Hydrido cluster complex, rhodium

It was of interest in this respect to determine which rhodium carbonyl complexes result from the former reaction and whether the parallel formation of any organic products derived from the hydrogenation of carbon monoxide Is also occurring. The latter possibility was considered because of the presence of hydrides In the cluster and the involvement of hydrido carbonyl complexes in the hydrogenation of carbon monoxide. Unfortunately, It was not possible to detect any organic products formed from this reaction even after the cyclic repetition of the transformations below (equation 14). 

These studies have Indicated that simple rhodium carbonyl complexes, e.g., mono- and binuclear species are Involved in the fragmentation and aggregation reactions of rhodium carbonyl clusters under high pressures of carbon monoxide and hydrogen. They indicate that it is possible to write formal equations for such reactions in the case of rhodium carbonyl anionic hydrido clusters (equation 25) and for the more particular situation when there are not hydrides present (equation 26)... 

A similar pattern has always been discussed for rhodium, with hydri-dotetracarbonylrhodium H-Rh(CO)4 as a real catalyst species. The equilibria between Rh4(CO)i2 and the extremely unstable Rh2(CO)s were measured by high pressure IR and compared to the respective equilibria of cobalt [15,16]. But it was only recently that the missing link in rhodium-catalyzed hydroformylation, the formation of the mononuclear hydrido complex under high pressure conditions, has been proven. Even the equilibria with the precursor cluster Rh2(CO)s could be determined quantitatively by special techniques [17]. Recent reviews on active cobalt and rhodium complexes, also ligand-modified, and on methods for the necessary spectroscopic in situ methods are given in [18,19]. 

The formation of this gold-rhodium cluster compound has been attributed to a mechanism involving partial reduction of the Au(PPh3)N03 species by the metal hydrido complex. 

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