Rhodium nuclear properties

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. 

There are several examples of well-characterized tri- and tetranu-clear hydroxo-bridged complexes of chromium(III) and cobalt(III). Penta- and hexanuclear aqua chromium(III) complexes have been prepared in solution, but their structure and properties are unknown. Oligomers of nuclearity higher than four have not been reported for cobalt(IIl), with the exception of some hetero-bridged heteronuclear species (193, 194). There appear to be no reports of rhodium(III) or iridium(III) complexes of nuclearity higher than two. 

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. 

The structure of these rhodium clusters with the unique rhodium centre, posed a general question that has underpinned much of the research in this area, namely, when do compounds of this type begin to exhibit the properties of metals This of course involves the question of what criteria are required to define metal properties. No satisfactory solution has as yet been found to the initial question but work in this area has established that there is a gradual variation in properties as the size of the metal polyhedra increases. It would now appear that a spectrum of properties are possible, and that for the very high nuclearity clusters, of the types discussed below, new and distinctive behaviour may be anticipated. If this proves to be the case then this enhanees the possible application of these materials. 

The steric properties of the porphyrin ring help control the nuclearity of the species. For example, the rhodium-tetraphenylporphyrin (TPP) complex in the equilibrium of Equation 4.36 is predominantly dimeric, while the tetramesitylporphyrin (TMP) complex in Equation 4.37 is predominantly monomeric. - Generally, the bond dissociation energies of the Rh-Rh bond in dimeric rhodium-porphyrin complexes He in the range of 8-25 kcal/mol. " ... 

Table III. Catalytic properties of (Rh+Pd)/Si02 catalysts, obtained using rhodium clusters of different nuclearity and reduced with H2 at 100°C, for benzene hydrogenation.

---