Rhodium atomic properties

The argument of each sine contribution in (6-8) depends on k, which is known, r, which is to be determined, and the phase shift (f(k). The latter needs to be known before r can be determined. The phase shift is a characteristic property of the scattering atom in a certain environment, and is best derived from the EXAFS spectrum of a reference compound, for which all distances are known. For example, the phase shift for zero-valent rhodium atoms in the EXAFS spectrum of a supported rhodium catalyst is best determined from a spectrum of pure rhodium metal as in Fig. 6.13, while RI12O3 may provide a reference for the scattering contribution from oxygen neighbors in the metal support interface. 

Some rhodium and platinum clusters exhibit dynamic properties, i.e., skeletal fluxionality. In the RhNMR of [Rh9P(CO)2i] , which has a capped antiprism structure, three signals possessing the intensity ratio 4 4 1 are observed at 183 K, while at 298 K there is only one signal. This experiment shows that, due to fluxionality of the skeleton, all rhodium atoms becomes equivalent. The same is indicated also by the presence of a decet in the PNMR spectrum of this compound. Similarly, [RhioS(CO)22] and [RhioE(CO)22] (E = P, As) are rigid at low temperatures but fluxional at high temperatures. 

The platinum-group metals (PGMs), which consist of six elements in Groups 8— 10 (VIII) of the Periodic Table, are often found collectively in nature. They are mthenium, Ru rhodium, Rh and palladium, Pd, atomic numbers 44 to 46, and osmium. Os indium, Ir and platinum, Pt, atomic numbers 76 to 78. Corresponding members of each triad have similar properties, eg, palladium and platinum are both ductile metals and form active catalysts. Rhodium and iridium are both characterized by resistance to oxidation and chemical attack (see Platinum-GROUP metals, compounds). 

Since ruthenium and rhodium are neighboring elements in the periodic table, a closer comparison of the properties of ruthenium-copper and rhodium-copper clusters is of interest (17). When we compare EXAFS results on rhodium-copper and ruthenium-copper catalysts in which the Cu/Rh and Cu/Ru atomic ratios are both equal to one, we find some differences which can be related to the differences in miscibility of copper with ruthenium and rhodium. The extent of concentration of copper at the surface appears to be lower for the rhodium-copper clusters than for the ruthenium-copper clusters, as evidenced by the fact that rhodium exhibits a greater tendency than ruthenium to be coordinated to copper atoms in such clusters. The rhodium-copper clusters presumably contain some of the copper atoms in the interior of the clusters. 

Because of- the similarity in the backscattering properties of platinum and iridium, we were not able to distinguish between neighboring platinum and iridium atoms in the analysis of the EXAFS associated with either component of platinum-iridium alloys or clusters. In this respect, the situation is very different from that for systems like ruthenium-copper, osmium-copper, or rhodium-copper. Therefore, we concentrated on the determination of interatomic distances. To obtain accurate values of interatomic distances, it is necessary to have precise information on phase shifts. For the platinum-iridium system, there is no problem in this regard, since the phase shifts of platinum and iridium are not very different. Hence the uncertainty in the phase shift of a platinum-iridium atom pair is very small. 

Field emission microscopy was the first technique capable of imaging surfaces at resolution close to atomic dimensions. The pioneer in this area was E.W. Muller, who published the field emission microscope in 1936 and later the field ion microscope in 1951 [23]. Both techniques are limited to sharp tips of high melting metals (tungsten, rhenium, rhodium, iridium, and platinum), but have been extremely useful in exploring and understanding the properties of metal surfaces. We mention the structure of clean metal surfaces, defects, order/disorder phenomena,... 

In order to vary the electronic situation at the carbene carbon atom a number of carbo- and heterocycle-annulated imidazolin-2-ylidenes like the benzobis(imida-zolin-2-ylidenes) [58-60] and the singly or doubly pyrido-annulated A -heterocyclic carbenes [61-63] have been prepared and studied. Additional carbenes derived from a five-membered heterocycle like triazolin-5-ylidenes 10 [36], which reveals properties similar to the imidazolin-2-ylidenes 5 and thiazolin-2-ylidene 11 [37] exhibiting characteristic properties comparable to the saturated imidazolidin-2ylidenes 7 have also been prepared. Bertrand reported the 1,2,4-triazolium dication 12 [64]. Although all attempts to isolate the free dicarbene species from this dication have failed so far, silver complexes [65] as well as homo- and heterobimetallic iridium and rhodium complexes of the triazolin-3,5-diylidene have been prepared [66]. The 1,2,4-triazolium salts and the thiazolium salts have been used successfully as precatalysts for inter- [67] and intramolecular benzoin condensations [68]. 

The coordination properties of this phosphine-alane were further investigated by Fontaine et al. toward the rhodium(III) precursor [RhCp Me2(DMSO)] (Cp =pentamethylcyclopentadienyl, DMSO = dimethylsulfoxide).78 According to the spectroscopic data, the DMSO molecule is retained in the corresponding complex 96, most probably by interaction of its oxygen atom with the pendant alane moiety (Scheme 55). Trimethylaluminium was added to 96 to trap DMSO and generate complex 97 related to 96. But the competition between the external and... 

Molecular recognition with rhodium complexes of functionalized porphyrins - The coordination abilities of rhodium(III) porphyrins outlined in Scheme 3 (paths b and g) were used to design porphyrins with special receptor properties, i.e. cis-or trans-5,15-bis(2-hydroxy-l-naphthyl)octaethylporphyrin [H2(npOEP)], trans-5,15-bis(8-quinolyl) porphyrin, or tetrakis(2-hydroxyphenyl) porphyrin. After Rh insertion, these porphyrins provide lateral OH or N donor groups at a fixed distance from the coordination site at the Rh atom. Unlike RhCl(TPP), RhCl(npOEP) activates acetone in a manner that an a-metallation of acetone takes place, yielding Rh(CH2COMeXnpOEP) [283]. 

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