Platinum metal clusters

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

This overview is organized into several major sections. The first is a description of the cluster source, reactor, and the general mechanisms used to describe the reaction kinetics that will be studied. The next two sections describe the relatively simple reactions of hydrogen, nitrogen, methane, carbon monoxide, and oxygen reactions with a variety of metal clusters, followed by the more complicated dehydrogenation reactions of hydrocarbons with platinum clusters. The last section develops a model to rationalize the observed chemical behavior and describes several predictions that can be made from the model. 

Other Cg hydrocarbons. The dehydrogenation of normal hexane and 2,3-di methyl butane also proceeds but not as voraciously on small platinum clusters. Figure 8 is a plot of the hydrogen content in the first adduct as a function of the size of the platinum metal cluster. The metal atom reacts via dihydrogen elimination to produce PtC6Hi2 products. The platinum trimer is now the smallest cluster that will produce a C H near one. The similarity of size dependent dehydrogenation of the normal hexane and the branched molecule suggest that these systems may not readily aromatize these alkanes. Further structural studies are needed to identify the reaction products. 

In summary a few "generalizations" have been found. First, size selective chemistry is strongly associated with chemisorption that requi res bond-breaking. Second, metal clusters react rapidly with ligands that molecularly chemisorb even when the eventual products involve dissociation of the ligand. Dehydrogenation of Cg-alkanes on small platinum clusters take exception to this. 

Platinum oxide cluster anions [Pt O]- and [PtM02] have been prepared by the reaction of the bare metal cluster anions [Pt ]- with N20 and 02, respectively (255). These platinum oxide cluster anions will oxidize CO to C02 and produce [PtJ, which can be reoxidized by N20 or 02. Thus a cyclic catalytic system for the oxidation of CO by N20 or 02 is produced. 

Electrospray ionization will often produce ions that are fully coordinated, stable, and nonreactive in the gas phase. These ions may be probed by removal of ligands to form coordinatively unsaturated ions that are generally reactive. The chemical activity of metal cluster ions differs markedly and often shows size specific enhanced reactivity or lack of reactivity. Silver cluster ions Ag are fairly inert similar to Ag+. Platinum cluster ions PL are quite reactive similar to Pt+. Often, large cluster ions only appear to react with one donor molecule such as benzene this may be due to low concentrations of reactants or short reaction times. Similar clusters may react with a larger number of smaller molecules, and so until more information is available, rules for the coordination behavior of metal clusters are as yet not available. 

Several mixed-metal clusters containing platinum and cobalt are known and some of them have been employed as methanol homologation catalysts.1 Among them, the title compound2 was first prepared unambiguously from the reaction of dichloro[l,2-ethanediylbis(diphenylphosphine)]platinum with sodium tetracarbonylcobaltate, Na[CO(CO)4]. The compound also may be prepared by the reaction of [l,2-ethanediylbis(diphenyl-phosphine)]bis(phenylethynyl)pIatinum with Co COJg.1... 

An important consequence of the nonutilization of tangential orbitals is that platinum clusters often do not obey the normal electron counting rules and appear to be electron deficient (19,21,29,58,75,76). Electron counts are usually intermediate between those found in normal transition metal clusters (58-68) and those observed in gold clusters (58,78), but no satisfactory general electron counting theory has been developed for Pt-containing clusters. In small Pt clusters constructed from PtL2 units, theoretical studies have shown that the total electron count depends on the relative orientation of the... 

Ketenyl complexes, in Ru-Os mixed-metal clusters, 6, 1080 Ketenylidenes, in iron cluster compounds, 6, 301 Ketenylidenetriphenylphosphoranes, with platinum, 8, 632 Ketimines... 

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