Rhodium, nuclear reactions

Fig. 3.6 (a) Decay scheme of and (b) ideal emission spectrum of Co diffused into rhodium metal. The nuclear levels in (a) are labeled with spin quantum numbers and lifetime. The dashed arrow up indicates the generation of Co by the reaction of Mn with accelerated deuterons (d in Y out). Line widths in (b) are arbitrarily set to be equal. The relative line intensities in (%) are given with respect to the 122-keV y-line. The weak line at 22 keV, marked with ( ), is an X-ray fluorescence line from rhodium and is specific for the actual source matrix... 

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... 

For the rational design of transition metal catalyzed reactions, as well as for fine-tuning, it is vital to know about the catalytic mechanism in as much detail as possible. Apart from kinetic measurements, the only way to learn about mechanistic details is direct spectroscopic observation of reactive intermediates. In this chapter, we have demonstrated that NMR spectroscopy is an invaluable tool in this respect. In combination with other physicochemical effects (such as parahydrogen induced nuclear polarization) even reactive intermediates, which are present at only very low concentrations, can be observed and fully characterized. Therefore, it might be worthwhile not only to apply standard experiments, but to go and exploit some of the more exotic techniques that are now available and ready to use. The successful story of homogeneous hydrogenation with rhodium catalysts demonstrates impressively that this really might be worth the effort. 

Like cobalt, rhodium also forms clusters of higher nuclearity by the oxidation of the trigonal prismatic anion [Rh6C(CO)l5]2, 24. Treatment of aqueous K2Rh6C(CO)l5 with ferric ion yields an as yet uncharacterized solid, which on dissolution in methylene chloride yields a number of crystalline products, the nature of which depends on which reaction time... 

In 1943, Hieber and Lagally reported that the reaction of anhydrous rhodium trichloride with carbon monoxide at 80°C, under pressure, and in the presence of silver and copper as halogen acceptors, gave a black crystalline product which, on the basis of elemental analysis, was formulated as Rh4(CO)n 75). The exact nature of this compound was established 20 years later by Dahl using three-dimensional X-ray analysis which led to its reformulation as Rh6(CO)i6 53). This discovery can be regarded as the birthday of the chemistry of high nuclearity clusters. 

Some use has been made of severe reaction conditions in which a metal salt or complex is reduced in the presence of a main group element compound [Eqs. (229)-(233)]83 375,378,384,385 to give extremely robust compounds. Unfortunately, little of the subsequent chemistry of these high-nuclearity species has been reported, except for spectroscopic investigations of the rhodium compounds that have shown the metal frameworks to be fluxional as well as the ligand complement. 

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. ... 

In rhodium clusters, e.g., [Rh13(CO)14H5. ]" (n = 2 and 3), only one of the H atoms is encapsulated and 3H nmr spectra show that this atom interacts with all 13 Rh atoms (103Rh, spin V2) and is thus migrating within the cluster. Proton transfer reactions of interstitial hydrides have been studied72 and more complex hetero-nuclear species73 can be made by reactions such as... 

Plutonium-noble metal compounds have both technological and theoretical importance. Modeling of nuclear fuel interactions with refractory containers and extension of alloy bonding theories to include actinides require accurate thermodynamic properties of these materials. Plutonium was shown to react with noble metals such as platinum, rhodium, iridium, ruthenium, and osmium to form highly stable intermetallics. Vapor pressures of phases in these systems were measured by the Knudsen effusion technique. Use of mass spectrometer-target collection apparatus to perform thermodynamic studies is discussed. The prominent sublimation reactions for these phases below 2000 K was shown to involve formation of elemental plutonium vapor. Thermodynamic properties determined in this study were correlated with corresponding values obtained from theoretical predictions and from previous measurements on analogous intermetallics. 

Applications of nuclear magnetic 179 resonance to the study of organo- (380) metallic compounds Reactions involving organometallic 20 compounds of rhodium, iridium, (77) palladium and platinum a survey of NMR studies... 

The selective production of methanol and of ethanol by carbon monoxide hydrogenation involving pyrolysed rhodium carbonyl clusters supported on basic or amphoteric oxides, respectively, has been discussed. The nature of the support clearly plays the major role in influencing the ratio of oxygenated products to hydrocarbon products, whereas the nuclearity and charge of the starting rhodium cluster compound are of minor importance. Ichikawa has now extended this work to a study of (CO 4- Hj) reactions in the presence of alkenes and to reactions over catalysts derived from platinum and iridium clusters. Rhodium, bimetallic Rh-Co, and cobalt carbonyl clusters supported on zinc oxide and other basic oxides are active catalysts for the hydro-formylation of ethene and propene at one atm and 90-180°C. Various rhodium carbonyl cluster precursors have been used catalytic activities at about 160vary in the order Rh4(CO)i2 > Rh6(CO)ig > [Rh7(CO)i6] >... 

With rhodium clusters the nuclearity of the species involved is doubtful. Chini et al 231) showed in a clean stoichiometric reaction that propene is converted into a 1 1 mixture of butanal and 2-methylpropanal by Rh4(CO)i2 and hydrogen the hexanuclear cluster Rh6(CO)J6 is formed quantitatively ... 

As reported in the same paper, it is likely that this unstable rhodium cluster is converted into the mononuclear rhodium-hydride species HRh(CO)x (x = 3,4), which are usually considered as the true catalyst system in the reaction mixture. These compounds represent extremely unstable intermediates, which would certainly recombine to form higher nuclearity rhodium species if alkene is not present in the reaction mixture. This mechanism is proposed for all the hydroformylation experiments carried out in the presence ofRh4(CO)iz... 

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