Osmium carbonyl cluster anions

Roberto, D., Cariati, E., Ugo, R., and Psaro, R., Surface-mediated organometallic synthesis High-yield preparations of neutral and anionic osmium carbonyl clusters by controlled reduction of silica-supported [Os(CO)3Cl2]2 and OsClj in the presence of Na2C03 or K2C03. Inorg. Chem. 35,2311 (1996). 

This review will cover the preparation of various osmium carbonyl clusters which led to very convenient surface-mediated syntheses of both neutral and anionic osmium carbonyl clustersJ A large part of this work has been performed in our laboratoriesj ... 

Surface-mediated syntheses of anionic osmium carbonyl clusters from neutral complexes, clusters, or salts require a surface of adequate basicity as the reaction medium. This has been achieved by two different approaches. The first is the use of an intrinsically strongly basic surface such as magnesia. The basicity of this surface can be easily varied by adjusting the temperature of the treatment. Increasing the temperature leads to gradual decarbonation and dehydroxylation of the surface with creation of very basic centers, and therefore to increased surface basicity.l Similar thermal behavior is observed with the less basic alumina surface.t l... 

We recently obtained evidence for a step-by-step process (Scheme 5) which explains the origin of the high selectivity observed in the controlled reduction of silica-supported [Os(00)3012)2 to generate different neutral or anionic osmium carbonyl clusters, in the presence of alkali metal carbonates (Schemes 1 and 2). ... 

The use of the surface of inorganic oxides as reaction media enables selective and high-yield syntheses of a variety of neutral and anionic osmium carbonyl clusters starting from simple and readily available materials. The procedures are simple and straightforward. Because the use of a solid as a reaction medium is not limited, as in solution, by boiling points, it is possible to work at atmospheric pressure even at relatively high temperatures. 

Similarly higher osmium carbonyl clusters are obtained from the pyrolysis of Os3(CO)22 (p. 351). Other syntheses involve carbonyl anions as intermediates (P. 171). 

Mingos rules. This hypoelectronicity or electron poverty (fewer than the Wade-Mingos 2n + 2 skeletal electrons) in the bare metal cluster anions Enz (z < n + 2) leads to deltahedra not only different from those in the deltahedral boranes but also different from those in hypoelectronic metal carbonyl clusters of metals such as osmium. 

A considerable range of iron carbonyl clusters is now known to contain antimony or bismuth, and later developments have seen this extended to ruthenium and osmium complexes. Whitmire (711) has described the reaction between NaBi03 and methanolic [Fe(CO)5], which affords the anionic triiron cluster [BiFe3(CO)10] (108), which contains a tetrahedral BiFe3 core. Cluster 108 is an example of a closo tetrahedral cluster with three first-row... 

Large metal clusters have been supported on many metal oxides. For example, the carbido-cluster anion [OsioC(CO)24] has been adsorbed directly onto MgO. This cluster anion can also be prepared in situ on MgO by the Reductive Carbonylation of adsorbed H2OSCI6. The adsorbed osmium cluster anion appears to be ionically bound to the surface. 

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. ... 

Sanders went on to synthesise a second generation of porphyrin hosts using guest amine-type molecules as templates. This enabled cyclic porphyrin dimers and trimers to be prepared. The trimer recognises and binds large polyoxometalate anionic clusters (e.g. PWj2O40 ) and anionic metal-carbonyl clusters containing osmium and carbon (e.g. OSioC(CO)24 -).-2 ... 

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