Cluster compounds shapes

As illustrated in the next section, the use of biological fingerprints, such as from a BioPrint profile, provides a way to characterize, differentiate and cluster compounds that is more relevant in terms ofthe biological activity of the compounds. The data also show that different in silico descriptors based on the chemical structure can produce quite different results. Thus, the selection of the in silico descriptor to be used, which can range from structural fragments (e.g. MACCS keys), through structural motifs (Daylight keys) to pharmacophore/shape keys (based on both the 2D structure via connectivity and from actual 3D conformations), is very important and some form of validation for the problem at hand should be performed. 

Pentafluorotellurium hypochlorite, 26 156 Pentagold heteronuclear cluster compounds, X-ray crystallography, 39 367-369 Pentahalide ion shapes of, 3 163 structure of, 3 150 Pentahaloselenates(IV), 35 251 Pentahalotellurates(IV), 35 251 Pentakisaminomonochlorocyclotriphosphazene, 21 52... 

There has been an increasing interest during the past decade in the physical study of metal clusters in the form of large molecules, with the metal cluster core surrounded by a number of ligands [l]-[23]. Such cluster compounds, with a well defined size and shape, are available in macroscopic quantities. This has made it possible to study the properties of individual clusters, and in some cases even of individual sites within such clusters [24, 25]. 

A theory which shows greater applicability to bonding in cluster compounds is the Polyhedral Skeletal Electron Pair Theory (PSEPT) which allows the probable structure to be deduced from the total number of skeletal bond pairs (400). Molecular orbital calculations show that a closed polyhedron with n vertex atoms is held together by a total of (n + 1) skeletal bond pairs. A nido polyhedron, with one vertex vacant, is held together by (n + 2) skeletal bond pairs, and an arachno polyhedron, with two vacant vertices, by (n + 3) skeletal bond pairs. Further, more open structures are obtainable by adding additional pairs of electrons. This discussion of these polyhedral shapes is normally confined to metal atoms, but it is possible to consider an alkyne, RC=CR, either as an external ligand or as a source of two skeletal CR units. So that, for example, the cluster skeleton in the complex Co4(CO)10(RCCR), shown in Fig. 16, may be considered as a nido trigonal bipyramid (a butterfly cluster) with a coordinated alkyne or as a closo octahedron with two carbon atoms in the core. 

Loren D. Lower and Lawrence F. Dahl Cube-shaped metal-cluster compound, Nig(CO)8(PPh3)6, the first transition metal analog of cubane... 

Figure 6.41 Anassembly comprised of a cubic [Er4(p.3-0)(p.3-0H)3] + and a diamond-shaped [Er2( X3-0H)2] unit (left) showing of one unit of the nanosized 36-Er wheel-like structure (middle) and the two-dimensional network of wheel-shaped cationic cluster [Er36( X3-OH)3o( X3-0)6(BDC)6] + (right) [25]. (Reproduced from Z.P. Zheng, Cluster compounds of the f-elements, in K.A. Gschneidner, Jr., J.C.G. Biinzli, and V.K. Pecharsky (eds.). Handbook on the Physics and Chemistry of Rare Earths, volume 40, 2010, with permission from Elsevier.)...

In addition to practical applications, metal cluster-derived catalysts, particularly intrazeolite metal cluster compounds, may aid in the identification of catalytically important bonding and structural patterns and thereby further our molecular understanding of surface science and heterogeneous catalysis. The ship-in-bottle technique for the synthesis of bulky metal-mixed metal cluster compounds inside zeolites and/or interlayered minerals has gained growing attention for the purpose of obtaining catalytic precursors surrounded by the interior constraint, imposing molecular shape selectivity. Such approaches may pave the way to offer the molecular architecture of hybrid (multifunctional) tailored catalysts to achieve the desired selectivity and stability for industrial processes. 

The TEC model developed by Teo also has been successfully applied to rationalize the geometries of a large number of cluster compounds. The TEC model combines Lauher s rule with Euler s theorem and adds an adjustable parameter This parameter X is equal to the number of electron pairs present in excess of that predicted by the 18-electron rule. " X has also been interpreted in terms of the number of missing antibonding orbitals. Given a value for X, determined by the shape of the cluster, an equation predicts the electron count for a cluster. Theoretical justification of the parameter X is based largely upon the classical molecular orbital calculations performed by Hoffmann and Lipscomb via the extended Hiickel method on the corresponding polyhedral boron hydride clusters The values... 

Since 1970 or before, chemists have relied on classical, detailed temperature-dependent line-shape analyses.Indeed, fundamental contributions to our understanding of the dynamics of fluxional metal complexes with vr-hydrocarbon ligands,tertiary phosphorus donors, as well as TT-allyl anions,all stem from these types of measurement. Their contributions to metal carbonyl dynamics and rearrangements in cluster compounds is even more pronounced, and we cite selected studies in this very large area of organometallic chemistry.For slow exchange... 

A final, really unexpected example of a zeolite-like material is the cluster compound Cui4gSe73(PPh3)3o which forms trigonal prismaticaUy shaped tri-phenylphosphonium coated copper selenide clusters of 4 nm length and a thickness of about 2nm [109]. This material crystallizes in a most amazing structure where the cluster molecules arrange in a way that they form uni-... 

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