Clusters oligomeric

Bridging oxide ligands are beginning to be detected in various oligomeric/cluster compounds [Mn3Q] (Section 41.4.3.3), [Mn202] and [Mt Oj] (Section 41.5.2) units are now known. 

Nucleophilic and electrophilic additions and oligomerization (cluster formation) have been demonstrated. Unusual coordination geometries and bonding may be adopted by the coordinated ligands. These in turn may lead to new reaction types for these ligands. 

Thus, it can be concluded that coordination of CO does not significantly effect the oligomeric clusters. Solvation by carbon monoxide does not succeed in breaking up the larger clusters to small species like dimers. 

The complexes derived from simple monodentate carboxylates are not stable, and only one mononuclear complex with a sterically hindered carboxylate has been successfully isolated, [VCKCbC Bujs] (13).103 Attempts to prepare carboxylates in organic solvents often led to the formation of oligomeric cluster compounds with a variety of interesting structural types.103 In aqueous solutions, these vanadium complexes appear to contain six-coordinate vanadium.60,104 Even carbonate has been found to form a weak vanadiumtV) complex in solution.105... 

Preparation of the primary oligomeric clusters has been discussed in terms of the association of the ions in electrolyte solutions or condensation-mediated ion clustering. Dependent on the electrolyte concentration and the physicochemical state (P, T, pS, pH, pi, pCl, pc, pe) of the system, the ions associate to polymeric species or then the ion clusters may reach the critical size that enables them to grow to primary particles of some 100 mn in size. As discussed, they are probably a coagulate of cemented crystallites of 1 to 10 mn size. However, to be useful for further processing, these must be stabilized to sols, i.e., to a stable dispersion of particles of colloidal (10 to 1000 mn) size. There are basically two approaches for the stabilization ... 

Stabilizing effects have been directly observed by pulse radiolysis studies of silver cluster coalescence supported on 4-nm silica colloidal particles (44). Very small oligomeric clusters absorbing at 290 nm and 330 nm are stable in the presence of oxygen and even Cu " or Ru(NH3)6Cl3 (E° = 0.2 Vnhe)-These supported the exhibition of higher redox potentials than for free oligomeric clusters. 

Although more than ten years have passed since the first compound with condensed NbeOi clusters was reported only one representative of oligomeric clusters is known, K4Al2Nbn02i. The [Nbn02o ]0io cluster found in this compound... 

The transformation of the whole uppermost layer of PbTH into a polymer with covalently bonded chlorine, results in the absence of unmodified oligomeric clusters from the surface of cycled polymer. 

Conceivably, metal tetrachalcolenes of the types exemplified in Figure 3 for metal tetrathiolenes can be synthesized. For discrete polynuclear metal complexes, one can either build up the oligomeric clusters by forming in-plane, metal-ligand o- bonds with tetrathiolene and other... 

Vanadium has also been incorporated into raesoporous MCM-41 materials by hydrothermal synthesis, impregnation and CVD [167]. UV-VIS DR spectra reveal predominantly mononuclear vanadium( V) oxide and oligomeric clusters... 

A typical chromatogram of the products from PIB is shown in Figure 2.2 each cluster of peaks corresponds to an oligomer group (characterised by MS), and the yield of each oligomeric cluster is measured as the summation of the components in the cluster. 

When it comes to isolated or oligomeric clusters, there is only a limited number of examples. They may all be derived from [R (Z)]X, 2X units (fig. 46). The most oxidized example is Cs2Lu[Lu6(C)]Cli8 with only 9 electrons for mtra-cluster bonding. It forms red transparent crystals. Other quaternary halides with isolated octahedra all with a single carbon atom or the dicarbon unit have 11 to 15 cluster-based electrons (box 8). 

We obtained (p (0)=h(F /F )P, starting from the assumption about equality for the initial stage of the process of equation (5.12), taking into account the ratio (/) ,= Vn/Vo- For the final stage of the process, namely after phase inversion, analogous assumptions lead to the expression

fractal characteristics of surface and volume of liquid oligomeric clusters in polymeric pattern. 

Figure 5-27. Stability range for discrete oligomeric clusters derived from a fragmentation procedure (see text). Representative compounds which contain these clusters are indicated.

The final product from the condensation of [Moe s] clusters via opposite faces of the octahedra is the Mos s chain. It occurs in the compounds AM03X3 for the univalent elements A = K, H, etc. Numerous compounds have been synthesized and structurally characterized [37, 38, 141] which contain such intermediate oligomeric clusters of stepwise increasing sizes as are shown in Figure 5>29. Those phases which have been characterized by single crystal structure investigations are summarized in Ihble 5-7. 

Oligomeric cluster a cluster that is built from clusters sharing common cluster atoms... 

TABLE 5 Cluster-Based Electron (CBE) and Mean Distances (in pm) for Oligomeric Cluster Complexes T(Rr Xx ... 

A. Henglein, P. Mulvaney, and T. Linnert, Chemistry of silver Agn aggregates in aqueous solution nonmetallic oligomeric clusters and metallic particles, Faraday Discuss. 92,31-44 (1992). 

The absorptions of stabilized oligomeric clusters of copper and of platinuirf have also been detected in the reduction of Cu-II and Pt-II salts, although no definite structures have yet been assigned to them. 

The bonding conditions as a function of the number of particles have been the subject of intensive investigation. This is ptirticularly of interest for oligomeric clusters. 

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