Cluster compounds carbide

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

There are a number of papers in the open literature explicitly reporting on the properties of boron cluster compounds for potential neutron capture applications.1 Such applications make full use of the 10B isotope and its relatively high thermal neutron capture cross section of 3.840 X 10 28 m2 (barns). Composites of natural rubber incorporating 10B-enriched boron carbide filler have been investigated by Gwaily et al. as thermal neutron radiation shields.29 Their studies show that thermal neutron attenuation properties increased with boron carbide content to a critical concentration, after which there was no further change. 

The formation of carbido-carbonyl cluster compounds with ruthenium and osmium appears to be common in pyrolysis reactions the basic reaction may be viewed as the transformation of the coordinated carbon monoxide to carbide and carbon dioxide. Small variations in... 

Francium, binary carbide not reported, 11 210 Franck-Condon effect, 16 69 energy, 21 180, 188, 189 envelopes, 16 80, 89, 90 hot bands, 16 90 factors, 32 47 principle, 21 179, 181 vibronic replica, 35 370 frd redon, 38 412, 414 Freeze quench EPR spectroscopy (FQ-EPR), CODH/ACS, 47 318 Fremy s salt, 33 106 Friedel-Crafts reaction, 17 194 cyclophosphazene, 21 65, 66 Frontier molecular orbitals, heteronuclear gold cluster compounds, 39 378-381 Frozen solutions, MOssbauer spectra in studies of, 15 101-103... 

Unusual Compositions. Three series of compounds deserve special consideration because they have developed from laboratory curiosities into classes of their own. These are the heterometallic clusters, the carbide clusters, and the tetranuclear cu-bane clusters. 

Up to 1999, only metal atoms [1-5], metal clusters [6,7], metal nitrides [55-57], and noble gas atoms [58-60] were observed to be encaged inside C60, C70, or various sizes of higher fullerenes. The experimental evidence for carbon atoms or metal-carbon compounds (carbides) being encapsulated inside fullerenes had not yet been observed. In 2000, Shinohara et al. succeeded in the first production, isolation, and spectroscopic characterization of a scandium carbide endohedral fullerene (Sc2C2) C84. Following this, the first experimental evidence based on synchrotron X-ray diffraction was presented and revealed that the Sc carbide is encapsulated in the form of a lozenge-shaped Sc2C2 cluster inside the D2d-C84 fullerene [8]. 

In the first experiments in which chemistry of metal clusters was demonstrated, the reactant was present in the carrier gas. The problem with this approach is that the reactant is also decomposed during the high-temperature vaporization process, and thus reactive radical fragments are present which may participate in the cluster growth process. Although not exploited, this may be a good method to synthesize metal cluster carbides, oxides, nitrides, sulfides, and hydrides, to name a few. It is not a viable mode of operation for the measurement of reactivity toward molecular species, but has been used to examine the stoichiometry of metal cluster compounds. ... 

To illustrate the wide and developing scope of hypercarbon chemistry by illustrating the variety of compounds now known to contain hypercarbon atoms (carbocations,organometallics, carboranes, metal-carbon cluster compounds," and metal carbides ).They include bridged metal alkyls such as alkyl-lithium reagents (LiR) in which the hypercoordinated nature of the metal-attached carbon atoms, and the roles that the metal atoms play in their chemistry, are often overlooked. 

Although less fully documented than osmium cluster chemistry, rhenium cluster chemistry has been subjected to many structural studies, including those on approximately 20 neutral or anionic carbonyls, particularly carbonyl hydrides [Rev(CO). H ] of nuclearities x = 2 to 6 (Fig. 7). In addition, some ten or more rhenium carbonyl carbides [Rev(CO)vH C] have been shown to contain a core carbon atom, usually occupying a central octahedral site. These systems offer scope not only to explore for rhenium the trends we have already shown for osmium, but also to study the effect on metal-metal distances (and so enthalpies) of such core carbon atoms, which formally donate all four of their valence shell electrons to the cluster bonding. To our knowledge only one rhenium carbonyl cluster compound, Re2(CO)io, has been subjected to calorimetric study to determine its enthalpy of formation. ... 

Cluster Compounds of Co, Rh, and Ir. In addition to the above-mentioned neutral cluster compounds, there is a large number of anionic carbonyl clusters and metal carbonyl carbides. Carbonyl carbides are formed when the interstice inside the metal cluster is sufficiently large to accommodate the carbon atom. Carbonyl carbides possessing at least four metal atoms are known. The most thoroughly investigated carbides are those of rhodium because they are very stable and resist air oxidation. Carbonyl clusters of group 9 elements containing even more than 20 metal atoms are now known [M6(CO)i5] - (M = Co, Rh, Ir), lM CO)uT. [M6(CO)i5C]"-(M = Co,Rh), [Co8(CO),sC] -, [Rh,(CO)i,] -, [Rh8(CO)i,C], [Ir8(CO)22]"-,... 

Metal carbonyl carbide cluster compounds may be formed either by the carbon-oxygen bond breaking of the CO molecule or by the decomposition of an organic compound present in the reaction mixture. Chloroform is a convenient source of the encapsulated carbon atom because in this case the synthesis may be carried out under mild conditions ... 

Cobalt and rhodium also form a series of high nuclearity carbido-cluster compounds. Contrasting with carbido compounds of the iron group which have a clear tendency to put the carbide atom in octahedral cavities, carbides of the group 9 often place it in trigonal prismatic cavities. As shown in scheme in Fig. 3.12, the parent compound [Co6(CO)i5C] of a series of encapsulated car-bido-cobalt species may be prepared by the reaction of Co3(CO)9CCl with [Co(CO)4]. The same scheme also describes some carbido-cobalt cluster interconversions. Rhodium carbide clusters are in general similar to the cobalt ones. 

Syntheses, crystallization, structural identification, and chemical characterization of high nuclearity clusters can be exceedingly difficult. Usually, several different clusters are formed in any given synthetic procedure, and each compound must be extracted and identified. The problem may be compounded by the instabiUty of a particular molecule. In 1962 the stmcture of the first high nuclearity carbide complex formulated as Fe (CO) C [11087-47-1] was characterized (40,41) see stmcture (12). This complex was originally prepared in an extremely low yield of 0.5%. This molecule was the first carbide complex isolated and became the foremnner of a whole family of carbide complexes of square pyramidal stmcture and a total of 74-valence electrons (see also Carbides, survey). 

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