Osmium metal atom cluster compounds

Five-metal-atom clusters have been obtained only with osmium. The binary carbonyl Os5(CO)i8 was initially formed in low yield (—10%) by the thermolysis of Os3(CO)12 (201). The yield of the compound may be... 

Metal carbonyl cluster compounds which contain three ruthenium or three osmium atoms in the cluster core are common.1 Potentially useful reagents for syntheses of these compounds are the triruthenium and triosmium dianions [M3(CO)h]2 (M = Ru, Os).2 Therefore, it is desirable to develop good synthetic routes to obtain [M3(CO)11]2- (M = Ru, Os) of high purity in high yields. A method that is particularly useful for generating [M3(CO)n]2 (M = Ru, Os) is the designed stoichiometric reduction of M3(CO)12 (M = Ru, Os) using an electron carrier such as potassium-benzophenone.3... 

The possibility of coordination of a two-electron ligand, in addition to arene, to the ruthenium or osmium atom provides a route to mixed metal or cluster compounds. Cocondensation of arene with ruthenium or osmium vapors has recently allowed access to new types of arene metal complexes and clusters. In addition, arene ruthenium and osmium appear to be useful and specific catalyst precursors, apart from classic hydrogenation, for carbon-hydrogen bond activation and activation of alkynes such compounds may become valuable reagents for organic syntheses. 

We have mentioned only in passing other cluster complexes in which a tetrahedral core of 1 carbon and 3 metal atoms is present. Such complexes in which the metal atoms are nickel, ruthenium, and osmium have been prepared XIII (81), XIV (82, 83), and XV (66, 82). Their chemistry remains largely unexplored, except for the transformations of compound XV in strong acid medium which we mentioned in the previous section. 

These metals are prolific formers of clusters, most of which obey the electron counting rules just presented. Osmium forms the greatest number, and is the only one to form clusters with more than six metal atoms. Indeed, osmium cluster compounds are so numerous (including many heteronuclear ones) that they afford examples of many subtle structural variations. 

Examples of the first type of these reactions are found in several systems but the reaction mechanisms may follow different pathways. One route is substitution of one of the carbonyl ligands as has been observed in some osmium and ruthenium complexes (458, 459). The other mechanism involves metal-metal bond fission (414, 428), and in some cases this means the formation of cluster compounds with a smaller number of metal atoms [Eq. (20)] (460). 

Metal carbonyl clusters containing four or more metal atoms are made by a variety of methods osmium in particular forms a range of binary compounds and pyrolysis of Os3(CO)i2 yields a mix of products (equation 23.15) which can be separated by chromatography. 

Although less fully documented than osmium cluster chemistry, rhenium cluster chemistry has been subjected to many structural studies, including those on approximately 20 neutral or anionic carbonyls, particularly carbonyl hydrides [Rev(CO). H ] of nuclearities x = 2 to 6 (Fig. 7). In addition, some ten or more rhenium carbonyl carbides [Rev(CO)vH C] have been shown to contain a core carbon atom, usually occupying a central octahedral site. These systems offer scope not only to explore for rhenium the trends we have already shown for osmium, but also to study the effect on metal-metal distances (and so enthalpies) of such core carbon atoms, which formally donate all four of their valence shell electrons to the cluster bonding. To our knowledge only one rhenium carbonyl cluster compound, Re2(CO)io, has been subjected to calorimetric study to determine its enthalpy of formation. ... 

Wider possibilities are also available with cluster-t)T3e MCMs. They are molecular compounds that include a framework of metal atoms, which are separated by short distances (not more than 3.5 A) permitting direct metal-metal interaction. Such a framework is enclosed within a set of polymerizable ligand groups. The first report in this regard relates to MCMs based on Co2(CO)g or Fc2(CO)9 and the methyl ester of j7-vinyldithiobenzoic acid as well as Wcyclohexyl-4-vinylthiobenzamide [57]. MCMs based on a carbonyl cluster of osmium 89, ruthenium [58] and rhodium [59] with 4-VPy 90 and allyldiphenylphosphine 91 are stable compounds (see Experiments 4-9 and 4-10, Section 4.6) [59,60]. 

Arenes can also bridge tw o metals, as shown in Figure 2.35. In one example shown, the arene bridges and sandwiches two palladium atoms. The arene in the dinuclear nickel complex also bridges two metals. The arene in the osmium complex binds symmetrically to each of the three metal atoms on the triangular face of a cluster. This compound provides a molecular view of the chemisorption of benzene on a metal surface. ... 

The methyl group in Os3(/i-CH3)(/x-H)(CO)io is asymmetrically bridged to osmium atoms through carbon and hydrogen and exists in equilibrium with the methylene compound. This is a classical example of a facile breaking of the C —H bond by metal atoms of a cluster. This cluster reaction represents a simple model of C —H bond cleavage occurring on the surface of the metal [equation (3.95)]. 

This observation may well explain the considerable difference between metal-olefin and metal-acetylene chemistry observed for the trinuclear metal carbonyl compounds of this group. As with iron, ruthenium and osmium have an extensive and rich chemistry, with acetylenic complexes involving in many instances polymerization reactions, and, as noted above for both ruthenium and osmium trinuclear carbonyl derivatives, olefin addition normally occurs with interaction at one olefin center. The main metal-ligand framework is often the same for both acetylene and olefin adducts, and differs in that, for the olefin complexes, two metal-hydrogen bonds are formed by transfer of hydrogen from the olefin. The steric requirements of these two edgebridging hydrogen atoms appear to be considerable and may reduce the tendency for the addition of the second olefin molecule to the metal cluster unit and hence restrict the equivalent chemistry to that observed for the acetylene derivatives. 

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