Osmium carbonyl clusters protonation

Retaining the theme of metal carbonyl clusters, capping considerations in transition-metal clusters have been discussed with reference to [Sb2Co4(CX))] g( A-CX))], and [Bi2Co4(CO)jQ( i-CO)]" 28. An infrared spectroscopic study of the formation of carbonyl rhodium clusters on a rhodium electrode produced by oxidation reduction cycles in acidic solution 2 has also been published. Electrochemistry with ruthenium carbonyls >21 osmium carbonyls 2 jg also reported. Muon spin rotation in a metal-cluster carbonyl compound has been communicated and, lastly, a proton spin-lattice NMR relaxation study of hydride carbonyl clusters has been reported. This provides a method for determining distances involving hydrido ligands... 

Oxidation of metal clusters may also be performed by reaction with Bronsted-acids through straightforward addition of protons to metal backbone. Thus, carbonyl clusters of ruthenium, osmium and iridium are stable in acids and may be pro toned without decomposition. The H-NMR spectra of these carbonyls in concentrated sulfuric or trifluoroacetic acid indicate the formation of cationic metal hydrides ... 

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

Some striking demonstrations of metal-metal bond lability are provided by cluster rearrangements due to protonation. This is the case for some anionic osmium clusters (cf. Section VI). It involves ligand activation for some tetrairon clusters (51-53). Thus, the clusters 9 and 11 open up upon protonation, and compensation for the lost iron - iron bonds in the products 10 and 12 comes from the bonding between one iron atom and a carbonyl oxygen. The relation of these unusual nucleophile-electrophile interactions to cluster-induced CO transformations is obvious. 

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