Cluster systems

Sellers H 1991 On modeling chemisorption processes with metal cluster systems. II. Model atomic potentials and site specificity of N atom chemisorption on Pd(111) Chem. Phys. Lett. 178 351-7... 

The dilithium triimidochalcogenites [Ei2 E(N Bu)3 ]2 form dimeric structures in which two pyramidal [E(N Bu)3] dianions are bridged by four lithium cations to form distorted, hexagonal prisms of the type 10.13. A fascinating feature of these cluster systems is the formation of intensely coloured [deep blue (E = S) or green (E = Se)] solutions upon contact with air. The EPR spectra of these solutions (Section 3.4), indicate that one-electron oxidation of 10.13a or 10.13b is accompanied by removal of one Ei" ion from the cluster to give neutral radicals in which the dianion [E(N Bu)3] and the radical monoanion [E(N Bu)3] are bridged by three ions. ... 

Ozin, Hanlan, and Power, using optical spectroscopy (49,121). In view of the marked temperature-effect observed for the cobalt system, we shall focus on this cluster system here. Evidence for cobalt-atom aggregation at the few-atom extreme first came from a comparison of the optical data for Co Ar — 1 10 mixtures recorded at 4.2 and 12 K (see Fig. 4). A differential of roughly 8 K in this cryogenic-temperature regime was sufficient to cause the dramatic appearance of an entirely new set of optical absorptions in the regions 320-340 and 270-280 nm (see Fig. 4). Matrix variation, from Ar, to Kr, to Xe, helped clarify atom-cluster, band-overlap problems (see Fig. 5). 

As holds for other cluster systems, certain magic cluster electron counts exist, which indicates for a certain cluster-halide ratio and interstitial present the filling of all bonding molecular orbitals and therefore the thermodynamically most stable situation. For main group interstitial atoms these are 14 cluster-based electrons whereas for transition-metal interstitials the magic number is 18 [1, 10-12]. All of these phases are synthesized by high-temperature solid-state chemical methods. A remarkable variety of different structure types has been... 

Fig. 5.8 Phase diagram for the quasi-ternary Zr-(CI,I)-B cluster system.

A broader and more important implication of the oxychlorides is the potential of expanding the ligand combination to other transition-metal cluster systems. The advances in soft-chemistry techniques open up new possibilities for the sta-... 

B. Influence of Solvation on the Composition of Protonated Clusters Mixed Cluster Systems... 

L Monomeric and Polymeric Organic Analogs of Boron Cluster Systems The polyhedral clusters of boranes and carboranes are groups of cluster systems that are present ubiquitously in organoboron polymers. As in the case with the... 

Dermota TE, Zhong Q, Castleman AW (2004) Ultrafast dynamics in cluster systems. Chem Rev 104 1861-1886... 

Figure 6.17 The molecular structure of the Fe19 crust cluster system. Reprinted from Powell, 1988, by courtesy of Marcel Dekker, Inc.

Photoionization ti me-of-fli ght mass spectrometry is almost exclusively the method used in chemical reaction studies. The mass spectrometers, detectors and electronics are almost identical. A major distinction is the choice of ionizing frequency and intensity. For many stable molecules multi photon ionization allowed for almost unit detection efficiency with controllable fragmentation(20). For cluster systems this has been more difficult because high laser intensities generally cause extensive dissociation of neutrals and ions(21). This has forced the use of single photon ionization. This works very well for low i oni zati on potential metals ( < 7.87 eV) if the intensity is kept fairly low. In fact for most systems the ionizing laser must be attenuated. A few very small... 

Unraveling the Origins and History of the Outer Halo Cluster System... 

The halo globular cluster system also provides valuable information, since accurate distances, and hence reliable ages, can be derived. Mackey Gilmore (2004) recently acquired and compiled a new, nearly complete, internally consistent set of photometric studies of the globular cluster population in both the Milky Way and its satellite galaxies they deduce, from analysis of HB morphol-ogy, age, abundance and structural information, somewhat more relaxed limits... 

Some general reviews on hydrogenation using transition metal complexes that have appeared within the last five years are listed (4-7), as well as general reviews on asymmetric hydrogenation (8-10) and some dealing specifically with chiral rhodium-phosphine catalysts (11-13). The topic of catalysis by supported transition metal complexes has also been well reviewed (6, 14-29), and reviews on molecular metal cluster systems, that include aspects of catalytic hydrogenations, have appeared (30-34). 

Practical systems based on Eq. (66), but using Ni(COD)2/RNC mixtures, were reported. Catalysis via Ni(CNR)3 or Ni(CNR)2 intermediates could not be completely ruled out in these cluster systems. The nickel isonitrile and acetylene clusters did not effect hydrogenation of the triple bond in nitrogen (394). 

A search for alternative energy supplies has triggered efforts to develop efficient homogeneous catalysts for Fischer-Tropsch-type syntheses via hydrogenation of carbon monoxide, a likely future key material available, for example, through oxidation of coal (33, 327, 328, 417, 418). Metal cluster systems have been used in attempts to emulate the presently used heterogeneous catalysts. The important reactions are methanation,... 

The above observations strongly indicate that O-protonation is an important step in this particular reaction for the reduction of coordinated CO. Recent studies in our laboratory provide other examples of proton induced reduction in metal cluster systems, and an example of proton induced CO reduction has recently been reported by Atwood (44). It thus appears that protons as well as Lewis acids are effective in the bifunctional activation of coordinated CO. 

There are several kinds of cluster ions, both cations and anions, observed in mass spectrometers. There are bare metal cluster cations and anions, binary clusters cations and anions M Em (where E is an element such as O or S), and other clusters involving ligands and metals. In this section, the bare metal cluster cations Mj and anions M will be discussed separately followed by the binary cluster cations M E+, then binary cluster anions M E , and finally other cluster systems having more than one metal atom. 

We take as our main themes in this review the structure, bonding, and reactivity of cluster systems, and will note any correlation between these aspects and work on heterogeneous systems. 

Information about internuclear distances in organic compounds has led to the view that the effective radius of an atom varies directly with bond order. This is understandable for elements like carbon, with a limited range of hybridized states, but less so for metallic (cluster) systems. The problem is threefold ... 

In all the above examples, the carbonyl group donates two electrons to the metal cluster unit. Four-electron donation by the carbonyl group has recently been observed although this appears to be a much less frequent mode of bonding in cluster systems, it has often been invoked to explain the properties of absorbed carbon monoxide on a metal surface. 

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