Transition metal cluster units

The Number of Skeletal Bonding Electrons (t -f a — 12) That Transition Metal Cluster Units Can Contribute ... 

TABLE 3.5. Skeletal Electron Contributions (r 4- x - 12)" that Transition Metal Cluster Units May Make... 

Number of Valence Shell Electrons (v) Transition Metal Cluster Unit ... 

TABLE 22.2 The Number of Skeletal Electrons Contributed by Transition Metal Cluster Units... 

An important consequence of the nonutilization of tangential orbitals is that platinum clusters often do not obey the normal electron counting rules and appear to be electron deficient (19,21,29,58,75,76). Electron counts are usually intermediate between those found in normal transition metal clusters (58-68) and those observed in gold clusters (58,78), but no satisfactory general electron counting theory has been developed for Pt-containing clusters. In small Pt clusters constructed from PtL2 units, theoretical studies have shown that the total electron count depends on the relative orientation of the... 

A simple realization of this concept is the removal of weakly bound (i.e., light transition metal) capping units from a triangle of strongly bound (i.e., heavy transition metal) cluster atoms by means of CO. This way H2Os3Co(CO) 0Cp is converted to CpCo(CO)2 and Os3(CO)12 (158), or H2Os3Rh(CO),0(acac) is converted to Rh(CO)2(acac) and H2Os3(CO)n (56). 

As is the case for transition metal cluster compounds in general, singlecrystal X-ray diffraction is normally the only technique available for the unambiguous structural characterization of heteronuclear Group IB metal clusters. Tables I, II, and IV-XIV indicate the mixed-metal clusters containing one or more ML (M = Cu, Ag, or Au L = two-electron donor ligand) units which have been studied by X-ray crystallography. Other... 

Seven contributions covering aspects of transition metal cluster chemistry and one concerning group-14 element clusters constitute this chapter on solid-state cluster chemistry. The general topics discussed are the structures of the cluster compounds and the electronic bonding schemes used to describe them. The importance of the bridges between the molecular units which form the solid-state cluster structures should be emphasized because of the general treatments of the electronic structures by use of modified molecular orbital calculations that are based on discrete cluster units. 

The cluster connectivities of the M6L14 and MaLis units in early transition-metal cluster compounds are discussed in Section Molybdenum, tungsten, and... 

Apart from their interesting structures, molecular clusters are intensively studied because of their physical properties. It has been shown for transition-metal clusters that small uniform particles of a certain size show interesting electronic, size-dependent effects (quantum size effects). In Au55(PR3)i2Cl6 a few electrons are trapped in a metallic state and single electrons tunnel between the cluster units. (SET, single electron tunneling).Such effects have yet to be observed in main-... 

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]". ... 

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