Rhodium , crystal structure

The ruthenium-copper and osmium-copper systems represent extreme cases in view of the very limited miscibility of either ruthenium or osmium with copper. It may also be noted that the crystal structure of ruthenium or osmium is different from that of copper, the former metals possessing the hep structure and the latter the fee structure. A system which is less extreme in these respects is the rhodium-copper system, since the components both possess the face centered cubic structure and also exhibit at least some miscibility at conditions of interest in catalysis. Recent EXAFS results from our group on rhodium-copper clusters (14) are similar to the earlier results on ruthenium-copper ( ) and osmium-copper (12) clusters, in that the rhodium atoms are coordinated predominantly to other rhodium atoms while the copper atoms are coordinated extensively to both copper and rhodium atoms. Also, we conclude that the copper concentrates in the surface of rhodium-copper clusters, as in the case of the ruthenium-copper and osmium-copper clusters. 

A mechanistic study by Haynes et al. demonstrated that the same basic reaction cycle operates for rhodium-catalysed methanol carbonylation in both homogeneous and supported systems [59]. The catalytically active complex [Rh(CO)2l2] was supported on an ion exchange resin based on poly(4-vinylpyridine-co-styrene-co-divinylbenzene) in which the pendant pyridyl groups had been quaternised by reaction with Mel. Heterogenisation of the Rh(I) complex was achieved by reaction of the quaternised polymer with the dimer, [Rh(CO)2l]2 (Scheme 11). Infrared spectroscopy revealed i (CO) bands for the supported [Rh(CO)2l2] anions at frequencies very similar to those observed in solution spectra. The structure of the supported complex was confirmed by EXAFS measurements, which revealed a square planar geometry comparable to that found in solution and the solid state. The first X-ray crystal structures of salts of [Rh(CO)2l2]" were also reported in this study. 

An X-ray crystal structure of [Rh(PF-PPh2)(COD)]PF6 reveals a distorted square-planar geometry around rhodium (Fig. 4.1). The Rh-phosphaferrocene bond length (2.25 A) is shorter than the Rh-(tertiary phosphine) bond length (2.30 A). We postulated that the strong -accepting capacity of the phosphaferrocene [9] may lead to effective Rh P back-bonding that decreases the conformational flexibihty of the metal-... 

Rhodium(II) forms a dimeric complex with a lantern structure composed of four bridging hgands and two axial binding sites. Traditionally rhodium catalysts faU into three main categories the carboxylates, the perfluorinated carboxylates, and the carboxamides. Of these, the two main bridging frameworks are the carboxylate 10 and carboxamide 11 structures. Despite the similarity in the bridging moiety, the reactivity of the perfluorinated carboxylates is demonstrably different from that of the alkyl or even aryl carboxylates. Sohd-phase crystal structures usually have the axial positions of the catalyst occupied by an electron donor, such as an alcohol, ether, amine, or sulfoxide. By far the most widely used rhodium] 11) catalyst is rhodium(II) acetate [Rh2(OAc)4], but almost every variety of rhodium] 11) catalyst is commercially available. 

At ordinary temperatures rhodium is stable in air. When heated above 600°C, it oxidizes to Rh203, forming a dark oxide coating on its surface. The gray crystalline sesquioxide has a corundom-like crystal structure. The sesquioxide, Rh203, decomposes back to its elements when heated above... 

Various X-ray crystal structures of metal-ligand complexes provided evidence of the geometry of the complexes in the solid state, even though the structure of these complexes may differ in solution. The hrst crystal structure of a bis(oxazoline)-metal complex was determined in 1994 by Brown and co-workers. " This group crystallized and elucidated the structure of V,V-bis-[2-((45)-(methyl)-l,3-oxazoli-nyl)]methane-bi(ri ethene)rhodium(I), 18a, as depicted in Figure 9.3. The key features of this crystal structure include the C2-axis of symmetry, the axial positions of the methyl groups and the orientation of the ethene molecules, orthogonal to the complexation square plane. In 1995, Woodward and co-workers were able to crystallize and determine the structure of benzylbis(oxazoline) with ruthenium... 

Fig. 1. The di-, tri-, and tetranuclear structures observed in X-ray crystal structures of hydroxo-bridged oligomers of cobalt(III), rhodium(III), iridium(III), or chromium(III) structures 4b, 7b, and 7c have never been observed, but the last two have been mentioned as possible structures for two of the known isomers of Cj4(OH)66+...

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