Transition metal clusters rhodium

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... 

The resonance at 190 ppm for CO on colloidal palladium is within the range reported both for terminal carbonyls in transition metal cluster carbonyls (12,13) and for CO adsorbed on supported rhodium and ruthenium (14-16 and Thayer, A. M. and Duncan, T. M., J. Phys. 

C-H Bond Activation. We have also examined the chemisorption of various hydrocarbons on different transition metal clusters. In this section we describe results obtained for methane activation on neutral clusters. First, we note that under our experimental conditions (low pressure, near room temperature, short contact time) methane activation readily occurs only on specific type and size metal clusters. For instance, we detect no evidence that methane reacts with iron(12), rhodium(26) or aluminum(2Z) clusters, whereas as shown in figure 5 strong size selective chemisorption is... 

Weber WA, Gates BC (1997) Hexarhodium clusters in NaY zeolite Characterization by infrared and extended X-ray absorption fine structure spectroscopies. J Phys Chem B 101 10423 Goellner JF, Gates BC, VayssUov GN, Rosch N (2000) Structure and bonding of a site-isolated transition metal complex Rhodium dicarbonyl in highly dealuminated zeolite Y. J Am Chem Soc 122 8056... 

A few reactions in which four components are reacted together in the presence of a transition metal cluster as the catalyst are known. These reactions involve an acetylene, an olefin, carbon monoxide and hydrogen, or a hydrogen donor and are catalyzed by the rhodium cluster Rh4(CO)i2. In a typical experiment, an acetone solution of diphenylacetylene is pressurized with ethylene (25 bar), CO (30 bar), and H2 (5 bar) at 150°C for 6 hr the reaction... 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

It is now clear that a variety of elements form clusters of atoms which are held together, at least in part, hy bonds between like atoms. Transition metals do this and the structures of some representative compounds are illustrated in Figure 3.3. Note that in Ta Cln the oxidation state of tantalum is +7/3. Non-integral oxidation states are also observed in other transition metal clusters. Some metal carbonyls also contain clusters of metal atoms for example, in Co4(CO)i2 there is a tetrahedron of cobalt atoms and in Rh6(CO)i6 the six rhodium atoms are arrayed at the comers of an octahedron. A measure of the stability of some of these clusters is the fact that C04 units survive conditions in a mass spectrometer which strip all the CO s from a Co4(CO)i2 molecule. At present, the metal cluster which contains the largest number of metal atoms, the champion, is [H2Pt38(CO)44]". ... 

Retaining the theme of metal carbonyl clusters, capping considerations in transition-metal clusters have been discussed with reference to [Sb2Co4(CX))] g( A-CX))], and [Bi2Co4(CO)jQ( i-CO)]" 28. An infrared spectroscopic study of the formation of carbonyl rhodium clusters on a rhodium electrode produced by oxidation reduction cycles in acidic solution 2 has also been published. Electrochemistry with ruthenium carbonyls >21 osmium carbonyls 2 jg also reported. Muon spin rotation in a metal-cluster carbonyl compound has been communicated and, lastly, a proton spin-lattice NMR relaxation study of hydride carbonyl clusters has been reported. This provides a method for determining distances involving hydrido ligands... 

Another example for an investigation of multinuclear transition metal clusters is the SOS-DFPT-IGLO study of the C shift tensors for interstitial carbides enclosed in carbonyl clusters. The interstitial shifts are important, both as a proof for the existence of an interstitial atom, and as a potential probe of electronic structure. Table 2 compares computed and experimental shifts. For the two rhodium clusters, it has been possible to compare not only isotropic shifts but the entire shift tensors, as an independent solid-state NMR study given the first tensor data for a number of interstitial carbides and nitrides. The overall agreement between computation and experiment is good, both for the isotropic shifts and for the available tensors. The largest deviation (43 ppm) was found... 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

Some general reviews on hydrogenation using transition metal complexes that have appeared within the last five years are listed (4-7), as well as general reviews on asymmetric hydrogenation (8-10) and some dealing specifically with chiral rhodium-phosphine catalysts (11-13). The topic of catalysis by supported transition metal complexes has also been well reviewed (6, 14-29), and reviews on molecular metal cluster systems, that include aspects of catalytic hydrogenations, have appeared (30-34). 

Hydrothermal methods, for molecuarlar precursor transformation to materials, 12, 47 Hydrotris(3,5-diisopropylpyrazolyl)borate-containing acetylide, in iron complex, 6, 108 Hydrotris(3,5-dimethylpyrazolyl)borate groups, in rhodium Cp complexes, 7, 151 Hydrotris(pyrazolyl)borates in cobalt(II) complexes, 7, 16 for cobalt(II) complexes, 7, 16 in rhodium Cp complexes, 7, 151 Hydrovinylation, with transition metal catalysts, 10, 318 Hydroxides, info nickel complexes, 8, 59-60 Hydroxo complexes, with bis-Cp Ti(IV), 4, 586 Hydroxyalkenyl complexes, mononuclear Ru and Os compounds, 6, 404-405 a-Hydroxyalkylstannanes, preparation, 3, 822 y-Hydroxyalkynecarboxylate, isomerization, 10, 98 Hydroxyalkynes, in hexaruthenium carbido clusters, 6, 1015 a-Hydroxyallenes... 

Transition metal ions, within the zeolite framework, may undergo a reductive carbonylation to give mononuclear monovalent carbonyl coumpounds M(I)(CO) and ultimatly to give zerovalent polynuclear carbonyl clusters. The rhodium(I)and iridium(i)carbonyIs were identified using spectroscopic and volumetric methods, the zerovalent rhodium and Iridium clusters M (CO)j were also synthetized in the zeolite matrix and their structure investigated using IR, NMR and spin labelling methods. 

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