Rhodium carbonyl cluster chemistry

S. Martinengo, G. Ciani, A. Sironi, and P. Chini, Analogues of Metallic Lattices in Rhodium Carbonyl Cluster Chemistry. Synthesis and X-ray Structure of the [Rh15 ( i-CO)14(CO)13]3 and [Rh14((j,-CO)15(CO)9]4 Anions Showing a Stepwise Hexagonal Close-Packed/Body-Centered Cubic Interconversion, J. Am. Chem. Soc. 100,7096-7098 (1979). 

Rhodium Carbonyl Cluster Chemistry Under High Pressures of Carbon Monoxide and Hydrogen... 

Vidal JL, Walker WE (1981) Rhodium carbonyl cluster chemistry under high pressure of carbon monoxide and hydrogen. 3. Synthesis, characterization, and reactivity of HRh(CO>4. Inorg Chem 20(l) 249-254... 

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. ... 

Rather different chemistry is displayed by small rhodium carbonyl clusters on the surfaces of these oxides. Much of this is dominated by the same equilibria as in solution, between [Rh(C0)2X]2> Rh4(C0)i2 Rh5(C0)] 5. As in solution, the species observed are very sensitive to the acidity and water content of the reaction media. Rh4(C0) 2 is substantially converted to Rh5(C0)i5 on a dried silica surface [62] this can be inhibited by a CO atmosphere. Treatment with water and air affords metallic particles and Rh sites, respectively [62,63]. A similar sensitivity to pretreatment is observed on an alumina surface. The proportions of the initially observed species also vary with the degree of hydroxylation of the alumina, with these being principally Rh6(C0)] 5 and Rh (C0)2 sites (Fig. A) [62,64-66] this provides an active water gas shift catalyst [44]. 

Another simple rhodium carbonyl complex also known to be Involved In the fragmentation and aggregation reactions of clusters Is Rh2(C0)s This species has been shown to participate In the reactions of neutral rhodium carbonyl species In either matrixes or solutions (equation 4), but It has not yet been Implicated In the chemistry of large anionic clusters. 

Chemistry similar to that described above for iridium clusters has also been observed for rhodium clusters. Several authors [16-18] have prepared [Rh6(CO)i6] in NaY zeolite [R1i4(CO)i2] has also been formed [18], and each of these has been decarbonylated with minimal changes in the metal frame, as shown by EXAFS spectroscopy [18]. Thus there appears to be some generality to the method of forming small clusters in zeolite cages by synthesis of stable metal carbonyl precursors followed by decarbonylation. However, the method is limited. Attempts to use it to prepare zeolite-supported platinum clusters that are structurally simple and uniform have apparently not been successfiil. The literature of platinum carbonyl clusters in zeolites is not considered here because it is still contradictory. 

For some clusters such as Rh species, the number of electrons available may not match the number apparently required by the structure adopted. Two examples in rhodium carbonyl chemistry are [Rh5(CO)i5] and [Rh9(CO)i9]. The former possesses 76 valence electrons and yet has a trigonal bipyramidal Rh5-core, for which 72 electrons are usual. However, a look at the Rh—Rh bond... 

The use of the cobalt triad carbonyls as catalysts continues to provide many papers for this report. Publications cover the silylformylation of 1-Hexyne catalyzed by diodium-cobalt carbonyl clusters the formation of hydroxycarbene cobalt carbonyl derivatives, the use of rhodium cluster carbonyls in the water-gas shift reaction Rh4(CO) 2> and Co3Rh(CO)] 2 catalysts for the hydrosilation of isoprene, cyclohexanone and cyclohexenone catalytic reduction of NO by CO and the carbonylation of unsaturated compounds The chemistry of iridium carbonyl cluster complexes has been extended by making use of capping reactions with HgCl2and Au(PPh3)Q... 

We have already alluded to the diversity of oxidation states, the dominance of oxo chemistry and the cluster carbonyls. Brief mention should be made too of the tendency of osmium (shared also by ruthenium and, to some extent, rhodium and iridium) to form polymeric species, often with oxo, nitrido or carboxylato bridges. Although it does have some activity in homogeneous catalysis (e.g. of m-hydroxylation, hydroxyamination or animation of alkenes, see p. 558, and occasionally for isomerization or hydrogenation of alkenes, see p. 571), osmium complexes are perhaps too substitution-inert for homogeneous catalysis to become a major feature of the chemistry of the element. The spectroscopic properties of some of the substituted heterocyclic nitrogen-donor complexes may yet make osmium an important element for photodissociation energy research. 

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