Cluster building blocks

In coating fullerenes with alkali metals, the stability of the cluster seemed to be determined primarily by the electronic configuration. The units C qM and C7oMg, where M is any alkali metal, proved to be exceptionally stable cluster building blocks. Coating a fullerene with more than 7 alkali metal atoms led to an even-odd alternation in the mass spectra, inter-... 

Answer. The [Zr6Cli2X]m+ core charge is —4 — (—5) = +1. If X is a main-group atom then we have 12 + vx — 1 = 14. vx = 3 = B. If X is a transition metal the same sum =18 and vx = 7 = Mn. The utilization of these cluster building blocks to generate nets and three-dimensional networks should be evident. 

Suggest a reasonable cluster core structure for RcxC(C0)24l2 that exhibits C2v symmetry with four types of metal positions. Suggest an isomer that has the same cluster building blocks but only two types of metal positions. 

Figure 29 Unit cells of Ba2Nb509 (a), BaNb40g (b), BaNb709 (c) showing Nbe cluster building block...

Recent years have witnessed great activity in studies of tetranuclear Mn compounds, largely because of their possible relevance to the putative Mn4 cluster at the water oxidation center (WOO of photosystem II (9, 21,22, 96-99) and because of their use as high-nuclearity cluster building blocks, the latter forming the basis of possible molecular magnetic materials (11, 12, 24, 27). 

FIGURE 939 (a) IZn O] cluster building block that comprises MOF-5. (b) Crystal structure of MOF-5. (Structures generated from CIF data). 

Table 15.7 Transition metal cluster building blocks (Cp = rf - C5H5)...

Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

Mercury has a marked ability to bond to other metals. In addition to the amalgams aheady mentioned (p. 1206) it acts as a versatile structural building block by forming Hg-M bonds with cluster fragments of various types e.g. reduction... 

Fig. 1. Iron-sulfur clusters basic building blocks. In most cases the iron is tetrahe-drally coordinated by sulfur from cysteinyl residues (and labile sulfur). Variability on coordination is allowed (see text). A, Rubredoxin type FeS4 (simplest cluster, no labile sulfur) B, plant-type ferredoxin [2Fe-2S] C, bacterial ferredoxin [3Fe-4S] D, bacterial ferredoxin and HiPIP [4Fe-4S] E, novel cluster [4Fe-2S, 20] ( hybrid cluster ).

The use of molecular biology methods, described in Section 5.3 seems to be especially worthwhile as it offers novel possibilities of optimization on process adjustment. Directed evolution leads to the formation of new biocatalysts with improved characteristics (selectivity, activity, stability, etc.). Incorporation ofnon-proteinogenic amino acids makes it possible to reach beyond the repertoire of building blocks used by nature. The prospect of bioconjugate preparation offers the possibility to form functional clusters of enzymes and to perform multiple synthetic steps in one pot. 

The importance of surface characterization in molecular architecture chemistry and engineering is obvious. Solid surfaces are becoming essential building blocks for constructing molecular architectures, as demonstrated in self-assembled monolayer formation [6] and alternate layer-by-layer adsorption [7]. Surface-induced structuring of liqnids is also well-known [8,9], which has implications for micro- and nano-technologies (i.e., liqnid crystal displays and micromachines). The virtue of the force measurement has been demonstrated, for example, in our report on novel molecular architectures (alcohol clusters) at solid-liquid interfaces [10]. 

Small gold clusters (<100 atoms) have become the subject of interest because of their use as building blocks of nanoscale devices and because of their quantum-size effects and novel properties such as photoluminescence, magnetism, and optical activity [427]. 

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