Giant clusters

FIGURE 13.8 The Johnson picture of fluxionality. A close-packed cuboctahedron of carbonyl groups defines a cavity in which the triangle of metals fits. This triangle can rotate within a fixed set of CO groups. 

Af—Af Multiple Bonds Chisholm has studied the reaction of M—M multiple bonds as shown in Fig. 13.9. In forming 13.28, it is not an M—M bond which is lost as would be the case for an Os carbonyl cluster. Instead two RO-to-metal ir-bonding interactions are lost (the lone pairs on O are 2e donors), and this allows the 2e of the two incoming nucleophiles to be accommodated. The carbonyl complex 13.29 is interesting because the v(CO) frequency is very low (—1600 cm ), and this is attributed to contributions from canonical forms of type 13.30. The system is an alkyne cyclotrimerization catalyst, probably via the sequence 13.31 — 13.32. 

Metal colloids have been known for years—red stained glass is gold in colloidal form in glass, for example. Aqueous colloids are preparations that contain metal particles of 20-1000 A diameter in which the metal does not precipitate because the surface of the particles is covered in a way that protects them from agglomeration the hydrophilic polymer polyvinylalcohol (PVA) is an effective stabilizing agent. In a typical preparation, an aqueous solution of a metal salt is reduced in the presence of PVA. Small metal clusters have a precisely defined nuclearity (number of metals) and structure, but colloid 

FIGURE 13.10 Electron micrograph of Moiseev s giant palkdium clusters on a carbon support (rcproduexi from ref. -KJa with p, rmisiiun of the Royal Society of Chemistry 1985) 

Unusual physical properties are sometimes seen for these particles. For example, Pt3(M phen) , Ox) shows two Pt NMR resonances that are as- 

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The last step, the formation of metal, takes place if in the reducing atmosphere (MX excess) a critical cluster size is reached (e.g., by heating the solution to more than 100°C), thus forming a giant cluster whose metal core is nearly identical to the bulk material and which can disproportionate to metal and Mm species without any further significant change of its bondings. 

Effects of acid strength and anion concentration have been studied as well as the effect of monodentate amines.564,565 Phosphines, in particular dppp, have been used as favorable ligands,566,567 but it has been shown that dppp is oxidized by nitrobenzene under the prevailing conditions.568 Giant clusters and heteropolyanions have been used.569,570 The role of nitroso... 

Moiseev, I.I. et al., Palladium-561 giant clusters chemical aspects of self organization on a nano level,... 

Vargaftik, M.N. et al., Catalysis with a Pd giant cluster phenol oxidative carbonylation to diphenul carbonate conjugated with reductive nitrobenzene conversion, J. Mol. Catal. A Chem., 108, 77, 1996. 

Moiseev, I.I., Tsirkov, G.A., Gekhman, A.E., and Vargaftik, M.N., Facile hydrogen- transfer reduction of multiple bonds by formic acid catalysed with a Pd-561 giant cluster, Mendeleev Commun., 7,1-3,1997. 

Kovtun G, Kameneva T, Hladyi S et al (2002) Oxidation, redox disproportionation and chain termination reactions catalysed by the Pd-561 giant cluster. Adv Synth Catal 344(9) 957-964... 

The contribution of other clusters is even lower. It is evident that, as the lifetime of mobile particles in the surface layer increases, higher clusters may accumulate through the successive addition of atoms to Mg5 to eventually form giant clusters and massive metal. 

YB66 crystallizes in space group Fm3c with a = 2344 pm and Z = 24. The basic structural unit is a thirteen-icosahedra Bi56 giant cluster in which a central B12 icosahedron is surrounded by twelve B12 icosahedra the B-B bond distances within the icosahedra are 171.9-185.5 pm, and the intericosahedra... 

Magic Nuclearity Giant Clusters of Metal Nanocrystals Formed by Mesoscale Self-Assembly... 

Figure 1. Schematic illustration of a (Mj5) 5 giant cluster. The structure of one of the nanocrystals is shown.

Figure 3. TEM image of Pd5(,i uanocrystals forming giant clusters. The numbers correspond to the proposed number of nanocrystal shells, n.

Figure 4. Histogram showing the distribution in the diameters of giant clusters.

Figure 5. Giant clusters of different magic nuclearities, (Pd the circles corresponding to the diameters of the clusters calculated on the basis of the effective volume of an individual nanocrystal.

Mesoscale self-assembly of metal nanocrystals into ordered arrays and giant clusters... 

Figure 8.23. TEM micrograph showing the giant clusters comprising Pd561 nanocrystals. The sample for TEM was prepared by the slow evaporation of a PVP- Pd561 hydrosol. (Reproduced with permission from reference [49].)...

The giant clusters could be reproducibly formed starting from Pd561 nanocrystals in water, ethanol and ethanol-water mixtures and from sols with very different concentrations of the nanocrystals. It is possible that the formation of the giant clusters is facilitated by the polymer shell that encases them. Unlike Pd nanocrystals coated with alkanethiols, which self-assemble to form ordered arrays, the polymer shell effectively magnifies the facets of the metallic core, thereby aiding a giant assembly of the nanocrystals. The surface properties of the polymer-coated nanocrystals are clearly more favorable in that the interparticle interaction becomes sufficiently attractive. 

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