Giant metal cluster

Schmid, G. Ligand-stabilized Giant Metal Clusters and Colloids. In Physics and Chemistry of Materials with Low-Dimensional Structures, Kluwer Academics The Netherlands, Longh, J. L. 1994 Vol. 18, pp 107. 

The chemistry of giant metal clusters is still at the start of its development. The studies outlined in this survey show that assembling of palladium and platinum atoms to form giant clusters proceeds via polynuclear complexes, which are precursors difficult to isolate for palladium but are more stable for platinum. 

Much larger clusters have, however, been found in a series of giant metal-cluster molecules, the metal cores of which are members of the series of so-called magic-number (full-shell) clusters depicted in Fig. They are obtained by surrounding... 

J1.9 Ligand-stabilised giant metal clusters and colloids... 

It has been shown, however, that in some cases, catalysts working efficiently even if they are used in very small quantities (0.1 ppm) such as the Speier catalyst H2PtCl6 or the Karstedt catalyst [Pt(CH2=CHSiMe20SiMe2CH=CH2)2], are indeed functioning in a colloidal nanoparticle form. Such nanoparticles are giant metal clusters containing several hundred atoms that are readily produced for instance by reduction of noble metal salts or complexes (see Chap. 2.2.1 and 20.7.3). 

Volkov, V. V. et al., Long- and short-distance ordering of the metal cores of giant Pd clusters, J. Cryst. 

In contrast to the usual Wacker-conditions, optimum rates and catalyst stability in the Pd/batophenanthroHne-catalyzed olefin oxidations was observed in the presence of NaOAc (pH s 11.5). Under such conditions, the catalyst-containing aqueous phase could be recycled with about 2-3 % loss of activity in each cycle. In the absence of NaOAc precipitation ofPd-black was observed after the second and third cycles. Nevertheless, kinetic data refer to the role of a hidroxo-bridged dimer (Scheme 8.1) rather than the so-called giant palladium clusters which could easily aggregate to metallic palladium. 

In the last three decades of the twentieth century, following Walter Hieber s retirement, four aspects of the research on mono- and polynuclear metal carbonyl complexes found particular attention. These were the preparation of highly reduced carbonyl metallate anions, the generation of stable metal carbonyl cations, the matrix isolation of uncharged metal carbonyls obeying or not the 18-electron rule and, last but not least, the giant metal carbonyl clusters. 

A. Muller, S. K. Das, C. Kuhlmann et al., On the Option of Generating Novel Type Surfaces with Multiphilic Ligands within the Cavity of a Giant Metal-Oxide Based Wheel Type Cluster Chemical Reactions with Well-Defined Nanoobjects, Chem. Commun. 2001, 655—656. 

The shell model for metal clusters, described above, has an important implication which will not have escaped the reader if electrons become delocalised from individual atoms and can roam freely over the whole cluster to form a closed shell, then this shell should be able to oscillate collectively, and should therefore exhibit giant dipole resonances analogous to those which were described in chapter 5 for free atoms. 

The second, and more important kind is the giant dipole resonance intrinsic to the delocalised closed shell of a metallic cluster. Such resonances have received a great deal of attention [684]. They occur at energies typically around 2-3 eV for alkali atoms, and have all the features characteristic of collective resonances. In particular, they exhaust the oscillator strength sum rule, and dominate the spectrum locally. 

Fig. 12.15. Example of a giant dipole resonance in a metal cluster with a closed shell, in this case a singly ionised K cluster with eight delocalised electrons (after C. Brechignac and J.-P. Connerade [714]).

The fact that they tend to be fairly symmetrical (at least when they occur below the ionisation threshold) is related to their time characteristics from the lifetime widths and resonance energies, one can deduce that the giant resonances in metallic clusters are many-body oscillations undergoing several periods. Giant resonances in metallic clusters can truly be considered as plasmons, and relate quite clearly to surface plasmons in solids. 

Clusters also demonstrate the ubiquity and generality of the basic principles of physics the stability of metal clusters is governed by a shell closure closely related to that of nuclear physics. Indeed, the collective, giant dipole resonances in clusters and in nuclei obey the same laws over changes of fourteen decades in scale size. 

The building block of each layer of 10 is the spherical icosahedral giant oxidized cluster cage of the Mo72Fe3o type but which now has a reduced metal-oxide-based cluster - the tetrahedral two-electron reduced Keggin [H2PM012040] ion-as nucleus (Figure 8). Like in the layer compound 8, each of the cluster-cluster composites is linked to four others via Fe-O-Fe bonds to form a layer structure. 

A giant heterometallic cluster with its 108 metal ions organized into a four-shell Russian doll-like structure has been reported recently (Kong... 

One of the challenging aims of cluster chemistry is to elucidate the factors controlling the formation of cluster molecules and small metal crystallites. Despite remarkable achievements in the synthesis and structural characterization of metal clusters, the pathways to the assembly of large numbers of metal atoms in the course of the synthesis of high nuclearity metal clusters remains rather mysterious. Some insight into this problem has been gained by recent studies of so-called giant clusters of palladium and platinum. 

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