Clusters, metal breakdown

Electric Breakdown in Anodic Oxide Films Physics and Applications of Semiconductor Electrodes Covered with Metal Clusters Analysis of the Capacitance of the Metal-Solution Interface. Role of the Metal and the Metal-Solvent Coupling Automated Methods of Corrosion Measurement... 

For cluster breakdown to occur, therefore, we must consider two stages, (i) The formation of an excited state geometry which contains CO bridges and must satisfy the criteria laid out above, i.e., there must be sufficient CO groups per metal atom within the cluster to produce a saturated unit. This will require an energy Et. (ii) The ejection of the saturated unit, which will require an additional energy E2. 

A characteristic feature of the carbon modifications obtained by the method developed by us is their fractal structure (Fig. 1), which manifests itself by various geometric forms. In the electrochemical cell used by us, the initiation of the benzene dehydrogenation and polycondensation process is associated with the occurrence of short local discharges at the metal electrode surface. Further development of the chain process may take place spontaneously or accompanied with individual discharges of different duration and intensity, or in arc breakdown mode. The conduction channels that appear in the dielectric medium may be due to the formation of various percolation carbon clusters. 

The reactions of HNCC with soft nucleophiles (CO, PR3, RCCR, RjCCRj, pyridine, NOj, and halides) result either in the formation of addition or substitution products, or in cluster breakdown. The nature of the species formed depends on the nucleophile (which may or may not undergo transformation on the cluster surface), the type of metal cluster, and the conditions employed in the reaction. Generally, clusters of the lighter elements tend to fragment even under mild conditions, while those of the heavier elements, which are more robust, often afford addition and substitution products. 

Simple nucleophilic addition at the metal centers of a cluster results in an increase in the number of electrons available for skeletal bonding. As a consequence, breakage of M—M bonds resulting in an opening of the cluster framework, sometimes followed by cluster breakdown, may be observed. 

The reaction of M3(CO)12 with both open-chain and cyclic poly-alkenes has attracted some attention, especially in the case of Ru3(CO)i2. In most of the examples reported, the organic fragment bonds to the metal framework in such a way as to interact with more than one of the three metal atoms (68-77). There are some exceptions to this general statement, however. One is the reaction of Ru3(CO)j 2 with cyclopentadiene, in which a mononuclear complex is obtained (78). In other cases, tetranuclear and hexanuclear compounds are obtained (79 81). Cluster breakdown has also been observed in the case of a rhodium complex upon reaction with ethylene (55) as shown in Fig. 3. 

Most of the reported reactions between tetranuclear clusters and alkynes involve mixed-metal cluster species. In these systems hydride and carbon monoxide substitution generally occurs [Eq. (11)] (194-200), although in some cases Me3NO has been used to activate the starting material (201, 202), and in still others cluster breakdown takes place even under mild reaction conditions (203). Rh4(CO)12 (204) and Ir4(CO)12 (205) retain their nuclearity in reactions with alkynes, but in the latter case the metal framework geometry is altered (Fig. 7). The use of [Ir4(CO)11Br] instead of Ir4(CO)12 in reactions with alkenes produces alkene-substituted tetranuclear complexes (189), as shown in Fig. 7. Few other homonuclear clusters have been found to react with alkynes (206-208). In the reaction between the tetranuclear cluster Cp2W2Ir2(CO) 0 and diphenylacetylene two independent processes... 

In the next chapter (Chapter 2), we estimate the fuse current of a conducting random network or the breakdown field of a randomly metal-loaded dielectric, using the percolation cluster models and their statistics. We also discuss here the breakdown probability distributions of such networks. All these theoretical estimates are compared with the extensive experimental and computer simulation results. 

Catalysis by Supported Metal-cluster Compounds. Further work has been reported recently on methods of chemically binding cluster compounds to supports and on the characterization of the resulting materials by various spectroscopic techniques. For example, the reaction of Rh6(CO)i6 with amine- and phosphine-modified silicas has been examined by infrared spectroscopy and has shown that cluster breakdown occurs giving L Rh(CO)2 and Lfn I (CO), where L comprizes the surface attached ligands. This behaviour is similar to that observed with Rh4(CO)i2 on unmodified silica where cluster breakdown occurs readily, particularly in the presence of traces of water and/or oxygen. ... 

The low reactivity of the rhodium clusters is surprising if one assumes that aryl group interchange can occur between vicinal metal centers as is proposed by Kaneda et al. (5). It is possible however that the rhodium clusters used here are rather coordinatively and structurally stable and do not promote aryl interchange at the employed temperatures. At the higher temperatures however, cluster breakdown to reactive mononuclear species and/or coordinatively unsaturated clusters is likely. 

The most extensive studies of the chemistiy of cluster complexes have been associated with the trinuclear cluster unit, as may be anticipated. A wide range of substitution reactions has been demonstrated for both Ru3(CO)i2 and Os3(CO)i2, with the full range of ligands normally employed in the study of metal carbonyl chemistry. In genera 1, the trinuclear osmium cluster is more readily maintained, ruthenium often giving rise to cluster breakdown, yielding mononuclear and binu-clear adducts. This reflects the increased bond enei of the metal-metal bond on descending the triad (see Table X later in this section). 

Amorphous alkali metal alkyls such as BuNa are typically insoluble in common solvents, but are solubihzed by the chelating ligand TMEDA (19.1). Addition of this ligand may break down the aggregates of lithium alkyls to give lower nuclearity complexes, e.g. ("BuLi TMEDA]2, 19.2. However, detailed studies have revealed that this system is far from simple, and under different conditions, it is possible to isolate crystals of either ["BuLi TMEDA]2 or ]("BuLi)4 TMEDA]oo (Figure 19.3). In the case of (MeLi)4, the addition of TMEDA does not lead to cluster breakdown, and an X-ray diffraction study of (MeLi)4 2TMEDA confirms the presence of tetramers and amine molecules in the sohd state. 

The breakdown of the resultant pentacoordinated intermediate is rate limiting, since the first step is an intramolecular reaction facilitated by a favorable activation entropy term. In this step, the metal-bound water functions as a general acid catalyst. The water bound to the lanthanide(III) ions has a pA a in the range 8-9, which should be further decreased in the bimetallic clusters since the second trivalent ion should further withdraw electrons from the... 

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