Rhodium, high nuclearity carbonyl

High nuclearity carbonyls Rh4(CO)i2 and Rhs(CO)i6 have been extensively used as precursors for the preparation of supported rhodium catalysts. Early studies reported the use of a great variety of supports that includes metal oxides [159-166], zeolites [101, 167], polymers [168] and modified-silica surface [169]. 

Fig. 14. Relationship between weighted average of 13CO chemical shift (SCO ) and fraction of negative charge per carbonyl group (A) in rhodium high nuclearity metal carbonyl clusters.

The incorporation of bridging germanium ligands into high-nuclearity transition metal clusters has been accom-plished. Thermal reaction of Ph3GeH with rhodium carbonyl yields a mixture of germanium/rhodium cluster... 

Interpretation of the EXAFS of [Os3(CO)i2] absorbed on silica in terms of a structure [Os3(CO)ioH(OS=)] has been confirmed by comparison with [OS3-(CO)ioH(OSiPh3)], which was prepared from the reaction between [Os3(CO)i2] and Si(OH)Ph3. The same species is formed by physisorption of [Os3(CO)ioH3] and of [Os6(CO)i8l on silica. Carbon monoxide inhibits the transformation to [Rh8(CO)i8] which occurs when [Rh4(CO)i2] is chemisorbed on highly divided silica. Both [Rh4(CO)i2] and [Rh8(CO)x8] are converted to high nuclearity rhodium particles when the chemisorbed carbonyls are treated with water vapour, but surface-bound [Rh(CO)2(OSi=)] is formed when the chemisorbed polynuclear carbonyls are treated with oxygen gas. The molecular compounds [Rh4(CO)i2] and [Rh8(CO)i8] can be regenerated in that sequence when [Rh(CO)2(OSi=)] is treated with CO. In this context, the observation that Na2[M(CO)5] reduces CO2 to form [M(CO)8] (M=Mo, W) and carbonate ion may provide a model for the surface process. ... 

Our Interest In understanding the behavior of rhodium carbonyl clusters in systems which catalytlcally convert CO H2 Into alcohols 3. prompted us to test the potential presence of mononuclear and bI nuclear rhodium carbonyl complexes In these systems. A positive characterization of these species under these circumstances would show a previously unknown behavior of rhodium carbonyl clusters under high pressure of carbon monoxide. It could also show the existence of a parallel behavior between the chemistry of these species under ambient and high pressures of carbon monoxide, and it may shed some light on the catalytic reactions occurring in those systems. 3. ... 

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