1,721,003 research outputs found
Aromaticity and Agostic Interactions as Stabilizing Factors in Trinuclear Rhenium Clusters with Low Electron Count
No full textA NEW INSIGHT FROM QUALITATIVE MO THEORY INTO THE PROBLEM OF THE FE-FE BOND IN FE2(CO)9
No full textA revision of EHMO calculations with FMO analysis has provided new qualitative insight into the bonding network in Fe2(CO)9 and reconciled the previous diverging interpretations. After separation of all of the Fe(CO)bridge bonding/antibonding levels (12 electrons for six FeC bonds) there remains a two-electron/two-orbital interaction (σ/σ{black star}) that is ultimately responsible for a direct (through-space) FeFe bond. However, predominant repulsive interactions via the CO bridges (through-bond) may hide the linkage
Metal-metal bonding network in tetranuclear planar clusters
No full textIn order to understand the trends of the M4 skeleton to deform in planar clusters of type M4(CO)16, extended Hückel calculations have been performed. Depending on the total electron count the shape transforms ideally from square to rhombus through the formation of one diagonal M-M bond (64 → 62e-). The nature of the annular and transannular M-M σ-interactions is illustrated
INTERMETAL BONDING NETWORK IN 2-DIMENSIONAL TETRANUCLEAR CLUSTERS
No full textThe present study deals with the small number of structurally characterized 64 → 60e- tetranuclear clusters containing metals of the groups 7 → 10 and having in common the primary feature of planarity. The M4 skeleton has a variety of shapes: square, rhombus, or more generally quadrilateral. Empirical electron counting rules do not always provide a sufficient description of the bonding in these two-dimensional compounds nor is the viewpoint unique for different cases. In order to determine the electronic distribution, hence the trends of the M4 skeleton to deform, extended Hückel calculations have been carried out for a number of models. The nature of the outer M-M σ interactions and the distribution of the metal lone pairs are illustrated for 64e- square clusters. Then, it is shown that 62e- M4(CO)16 rhombuses correlate ideally with 64e- squares through the formation of one diagonal bond. Five M-M bonds are also ascertained in the 62e- quadrilateral Os4(CO)15 that is short one ligand. In any case, the fifth bond is shown to involve "t2g" orbitals in an unforeseen way. The distribution of the M-M bonds in 64e- clusters, containing π-donor coplanar phosphido bridges, is irregular. In Ru4(CO)13(μ-PPh2)2, formed by two condensed triangles sharing one side, only two outer consecutive Ru-Ru linkages have bond order 1, whereas the other three (including the quadrilateral's diagonal) are assigned bond order 2/3. In Ru4(CO)10(μPPh2)4 the M4 bonding network is something intermediate between two limiting viewpoints. The first one assigns a total of three bonds to the four sides of the Ru4 rhombus. The fourth linkage, coinciding with one diagonal, is best described as an uncommon four electron/three orbitals bond. The other description indicates that a fourth cyclic M-M bond is formed at expenses of part of the M-P bridge-bonding network. The theoretical result has also an experimental confirmation. Finally, there are planar tetranuclear clusters having all of the metal atoms square planarly coordinated by ligands and a reduced number of M-M connectivities. Among the latter, Ni4(μ-SR)8 (64 electrons) has a square Ni4 skeleton but no M-M bond, whereas Pd4(μ-CO)4(μ-CH3CO2) 4, with 60 electrons, is a parallelogram with two M-M bonds at those sides doubly bridged by the carbonyl ligands
DFT mechanistic proposal of the ruthenium porphyrin-catalyzed allylic amination by organic azides
No full textA DFT-based theoretical analysis describes the allylic amination of cyclohexene by 3,5(CF3)2phenylazide
catalyzed by [Ru](CO) ([Ru]= Ru(TPP), TPP = dianion of tetraphenylporphyrin). The activation of an azide molecule (RN3) at the free ruthenium coordination site allows the formation of a monoimido complex [Ru](NR)(CO) with the eco-friendly dismissal of a N2 molecule. The monoimido complex can undergo a singlet→triplet interconversion to confer a diradical character to the RN ligand. Hence, the activation of the allylic C−H bond of cyclohexene (C6H10) occurs through a C−H···N interaction over the transition state. The formation of the desired allylic amine follows a “rebound” mechanism in which the nitrogen and carbon atom radicals couple to yield the organic product. The release of the allylic amine restores the initial [Ru](CO) complex and allows the catalytic cycle to resume by the activation of another azide molecule. On the singlet PES, the CO ligand may however be eliminated from the monoimido complex [Ru](NR)(CO)S, opening the way to an alternative catalytic cycle which also leads to allylic amine through comparable key steps. A second azide molecule occupies the vacant coordination site of [Ru](NR)S to form the bis-imido complex Ru(TPP)(NR)2, which is also prone to the intersystem crossing with the consequent C−H radical activation. The process continues until the azide reactant is present. The interconnected cycles have similarly high exergonic balances. Important electronic aspects are highlighted, also concerning the formation of experimentally observed byproducts
Electrochemistry of the two-dimensional heteronuclear [Fe3Pt3(CO)15]n clusters (n=2-, 1-, 0): MO treatment of the skeletal adjustments in 86-84e− congeners
No full textElectrochemical studies show that it is possible to move step-wise and reversibly between the redox congeners of the series [Fe3Pt3(CO)15]n, n=2-/1-/0. By contrast, a multielectron reduction of the dianion leads to an irreversible demolition of the species. When [Fe3Pt3(CO)15]2- is treated with one or two equimolar amounts of the oxidant [Fe(C5H5)2]+, the oxidized species (n=1- and n=0) can be also obtained. It can be established or extrapolated from the already known structures of the dianion and the monoanion that the successive oxidations strengthen the inner Pt-Pt linkages of the overall quasiplanar Fe3Pt3 skeleton. MO analysis, by establishing the antibonding nature of the frontier level from which the electrons are added or subtracted, allows the correlation of the bonding features of the inner Pt3 skeleton with the redox propensities of the system
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
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