exceed the superconducting coherence length.

Synthesis and structure of complexes formed in the reaction between dicobalt octacarbonyl and tetramethyl-biphosphine disulfide

Co2(CO)8 and Me2P(S)P(S)Me2 react to form the two cluster complexes: Co4(CO)9S(PMe2)2) (1) and Co3(CO)7S(SPMe2) (2). The strucure of1 and of the disubstituted triphenyl phosphine derivative of2. Co3(CO)5(PPh3)2S (SPMe3) (2a) were determined. Compound1 contains a quasi-planar rhomboidal Co4 cluster formed by two Co3 isosceles triangles sharing a Co-Co edge. One triangle is capped by a sulfur atom, the other triangle has two edge-bridging PMe2 moieties. Electron counting gives 64 electrons corresponding to a planar system; the distribution of long Co-Co distances, in particular in the triangle bearing PMe2 bridges, suggests that the excess electrons are located on Co-Co antibonding ortibals. Compound2a contains a Co3S cluster with one side bridged by a SPMe2 unit forming a four-membered Co2SP ring. The substitution of two CO groups with two PPh3 causes a large deformation of the cluster Co-Co bondscis to these two phosphorus atoms. Crystal data for1, space group P1,a = 9.728(2) A,b = 10.288(2) A,c = 11.860(3) A,α = 86.41(2)°,β = 76.20(2)°,γ = 80.37(5)°,Z = 2, 5300 reflections,R = 0.0398; for2a, space group P1,a = 9.78(3) A,b = 13.05(4) A,c = 18.28(6) A,α = 93.23(3)°,β = 99.17(2)°,γ = 97.26(6)°,Z = 2, 2976 reflections,R = 0.0579.

X-Ray Absorption Spectroscopy of Cobalt(II) Multinuclear Surface Complexes and Surface Precipitates on Kaolinite

Abstract We have examined the local atomic structure of multinuclear complexes and surface precipitates of Co(II) sorbed to kaolinite (Al2Si2O5(OH)4), a common clay mineral, using extended X-ray absorption fine structure (EXAFS) spectroscopy. Structural information from quantitative analysis of EXAFS spectra is used to examine differences, on a molecular level, among Co species sorbed on kaolinite at different coverages from solutions under- and over-saturated with respect to solid Co(OH)2 phases, and between sorbed species and Co hydroxide solids precipitated from solutions in the absence of kaolinite. The EXAFS results indicate formation of oxide- or hydroxide-bridged multinuclear surface complexes on kaolinite at surface coverages well below a statistical monolayer from undersaturated solutions. As Co surface loading on kaolinite is increased, the number of Co second-neighbor atoms at 3.10-3.12 A increases from ≈2 to 6, the number of second-neighbor Co backscatterers in crystalline Co (OH)2(s), and interatomic distances determined by EXAFS remain constant. These observations suggest that hydroxide-like multinuclear complexes form at low surface coverages in addition to monomeric surface complexes, perhaps as precursors to surface precipitation. At high surface coverages and apparent solubilities (i.e., solution ion activity products) both below and above saturation with respect to stable Co(OH)2(s), the spectra of the sorption products differ from the spectra of Co solids precipitated in the absence of kaolinite, including dry, crystalline Co(OH)2(s) (pink form) and wet, freshly precipitated Co(OH)2(s) (blue and pink forms). Compared to the precipitates, sorption samples have Co-Co distances shorter by 0.050.07 (±0.02) A, lack small beat patterns in normalized spectra, and have reduced backscattering amplitudes from multiple scattering among Co atoms at ≈ 3 and 6 A. Long-term equilibration of oversaturated sorption samples (21 or 45 days) resulted in a decrease in backscattering amplitudes for both single- and multiple-scattering paths among neighboring Co atoms relative to short-term (24-36 h) equilibration. These differences are attributed to a decrease in the average number of Co atoms in a hydroxide-like complex and/or to an increase in disorder in the local atomic structure. This is not expected in typical aging of a freshly precipitated solid phase, where average particle size and degree of long-range order normally increase with time, suggesting that kaolinite influences the sorption product. The contraction in interatomic distances and disruption of long-range structure indicated by EXAFS for sorption samples can be accounted for by small particle sizes of the new surface phase, structural misregistry on an atomic scale between a Co hydroxide phase and kaolinite, or dissolution of the kaolinite substrate and substitution of minor impurities into a Co hydroxide phase. The apparent reduced solubilities of the sorption products relative to bulk solubilities of Co(OH)2(s) phases may be related to Ostwald-step processes, the formation of an impure Co hydroxide phase, or a lower near-surface solvent dielectric constant.

β-Dicobalt pyrophosphate

A new high-temperature or β form of dicobalt diphosphate, Co 2 P 2 O 7 , has been isolated in space group A2/m (non-standard setting of C2/m). As in other high-temperature forms of transition-metal pyrophosphates, metal atoms lie on planes in a two-dimensional hexagonal array [Co-Co distances 3.461 (4) and 3.284 (3) A] with adjacent metal atoms linked by two O-atom bridges. Co atoms are octahedrally coordinated with final distortions [average Co-O eq 2.14 (1), Co-O ax 2.32 (1) A]