Other Cluster Systems

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... 

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. 

It has been our aim to try to fully understand the kind of chemical reactions which this cluster undergoes as well as their mechanistic course, with the expectation that this knowledge will prove applicable to other cluster systems. Complete details of these various studies will be provided in future, separate publications, but it is the purpose of this article to summarize the more important results and to present the overall reactivity picture of I FeRujCCO)] as we now perceive it. 

More generally, in terms of electron counting, alkyne-substituted clusters may be treated in a manner similar to other cluster systems. In these schemes the ligands are considered primarily as species which donate electrons to the cluster framework and do not have specific donor and acceptor properties of their own. Because of possible... 

Another way of obtaining insight into the bonding of met-cars, nanocrystals, and other cluster systems consists in investigating their mechanisms of decomposition. The method most widely used for met-cars and nanocrystals is photodissociation, in... 

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-... 

Small ey(Ib) that cobalt and niobium clusters react in a very selective fashion with deuterium was rapidly followed by similar studies on a variety of other metal systems. Three groups simultaneously reported similar dramatic behavior for iron c usters( 2b, 3b). Di hydrogen or di deuteri urn addition reactions have been reported for vanadium( 3e), i ron(lc,2a-d,3b), cobalt(lb),... 

Lewis Bases. A variety of other ligands have been studied, but with only a few of the transition metals. There is still a lot of room for scoping work in this direction. Other reactant systems reported are ammoni a(2e), methanol (3h), and hydrogen sulfide(3b) with iron, and benzene with tungsten (Tf) and plati num(3a). In a qualitative sense all of these reactions appear to occur at, or near gas kinetic rates without distinct size selectivity. The ammonia chemisorbs on each collision with no size selective behavior. These complexes have lower ionization potential indicative of the donor type ligands. Saturation studies have indicated a variety of absorption sites on a single size cluster(51). 

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. 

Defect clusters are similarly prominent in hydrated phases. For example, anatase nanocrystals prepared by sol-gel methods contain high numbers of vacancies on titanium sites, counterbalanced by four protons surrounding the vacancy, making a (Vxi 4H ) cluster. In effect the protons are associated with oxygen ions to form OH- ions, and a vacancy-hydroxyl cluster is an equally valid description. Similar clusters are known in other hydrated systems, the best characterized being Mn4+ vacancies plus 4H in y-Mn02, known as Reutschi defects. 

Many other clusters have been postulated to occur in these systems, together with a large number of ordered structures in which the clusters are arranged on regular crystallographic positions. 

Further studies on this system will include IR-SEC experiments under an atmosphere of CO to verify its catalytic activity for CO reduction and to aid in formulating a mechanism for the reaction. Other multimetallic systems used as CO reduction catalysts such as mthenium-, iridium-, and cobalt-based complexes, or metal clusters used as models in the active sites of biological systems, many of which have complex redox behavior can also be investigated using the IR-SEC technique. 

Different bismuth clusters are apparently formed in reactions between bismuth and bismuth trichloride-aluminum trichloride mixtures, from which the salts (Big) " (AICl4 )3, and (Bi8) + (AICl4 )2 have been isolated 43). A trigonal-bipyramidal structure was predicted for Big (a closo 6 skeletal bond pair system cf. C2B3Hg), and a square anti-prismatic structure for Bi8 , as appropriate for an arachno 8-atom, 11 skeletal bond pair cluster. Similar polyhedral shapes appear likely for other clusters not only of bismuth but also of other heavy main group metals (the anion Pbg ", for example, is isoelectronic with Bi9 " ). 

Norepinephrine is widely distributed throughout brain in a nonuniform pattern, discrete from that of other amine systems such as dopamine and serotonin, although overlapping in several areas. The majority of noradrenergic axons and nerve endings found in the brain originate from the locus co-eruleus, a small, well-delineated cluster of cell bodies located in the pontine brain stem, just below the floor of the fourth ventricle. Given the wide distribution of norepinephrine in the brain [widely documented in the psychiatric literature), remarkably few neurons, estimated as 12,000 neurons on each... 

Very few examples are known, apart from solid state structures, where the sulfide acts as a bridge to more than four metal atoms. Two Rh clusters [Rh17(S)2(CO)33]3 and [Rh,0S(CO)22]2 have been reported in the literature77,78 where the sulfide is found interstitially in the center of the metal cluster and has contacts to nine and eight Rh atoms, respectively. In the former example an almost linear S—Rh—S unit (with d(Rh—S) as short as 2.16 A) is encapsulated in an Rh16 cluster, with four Rh—S contacts at about 2.33 A and four more at about 2.8 A.77 Other related systems are, for example, the [M6S,7]4 ions (13 M = Nb, Ta) which have been recently prepared by Holm and co-workers.80 Here, among other types of sulfide coordination, a /vS has been found in the base of the bell-shaped ions. 

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