Heteroatom clusters

Heteroatom cluster addition reactions are exemplified by the following ... 

Heteroatom Cluster Compounds Incorporating Polyhedral Boranes as Ligands... 

In the following section, we give a survey of the known types of heteroboranes in question and discuss the structures in terms of the Wade-Williams rules. Readers, who are interested in the synthesis of heteroatom clusters, in their skeletal transformations, or in reactions at the ligand sphere, are referred to the cited literature. conjuncto- Boranes with heteroatoms in the skeleton are not considered in this brief discussion. 

Fig. 3.4-15. The planarized hexagonal aperture of five types (a)-(e) of ar-10

Prior to any work on heteroatom clusters the notion was expressed (20) that heteroatom placement within the polyatomic clusters would lead to a decrease In delocalization and bonding and thence stability. Although this may lessen stability the substitution clearly does not preclude It. Furthermore, many of the likely polyhedra already have Inequlvalent atom positions, the 5, 7, 9 and 10 atom examples already considered here for example, and mixed species especially with elements from different groups may be quite stable within the discrimination provided by Inequlvalent positions. Even the nominally equivalent atom positions In a tetrahedron can obviously accommodate substantial differences. Additional examples of mixed element polycations are certainly to be expected. An Inadequate foresight was revealed In a review of polycations (20) written for a 1974 award symposium, about one year before the crypt discoveries, by the expectation that polycations should be more stable than polyanions for the metallic elements. In hindsight, metallic behavior Is a property of the dense solid state and has little to do with the stability of small clusters where electronic and geometric factors are far more important. 

In summary, the bonding situation found in the deltahedral stmctures of bare Zintl ions, which follow Wade s mles, changes smoothly to that observed in ligand-stabilized metalloid and heteroatomic clusters. Therefore, they can be considered as a subclass of intermetalloid clusters. The accessibility of larger intermetalloid... 

In case < 0, bonding of like atoms is energetically preferred, leading in principle to separation of the alloy into a mixture of A and B rich solid-solution phases, each with nearly homoatomic SRO clusters, compared to short range A-B mixed or heteroatomic clusters in the former case of V >0. In other words, the tendency to order (or phase-separate) is manifested to some, local degree also in most solid solutions, where the distribution of atoms in the crystal is not entirely random, and should be incorporated too in any theoretical quantitative evaluation of surface segregation phenomena. Moreover, many alloys of... 

S-adenosylmethionine (SAM or AdoMet) superfamily, aconitase, and others), enzymes that contain Fe-S heteroatomic clusters (nitrogenase iron-molybdenum cofactor (FeMoco), carbon monoxide dehydrogenase (CODH), and acetyl CoA synthase (ACS)), and enzymes that contain unique ligation sets around specialized iron centers ([NiFe] and [FeFe] hydrogenases) (Figure 1). ... 

A more complete mini-periodic table of NPFM bonding wiU have to include the heavy alkali metals (K, Cs, Rb, Fr), and the group III metalloids, such as B, Al, and so on. In view of the importance of the ionic stractures, the heteroatomic clusters may be even more strongly bonded. Thus, even though the principles of NPFM bonding are clear, still some future work thus lies ahead. 

Binary aggregates are usually called heteroatom clusters when m = 1 (or some-... 

Early interest in heteroatom clusters having alkali metals as the host was academic rather than dictated by precise observations. The main question regarded the extent to which the jellium-derived shell model retained its validity. However, this question was approached on the basis of oversimplified structural models in which the heteroatom (typically a closed-shell alkali-earth such as Mg) was located at the center of the cluster [235, 236]. In this hypothetical scheme, the perturbation of the electronic structure relative to that of the isoelectronic alkali cluster is somewhat trivial for instance, in the Na Mg system the presence of Mg would only alter the sequence of levels of the shell jellium model from Is, Ip, Is, 2s,. .. (appropriate to sodium clusters) to Is, Ip, 2s, Id,. .. (see also [236]). This would lead to the prediction that Na6Mg and NasMg are MNs. 

Finally, let us mention a recent calculation of the special heteroatom cluster AliaNa [243] (see Appendix C), which corresponds to a 40e system, namely, an MN of the shell model, and is isoelectronic to the Ali3 anion discussed in Section 3.8. The result is not... 

The vast majority of metal-rich rare earth-metal halides form clusters R that incorporate an atom Z, or a small atom group, in the center of the cluster, hence heteroatomic clusters ZR need to be considered. These clusters are surrounded by ligands, usually haUdo ligands X of the triad chloride, bromide, and iodide. Figure 10 depicts such an isolated cluster complex [ ZR X i2X 6] ... 

The rare-earth elements, R, may form clusters of six to eight R atoms encapsulating an endohedral transition metal atom T to compensate for the low number of valence electrons of the R atoms (only three). These heteroatomic clusters, TR,, are surrounded by halide ligands, X, to build cluster complexes, TR ... 

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