Cluster ligands

Pr )2(PPh3)7](PFg)2-3249 and [Au, i(PPlioIV1 e)i(l]3, 3250 The structure of the mixed-ligands clusters corresponds to a C3v metal skeletal framework and the homoleptic to a centered bicapped square antiprism (Figure 32). Other stoichiometries are known as, for example, [AunCl2(PPh3)8]Cl.3251... 

In a similar fashion, the introduction of angle-dependent electron densities into the EAM suggests that this formalism may be successfully extended to chemical reactions. This would allow the study, for example, of the reaction of a metal-ligand cluster with a metal surface. This would enhance the applicability of the EAM, and would increase the realm of processes which computer simulations can effectively model. 

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The thermolysis of 4 with Ru3(CO)12 3 in bis(2-methoxyethyl)ether under reflux for 3 h affords the decanuclear cluster [RuioC(CO)24]2 218 in 81% yield (Scheme 36).124 The same cluster has also been isolated from the thermolysis of 3 with mesitylene.95 The structure of 218 has been established by single crystal X-ray diffraction, which shows that the metal skeleton consists of a tetra-capped octahedron decorated with terminal carbonyl ligands. Cluster 218 reacts with CO in dichloromethane under ambient conditions to regenerate 4 and 3 in quantitative yield.109 The decanuclear cluster 218 also undergoes a reversible reaction with two equivalents of iodine to afford [RuioC(CO)24I] 219.109 At higher temperatures further reaction occurs with iodine to produce a species tentatively characterized as the hexamer [Ru6C(CO)i6I2] 220. 

Two structures of the dianion have been reported. The tetramethylam-monium salt, 18a, apparently differs from the tetraphenylarsonium salt 18b in the precise disposition of the bridging carbonyl ligands. Cluster 18b is reported to contain four bridging carbonyls (40), whereas 18a has only three... 

The C-C bond in organic chemistry makes possible the formation of chains and rings. However, a feature of d block chemistry is the formation of M clusters (n 3) in which each M atom is bonded to at least two others, as well as to other ligands. Clusters may be divided into two categories ... 

To conclude this section, we note that an enormous amount of material on the use of the variant of template synthesis, examined in this section, for obtaining heterometallic clusters has been accumulated [499,500]. It is emphasized that the use of clusters as metal ligands is the most suitable approach in order to synthesize such compounds with programmed structure and composition. Especially, many such reactions have been carried out at the expense of sulfur atoms of ligand cluster structures, for instance those reported in Refs. 501 and 502. 

In theory, pyrolysis of carbonyl-based heteronuclear cluster compounds should, by analogy with homonuclear carbonyl cluster chemistry, cause elimination of carbon monoxide followed by cluster aggregation. In practice it is difficult to predict the course of these reactions, which can involve simple loss of ligand, cluster aggregation, or degradation, and reactions are often complex and result in low yields. A few... 

Fe-Heterocycles and iron complexes with heterocyclic ligands 93 JOM(457)63. Ruthenium and osmium complexes with heterocyclic ligands, clusters including Ru- and Os-heterocyclic fragments 93JOM(457)121. 

The presence of zinc at the catalytic site of liver alcohol dehydrogenase suggests comparison with other zinc-dependent enzymes [164], and three are shown in Fig. 37. The hand and geometry of the zinc environment is invariant with respect to the proton abstractor (L4 and Fig. 37), the zinc atom, substrate site, water position, and protein ligand cluster. Ligand LI is in each case histidine, and the planes of the... 

IMS-MS applications published in the literature can be grouped into two categories. The first contains studies of conformations of flexible molecules. Such flexible molecules include synthetic polymers and biopolymers such as peptides, proteins, and oligonucleotides. The studies of the second category deal with the geometry of cluster ions such as carbon clusters, semiconductor clusters, metal clusters, salt clusters, ion-ligand clusters. The major conclusions regarding structure of these systems are reviewed in Sect. 5. 

The degree of substitution is determined by the initial ratio of (1) to ligand, and up to three (or in one instance, four) CO groups are replaced straightforwardly. This method can be employed sequentially, allowing for the preparation of mixed-ligand clusters. More forceful conditions must be employed to replace more CO ligands. 

Reaction of S1H4 with a mixture of iron carbonyl complexes (equation 53) leads to a mixed ligand cluster (42) in good yield in which the average Si Fe(CO)3 distance is 2.306 A and the Si-Fe(C02Cp) average distance is 2.250 A. ... 

K are those coefficients evaluated for the case that the effects of a medium external to the n-ligand cluster are neglected. Those parameters can be obtained by calculating free energy changes for these reactions with standard programs to treat electronic structure. 

The implications of this result for spectroscopic predictions are considerable. The composition and symmetry of the HOMOs and LUMOs have changed considerably from the NR case, and we see that the HOMO-LUMO transition is now essentially irrelevant with respect to the IR emission spectra. In the simplified one-electron picture, we predict absorption in the extreme IR up to 0.7 eV for C1-, 1.0 eV for Br-, and 0.2 eV for the I-ligand clusters. Dipole allowed one-electron transitions in the IR emission region are predicted to have a threshold at 2.8 eV for C1-, 2.7 eV for Br-, and 1.1. eV for I-ligand clusters. From the DOS diagrams, we can predict intense absorption into the antibonding peak located 4.1, 3.7 and 3.0 eV above Ef for X=C1, Br and I respectively. 

Figure 58 shows the structure of PtOs3(p-H)2(CO)10 P(cyclo-C6H11)3, which is the first of a series of closo tetrahedral, mixed-ligand clusters. The Pt-capped tetrahedron bears two hydride ligands localized on the longest Pt-Os (Pt-Osl) and Os-Os (Os2-Os3) edges, respectively [123],... 

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