1,721,026 research outputs found
. Stabilization of trivalent nickel through 1:2 coordination by cyclic terdentate ligands CH2CH2NH(CH2)2NH(CH2)2X (X = NH, O, or S)
No full textThe octahedral 6-coordinated complexes [Ni(L)2]2+ (L = 1-oxa-4,7-diazacyclononane; L1 = 1-thia-4,7-diazacyclononane) (donor set: N4O2 and N4S2, resp.) undergo a reversible 1-electron oxidn. in MeCN and DMSO solns. to give authentic Ni(III) species. These Ni(III) complexes present a distorted stereochem. arrangement, with the O and S heteroatoms occupying the axial sites of an elongated octahedron, and they persist in soln. for hours. Comparison with the corresponding complexes of 1,4,7-triazacyclonanane (L2) (donor set: N6) indicates that the relative stability of the trivalent species, expressed by the value of E1/2(Ni(III)-Ni(II)), decreases along the series: L2 > L1 > L, the trend reflecting the decrease in the strength of the Ni-X interactions (X = N, S, or O). The corresponding Ni(III) complexes of the open-chain analogs of the macrocycles L3 or L1 form at comparable potentials, but persist in soln. only in the time-scale of the voltammetry expt., thus demonstrating the existence of a purely kinetic macrocyclic effect in the stabilization of high oxidn. states of metals
Experimental electron density in a transition metal dimer: Metal-metal and metal-ligand bonds
No full textThe accurate experimental electron density distribution of Co2(CO)6(AsPh3)2 has been determined through X-ray diffraction at T = 123 K. Metal-metal and metal-ligand bonds have been investigated by means of deformation densities and the quantum theory of atoms in molecules. The 'expected' lack of charge accumulation in the deformation density map is 'contradicted' by the presence of a bond critical point and a bond path line linking the two Cobalt atoms, in agreement with theoretical predictions on similar compounds. A careful analysis of the properties of ρ(r) at the bond critical points and of the Laplacian distribution along the bond paths has allowed the full characterization of all bonds in the title compound and, in particular, to discard the apparently straightforward classification of Co- Co as a closed-shell interaction. The radial shape of the atomic Laplacian makes (covalent or polar) shared interactions similar to donor-acceptor ones when at least one 'heavy atom' is concerned. Thus, even if it is possible to recognize the shared character of Co-Co and As-C bonds by comparison with the donor-acceptor C-Co and As-Co interactions, this distinction is grounded on subtleties which could possibly not suffice without some a priori chemical insight
Experimental electron density studies for investigating the metal π- Ligand bond: The case of bis(1,5-cyclooctadiene)nickel
No full textThe accurate experimental electron density of crystalline bis(5- cyclooctadiene)nickel, Ni(COD)2, has been determined by X-ray diffraction (T = 125 K, 17 051 reflections measured for 2θ ≤ 96°) and it has been interpreted in terms of quantum theory of atoms in molecules (QTAM). The data were measured with a CCD area detector, whose performances were tested. This experimental electron density study of a π-ligand η2-coordinated to a metal atom is intended to test the Dewar-Chatt-Duncanson (DCD) bonding formalism providing further information about the π-complex vs metallacycle dichotomy. QTAM analysis, within the NiC2 triangle, shows a ring structure with a bond critical point between the two C(sp2), two bond critical points between Ni and each carbon, and one ring critical point in its center. Topology speaks for a π-complex with a concave ring structure and the overall bonding picture is in agreement with the DCD model: σ-donation and π-back-bonding are recognized in the Ni-C bond paths, which are inwardly curved (σ-donation) but well separated (π-back-donation)
Site preference of ligand and metal substitution in trigonal-bipyramidal metal carbonyl clusters
No full textGimarc's topological charge stabilization rule (TCS), for addressing site preference, has been extended to the realm of metal carbonyl clusters using Allen's energy indexes (Els) instead of charges. EIs have been computed within the extended Hückel (EH) approximation and, in order to assess an internal electronegativity scale for transition metals and to allow comparisons across the periodic table, a homogeneous set of EH parameters has been determined. EIs have been shown to behave similarly to charges on "clusters" with a rigid ligand stereochemistry like carboranes but, when one deals with metal carbonyl clusters and their intriguing ligand mobility, EIs are definitely superior to charges. EIs do address both "skeletal" and "ligand" site preferences according to the following rules of thumb: (i) more electronegative metal atoms occupy higher El sites of the uniform reference frame (URF) (i.e. the one with the lower local electronegativity) and (ii) (nucleophilic) ligand substitution preferentially occurs on the lowest energy index site of the URF (i.e. the one with the largest electrophilic character). Previous attempts to rationalize site preference in metal clusters were mainly concerned with the relative strengths of metal-metal and metal-ligand bonds and substantially disregarded electronegativity differences between different metal atoms. In contrast, we have shown that the latter are important whenever the actual URF allows for different EIs on different metal sites. In particular, differences in electronegativity have been shown to be important for M5L15and M5L12but not for M5L12bipyramidal metal carbonyl clusters. © 1997 American Chemical Society
An unprecedented triply interpenetrated chiral network of 'square-planar' metal centres from the self-assembly of copper(II) nitrate and 1,2-bis(4-pyridyl)ethyne
No full textCopper(II) nitrate reacts with 1,2-bis(4-pyridyl)ethyne (bpethy) in ethanol to yield mixtures containing, beside to a one-dimensional ladder-like polymer, a networked [Cu(bpethy)2] species, with a triply interpenetrated chiral frame based on pseudo-square planar metal centres, showing a topology theoretically predicted but never observed before, and containing uncoordinated ligands as guest molecules
Spinel type twins of the new cubic Er6Zn23Ge compound
Get PDFThe crystal structure of the new Er6Zn23Ge intermetallic compound was established by X-ray diffraction analysis on a twinned crystal (space group Fm-3m, Wyckoff sequence: f2edba, cF120-Zr6Zn23Si, a=12.7726(6) Å). The crystal is composed of two nearly equal size domains, whose mutual orientation is described by a 180° rotation around the cubic [111] axis, i.e. a spinel-type twinning law, not common for intermetallics. Applying the nanocluster approach, Er6Ge octahedra and centered two-shell Zn45 clusters were found as structural building blocks, filling the crystal space in a NaCl-like arrangement. This description was adopted to interpret the twinning in terms of stacking faults in the fcc cubic close packed arrangement. Moreover, the assembly of the nanocluster units is proposed as a possible mechanism for crystal growth and twin formation, in agreement with the principle of the interface energy minimization. Experimental conditions such as supersaturation and co-formation of other phases are also considered as favorable factors for Er6Zn23Ge twin formation
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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