Group 8 VIII clusters

Detailed vibrational studies have been carried out on several carbonyl cluster compounds. Both single crystal and Raman solution data have been obtained for M3(C0)i2 (M = Ru or Os), and small changes in frequency of the two-mode v(C0) A vibrations of these two carbonyls in mixed crystals have been used as indicators of the molecular geometry changes by which each species adapts to sites in the mixed crystal. Intermediates formed on the interaction of several Group VIII cluster carbonyls with oxide surfaces have also been characterised by spectroscopic means. 

We plan to make studies on palladium-copper, iridium-copper, and platinum-copper catalysts to extend our investigation of the effect of varying miscibility of the components on the structural features of the bimetallic clusters present. With these additional systems, the whole range from complete immiscibility to total miscibility of copper with the Group VIII metal will be encompassed. 

The dominant role of copper catalysts has been challenged by the introduction of powerful group VIII metal catalysts. From a systematic screening, palladium(II) and rhodium(II) derivatives, especially the respective carboxylates62)63)64-, have emerged as catalysts of choice. In addition, rhodium and ruthenium carbonyl clusters, Rh COJjg 65> and Ru3(CO)12 e6), seem to work well. Tables 3 and 4 present a comparison of the efficiency of different catalysts in cyclopropanation reactions with ethyl diazoacetate under standardized conditions. 

Butterfly Cluster Complexes of the Group VIII Transition Metals Sargeson, Alan M., see Hendry, Philip Sanon, G., see Fleischauer, P. D. Sawyer, Donald T., see Sobkowiak, Andrzej Sawyer, Jeffery F., and Gillespie, Ronald J., The Stereochemistry of SB (HI) 35 437... 

XIII. Clusters of Ni, Pd, Pt and Mixed Clusters Containing Group VIII C Metals... 

E. Sappa, A. Tiripicchio, A. J. Carty, and G. E. Toogood, Butterfly cluster complexes of the group VIII transition metals, Prog. Inorg. Chem., 35, 437 (1987). 

Butterfly Cluster Complexes of the Group VIII Transition Metals. 35 437... 

Alkynes containing methylene (—CHi—) functionalities a to the triple bond readily undergo thermal C-H activation under conditions necessary to initiate reaction with group VIII metal carbonyls (54). The allenyl clusters obtained isomerize thermally via a 1,2-hydrogen atom shift to afford the thermodynamically more favorable 1,3-dimetalloallyl clusters (Table 1) (49,55). A similar chemistry has been recognized for osmium clusters, albeit under more severe conditions. 

Empiricism in early catalytic studies had shown that combining two or more metals could alter catalyst activity and stability. Electronic and structural interactions between the catalyst components were thought to be responsible for the observed changes by modifying the reactant strength of adsorption (7/5, 270). Electrocatalytic research exploited this idea for fuel oxidations and fundamental studies of electrocatalytic activity (78, 79, 80, 113, 271-274). Primarily, clusters of group VIII and group IB metals have... 

Ruthenium-copper and osmium-copper clusters are examples of bimetallic clusters in which one component is from Group VIII and the other from Group IB of the periodic table. These clusters are of particular interest because copper is virtually completely immiscible with either ruthenium or osmium in the bulk (7). Copper has the face-centered cubic structure in the metallic state, whereas ruthenium and osmium both exhibit hexagonal close-packed structures (8). 

Direct experimental verification of very highly dispersed bimetallic clusters is complicated by limitations in the ability of physical methods to obtain structural information on such systems. In such a system, however, a catalytic reaction can serve as a sensitive probe to obtain evidence of interaction between the atoms of the two metallic components. For supported bimetallic combinations of a Group VIII and a Group IB metal, the hydrogenolysis of ethane to methane is a useful reaction for this purpose. In the case of unsupported bimetallic systems of this type, as discussed previously, the interaction between the Group VIII metal and the Group IB metal results in a marked suppression of the hydrogenolysis activity of the former. 

Another catalyst system that could be considered in the bimetallic cluster category is supported platinum-rhenium (5), which represents still another type of system in the sense that a Group VIIA metallic element (rhenium) is incorporated with the Group VIII metal component. Platinum and rhenium have different crystal structures (fee vs. hep) (8) and do not exhibit complete miscibility in the bulk (Ref. 45, p. 820). However, these factors may have limited... 

Fig, 6.7. a Site time yield in the processes described in the figure as a function of radius of the metal cluster involved in the catalytic processes. No particle size effect is observed down to small clusters, b Particle size effect for Pt clusters dispersed on Si02 in the recombination reaction of oxygen and hydrogen, c Particle size effect for Fe clusters dispersed on MgO for the ammonia synthesis at atmospheric pressure and 570° K. d Relationship between catalytic activity, dispersion of the metal cluster on support and bulk metal character of the cluster for metals of the group VIII... 

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