Formation of dusters

The formation of dusters finm atoms and their subsequent stabilization with ligands, in our case we consider mainly phosphorus and nitrogen containing molecules, is accompanied by a surprising phenomenon most of the dusters obtained by this method have a so-called full shell structure. This means that the... 

As was shown in the preceeding section, tertiary phosphines are capable of promoting the formation of dusters of the electron-rich transition metals. At the same time, they are able to suppress the formation of metal pnictides or chalco-genides that is always observed in the reaction of phosphine complexes of cobalt or nickel chlorides with E(SiMe3)2 (E = PPh, AsPh, S, Se). 

RoUgen, F.W. Ott, K.R On the Formation of duster Ions and Molecular Ions in Held Desorption of Salts. Int. J. Mass Spectrom. Ion Phys. 1980,32,363-367. 

Formation of Mixed-Ligand TetranuclearCold(l) Nitrogen dusters 19 K (4-Mcl h-fonii)j... 

Aggregation of the atoms or microclusters may give metal nuclei. The micro-cluster itself may work as the nucleus. Although the size of microcluster or nucleus is not clear, the nucleus may consist of 13 atoms, which is the smallest magic number, This idea may be supported by the structural analysis of PVP-stabilized Pt nanoparticles (64) and other systems. In fact, a particle of 13 atoms is considered an elemental duster. In the case of preparation of PVP-stabilized Rh nanoparticle dispersions by alcohol reduction, formation of very tiny particles, the average diameter of which is estimated to be 0.8 nm, was observed (66). These tiny particles in the metastable state may consist of 13 atoms each and easily increase in size to the rather nanoparticles with average diameter of 1.4 nm, i.e., the particles composed of 55 atoms. This observation again supports the idea that the elemental cluster of 13 atoms is the nucleus. 

Transition from non-metallic clusters consisting of only a few atoms to nanosized metallic particles consisting of thousands of atoms and the concomitant conversion from covalent bond to continuous band structures have been the subject of intense scrutiny in both the gas phase and the solid state during the last decade [503-505]. It is only recently that modern-day colloid chemists have launched investigations into the kinetics and mechanisms of duster formation and cluster aggregation in aqueous solutions. Steady-state and pulse-radiolytic techniques have been used primarily to examine the evolution of nanosized metallic particles in metal-ion solutions [506-508]. 

The role of tbe additives, which may be used in bi- or muitnnetaltic combinations (Re, Ir, Ru, Ag, Au, Ge etc.) is poorly umterstood. They help to enhance the properties of platinum hy keeping it in a suitable state of dispersion, and to modulate the acidic character of the support. They appear to oppose the sintering of metallic crystallites by the formation of alloys or polymetallic dusters. 

Figure 1. Schematic representation of the synthetic protocol used for formation of open-chain dusters. The complex is stabilized by various ligands such as polypyridyl or rr-acceptor carbonyl or phosphine groups.

The addition of two neutral Mo3( -type hub-caps to the surface inside the cavity of the Moi76 -type wheel leads to the formation of a Mo24g duster and corresponds to a nudeation process under boundary conditions [3a] (see Figure 14.9). This reaction can be regarded as a model for both crystal growth, especially for the related initial nudeation process, which is still not dearly understood, and also for metal-centre uptake processes in biological systems. In particular, such... 

Consider a gas composed of single molecules of a condensable vapor (monomers), dusters of molecules of a condensable vapor distributed in size, and an inert carrier gas such as air. When the saturation ratio is less than unity, no net growth occurs, and the rates of formation and decay of dusters of any size are equal. This statement can be written using the formalism for a chemical reaction ... 

Some additional siabili/.ing factor is necossary for the formation of measurable quantities of hydride dusters. It could be the formation of complexes with molecules in the matrix formed on condensation at low temperature. Alkanes cannot coordinate to magnesium, and therefore should not aid hydride cluster formation. The presence of molecules such as alkenes or aromatics that can form r-complexes with magnesium, however, could exert a favourable inllu-ence on the cluster hydride yield. In solutions, there are possibilities for stabilization by association of magnesium with solvent molecules. 

A specific feature of reactions occurring in the autoclave is that the least soluble compounds are always precipitated from the homogeneous phase of the reaction. As a result, the equilibrium of the reaction is always shifted to the formation of these very insoluble compounds. Thus, it becomes dear that by varying the composition of the reaction mixture (mainly due to the introduction of new cations and anions) practically all types of the cluster forms being generated in the given system can be obtained in the solution. This is a clear advantage of the hydrothermal technique for duster synthesis in the autoclave. 

FIGURE 11.10 Free energy of duster formation, AG(nA.nB). for binary nucleation (a) schematic diagram of saddle point in the AG surface, (b) AG surface for HjSO -HjO system at 298 K. 

A 26-kb gene duster encoding enzymes for synthesis of the blue antibiotic actinorhodin by Streptomyces coelicolor has been cloned and sequenced The three large 10-kb genes required for formation of the broad-spectrum antibiotic erythromycin by Saccharopolyspora erythraea have also been cloned and sequenced. In both cases, the genes... 

It is still uncertain whether any of these three-iron dusters have any physiological role at all. It is evident that 3Fe and [4Fe-4S] clusters are interconverted readily, and a 3Fe cluster may lie on the biosynthetic route that leads to the formation of [4Fe-4S] clusters. There is some evidence that the 3Fe cluster of Fdll can participate in electron transfer between redox proteins, but again this has no ultimate significance, as a variety of added mediator complexes are able to carry out such a function. It has been suggested recently that the 3Fe centre in aconitase is capable of promoting partial catalytic activity of the enzyme. ... 

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