Osmium large clusters

There are families of metal cluster compounds (Fig. 6.40) containing metal clusters surrounded by ligands (Lewis Green, 1982). In small cluster compounds, the electrons are paired, but in large clusters there will be closely spaced electronic levels, as in metal particles. In such clusters, quantum size effects would be expected. Benfield et al (1982) have found intrinsic paramagnetism in H20sio(CO)24 below 70K as expected of an osmium particle of approximate diameter of 10 A the excess paramagnetism increases with cluster size in osmium compounds (Johnson et al, 1985). 

Osmium forms stronger metal-metal bonds than either iron or ruthenium consequently, there are a large number of osmium carbonyl clusters see Cluster) and these complexes have a large body of reaction chemistry. The triosmium cluster are prepared from Os3(CO)i2. Most are diamagnetic... 

The gold phosphine cationic units tend to attach themselves to the periphery of the osmium cluster anion in the condensation process, and do not form part of the central core as do some other cationic capping groups. The reaction chemistry observed for the mixed osmium-gold clusters is largely centered on the osmium core and does not involve the gold centers directly. 

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 ... 

The Osmium cluster Os3(CO)12 and clusters in the presence of various phosphines and triphenylphosphite have been utilized for the hydrogenation of cinnamaldehyde and crotonaldehyde (Table 15.7) [36]. The results show that good yields of unsaturated alcohols can be obtained by using a large excess of phosphine at elevated hydrogenation temperatures. 

This survey of higher Os clusters covers their direct formation from Os3 clusters only. Studies of the structures, properties, and reactions of high nuclearity osmium clusters encompass a large area, too big to be considered in this article. 

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. 

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. 

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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