Clustering, series formation

The derivatives of the inorganic phosphorus acids are usually intractable powders which are unsuitable for X-ray diffraction, but those of the organic phosphorus acids are often crystalline and a series of complex solid-state structures which have been described as drums, cubes, oxygen-capped clusters, butterfly formations, crowns, and extended clusters32-51 have been identified. 

A distinct core-shell dimensional enhancement was observed as a function of the sum of the core-shell generation values used in the construction of the series (e.g., G4/G3, polydispersed dendrimer cluster/gel formation observed for 1 1 reaction ratios described in our earlier work [55]. 

A related unprecedented double insertion of electron-deficient alkynes has also been reported in the reactions of the linear Pt2Pd heterotrimetallic complex 64 with 65 (RO2CCSCR) (Scheme 24) [95,96]. A series of unsymmetri-cal A-frame clusters 68 has thus been obtained in which a first insertion of the alkyne takes place site-selectively into the Pt-Pd bond vs the Pt-Pt bond (66). After a zwitter-ionic polar activation of the resulting inserted alkene (67), a subsequent reaction with the phosphine unit of the dpmp allows one to obtain the products 68 via the nucleophilic migration of the terminal P atom from the Pd center to the CH terminal carbon (formation of the P-C bond). 

Methanatlon Studies. Because the most effective way to determine the existence of true bimetallic clusters having mixed metal surface sites Is to use a demanding catalytic reaction as a surface probe, the rate of the CO methanatlon reaction was studied over each series of supported bimetallic clusters. Turnover frequencies for methane formation are shown In Fig. 2. Pt, Ir and Rh are all poor CO methanatlon catalysts In comparison with Ru which Is, of course, an excellent methanatlon catalyst. Pt and Ir are completely inactive for methanatlon In the 493-498K temperature range, while Rh shows only moderate activity. 

A particularly elegant example of cluster formation involving chiral recognition and retention of chirality through an increasingly complex hierarchical series of clusters is that of rubrene on Au l 1 1 [9] illustrated in Figure 1.5... 

The resulting [Ca/Fe] versus [Fe/H] plot is shown in Fig. 1, where except for a few outliers that will have to be manually inspected, a clear trend appears [Ca/Fe] slowly rises with [Fe/H] until it reaches a maximum and then declines again for the most metal-rich stars (RGB-a according to [4]). This nicely confirms a previous finding by [8] and [9]. If the metal-rich stars have evolved within the cluster in a process of self-enrichment, the only way to lower their a-enhancement would be SNe type la intervention. No simple explanation is provided for the rise of [Ca/Fe] at low [Fe/H], although a series of star formation bursts should be the likely cause. 

Metal Preferences. LVC s are formed mainly by transition metals to the right in the periodic table (especially elements in Group 8). This is in part due to the availability of d electrons that can be used in back-donation to the 7r-accepting ligands. Moreover, the formation of LVC s is not particularly "row-sensitive" by which I mean that the first-transition-series metals, Fe, Co and Ni, tend to form most of the same cluster compounds as their congeners, Ru, Rh, Pd and Os, Ir, Pt. 

The fact that most of the available EM-values for large-ring formation in diverse cyclisation series cluster in the fairly limited range 0.05M-0.01 M suggests a virtually complete cancellation of contributions arising from the... 

The formation of the saturated monometal fragments will depend upon the ratio of CO M in the cluster and also, to a lesser extent, upon the arrangement of the carbonyl ligands about the central Mm cluster unit. Clearly, the higher the CO M ratio, the greater the probability of ejecting the appropriate M(CO) saturated unit. Consider the osmium series Osm (m = 1-8) ... 

Some post-transition elements (or the corresponding radicals) containing 3 or more electrons in their valence shell are able to assist the formation of clusters by bonding to several metal atoms. Typical examples of this behaviour are the extraordinarily easy syntheses of large series of compounds such as Co3 (CO)9 (p3-E) (E = Al, CR, CX, GeR, P, As, PS, S, Se, PR, SR) 201 209) and Fe3 (CO)9 (p3 -E)2 (E = S, Se, Te, NR, PR). This type of stabilization is usually found in trinuclear clusters although a few examples in tetranuclear clusters are known, for instance ... 

In mixed metal clusters, redistributions can occur in the mass spectrometer so that the highest observed peak arises from such a product. This is clearly exemplified by the heterometallic dodecacarbonyls containing metals of the cobalt triad. The thermal stability of this series of clusters decreases as the rhodium content increases and facile redistributions result in the formation of tetranuclear species containing less rhodium the very thermally stable Q I CO) does not rearrange184. ... 

Fig. 2 (a) Schematic of the formation of silver clusters using DNA oligonucleotide as scaffold. After complexation of DNA with silver cations, the mixture is reduced with NaBtL, and the fluorescent cluster is formed, (b) Absorption spectra of silver clusters acquired every 30 min using [5 -AGGTCGCCGCCC-3 ] = 10 uM. [Ag+] = 60 uM. and [BfLj-] = 60 pM. The foremost spectrum was acquired 9 min after adding the BH4, and it has at 426 nm. The inset spectrum shows the last spectmm in the series (692 min), with peaks at 424 and 520 nm. (c) Induced circular dichroism spectra. The cell path length was 5 cm. The spectra were collected 2 min (A, dashed-dotted line), 20 min (B, dotted line), 40 min (C,fine dashed line), 60 min (D, coarse dotted line), and 150 min (E, solid line) after adding the BH [32]... 

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