Anionic species molecular clusters

This structural variation notwithstanding, only a few cationic transition-metal ions react efficiently with molecular oxygen under gas-phase conditions (see below). In contrast, many anionic metal complexes and clusters are readily oxidized by O2 to afford various metal-oxide anions [19]. From a conceptual point of view, however, anionic species appear to be inadequate reagents for the activation of hydrocarbons, because they generally require electrophilic attack. At present, only a few oxidations by transition-metal oxide anions have been reported to occur in the gas phase, and they are mostly limited to relatively polar substrates, such as the CH3OH CH2O conversion [20]. Because of the lower reactivity of hydrocarbons, their C-H bond activation by metal-oxide anions is likely to be limited to radical pathways driven purely thermodynamically, i.e., when Z)(0-H) exceeds Z)(C-H) of the substrate [21]. As radical-type pathways are prone to create selectivity problems, and over-oxidation is particularly difficult to control, the anionic route appears less attractive as far as partial oxidation of alkanes is concerned. 

A series of alkali metal and alkaline earth metal-arsenic clusters formulated as M[c-Ass] (M = Li, Na, K, Rb, Cs) andM [c-Ass]+(M = Be, Mg, Ca, Sr, Ba) were investigated by Xu and Jin [173] using DFT methods. All M[c-Assj andM [c-Ass]" clusters adopt a pentagonal pyramidal structure with Csv symmetry, its basal plane involving the planar pentagonal [c-Ass] anion. From molecular orbital and NICS analysis, it was established that each of these species had three delocalized n MOs that satisfied the 4h - - 2 electron counting rule and therefore exhibit n-aromatic character. 

Although Ni(CO)4 was discovered many years ago, no neutral Ni2(CO)x compound has ever been synthesized in macroscopic amounts. However, several communications report ionic species such as [Ni2(CO)8l+, [Ni2(CO)7], and [Ni2(CO)6]+, where structures with one or two bridging carbonyls are proposed.2418 Plausible structures for neutral Ni2(CO)x (x = 5, 6, 7) have been investigated by theoretical methods, and decomposition temperatures well below room temperature have been predicted.2419,2420 Tetra-, penta-, and hexanuclear nickel carbonyl clusters have been investigated by means of molecular orbital theory. It is found that the neutral forms are more stable than the corresponding anionic forms but the anionic forms gain in stability as the nuclearity rises.2421 Nickel carbonyl cluster anions are manifold, and structural systematics have been reviewed.2422,2423 An example includes the anion [Ni9(CO)i6]2- with a close-packed two-layer metal core.2424... 

FAB is well suited for the analysis of ionic analytes. In positive-ion mode, the spectrum is usually dominated by the cationic species, C which is accompanied by cluster ions of the general composition [CnH-An-i]". Thus, the distance between these signals corresponds to the complete salt CA, i.e., yields its molecular weight. This behavior is perfectly analogous to FD (Chap. 8.5.3). In negative-ion FAB, the anion A" will cause the base peak of the spectrum, and accordingly, cluster ions of the type [Cn.iH-AJ" are formed in addition. Consequently, both cation and anion are usually identified from the same FAB spectrum, irrespective of the chosen polarity. Nonetheless, it is common practice to select the polarity of the more interesting ion for the measurement. 

The initial nucleation stage of the complex-decomposition mechanism is probably similar to the simple free-anion mechanism. Either ionic or molecular metal species (ion-by-ion) or Cd(OH)2 (cluster) adsorbs on the substrate. However, instead of conversion of the hydroxide to sulphide by topotactic reaction with sulphide ions, the chalcogenide precursor (in almost all studies of this mechanism, that is thiourea) adsorbs on the Cd(OH)2 surface to form a hydroxide-thiourea complex, which then decomposes to CdS. 

Specific solute-solvent interactions, which are not explicitly taken into account in continuum models, can play a significant role especially for charged species. A typical cluster formed by the glycine anion radical with four water molecules is shown in figure 16. The analogous supermolecule obtained for the neutral form is quite similar, except for the much weaker interaction energies involved in the neutral species. The number and the position of the solvent molecules are determined by molecular dynamics simulations performed by the AMBER force field [141]. 

Polyoxometalates (POMs) are discrete molecular structures composed of metal cations bridged by oxide anions. They are not multi-metal species with metal-metal bonding, one conventional definition of clusters, but they are clusters in the generic sense of the word. A number of general reviews on POMs have appeared in recent years.1-13 Since a great majority of these structures are anionic, and thus salts with obligatory cations, and constituted primarily of early-transition-metal (d-block) elements in their highest oxidation states, the... 

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