ON THE THERMAL GROWTH AND DECOMPOSITION OF RHODIUM CARBONYL CLUSTERS

Article abstract of DOI:10.1016/S0022-328X(00)98532-2

The growth of anionic rhodium carbonyl clusters under pyrolytic conditions depends upon the counterion.The appearance of complexes of higher nuclearity was least favorable with the cesium salts while it occurred more readily with the ammonium salts following the trend: ⁺ (R=alkyl) < ⁺ (R'=benzyl) < ⁺ < ⁺.A redox reaction between the rhodium carbonyl anion and the ammonium cation is implicated in the thermal growth of clusters.The electron-transfer process between these two moieties may involve the initial fragmentation of the clusters to form ^-, and the reduction of the ammonium counterions by ^- at high temperatures, although the direct electron transfer from the polynuclear species to the cation cannot be ruled out yet.It appears that electron transfer precedes or, that it is parallel to, the fragmentation of the clusters while the aggregation reactions of the resulting fragments give the observed products.The formation of metallic rhodium from either mono- or poly-nuclear complexes, and the sequential transformations found for these species are in agreement with the expected thermodynamic preference for high nuclearity species.The existence of the independent effect of the amines and the alkali cations when both of them are present resulted in the inhibition of the formation of metallic rhodium and retarded the growth of the clusters.These results correlate with the enhanced rhodium solubility noted for some catalytic systems that are based on rhodium carbonyl clusters with amines and cesium salts added, and which are used to convert H2/CO mixtures into polyalcohols.