Transition metal clusters electron deficiency

Wade has taken into account the fact that transition metals are electron-deficient in average-size clusters in order to rationalize their geometries and electron counts. He proposed that the metals must share their available electrons to form the cluster skeleton. Wade also noticed that the situation is analogous for polyborane clusters, a very rich and varied family of general formula BnH/ , the starting point of this analysis.In these compounds, each boron atom, that has 3 valence electrons. 

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... 

The structural similarities between these clusters and a piece of metal are obvious, and the same tetrahedral and octahedral sites present in a close-packed metallic lattice are also found in molecular transition metal clusters. Solid-state metal alloys are industrially important materials the simplest can be regarded as a metallic lattice containing individual main-group atoms in a set of interstitial holes. Likewise, there are numerous examples of discrete molecular clusters containing either electron-deficient, electron-precise, or electron-rich atoms in their interstitial cavities. 

The analogy between boranes and carboranes and transition metal clusters rests on the assumption that the fragments M(CO)x as well as the fragments BH in the boron derivatives contribute with three orbitals to the formation of the metal polyhedron. This analogy between electron-deficient boron species and the electron-rich metal clusters is fundamentally empiric. However there are some theoretical calculations for specific cluster species that make some clarity about the degree of validity of such an analogy. 

The chemistry of the electron-deficient boranes and carboranes which has indeed been employed as a useful model in the description of bonding in transition metal clusters appears to be a singular case among main group elements. From the point of view of the cluster chemistry, the properties of boron appear to meet with those of the transition metals. 

That transition metal-carbonyl clusters, which contain an apparent abundance of electrons, might have much in common with boranes and carboranes, notorious for their deficiency of electrons, appears at first sight improbable. However, the structural and bonding relationship between them becomes apparent if one considers certain metal-carbonyl clusters for which localized bond treatments are unsatisfactory. 

Little is known about the chemical nature of the recently isolated carbon clusters (C o> C70, Cg4, and so forth). One potential application of these materials is as highly dispersed supports for metal catalysts, and therefore the question of how metal atoms bind to C40 is of interest. Reaction of C o with organometallic ruthenium and platinum re nts has shown that metals can be attached directly to the carbon framework. Ihe native geometry of transition metal, and an x-ray difi action analysis of the platinum complex [(CgHg)3P]2Pt( () -C6o) C4HgO revealed a structure similar to that known for [(C4Hs)3P]2Pt( n -ethylene). The reactivity of C40 is not like that of relatively electron-rich planar aromatic molecules su( as benzene. The carbon-carbon double bonds of C40 react like those of very electron-deficient arenes and alkcnes. 

There are some interesting observations too concerning the structures of polyhedral molecules. Very often they are electron deficient in the sense that there are fewer than two electrons for each close contact. The heavy atoms forming the skeleton of the molecule may be either main group atoms (for example in the boranes and carboranes) or transition metal atoms (metal cluster compounds) or both (metallocarboranes). 50 shows the structures expected from Wade s rules for five atom polyhedral molecules with six, seven and eight pairs of skeletal electrons. There are a total of fifteen skeletal orbitals... 

For the catalyst containing osmium alone on silica, the osmium clusters behave as if they are more electron deficient than pure metallic osmium that is, there appear to be more unfilled d states to accommodate the electron transitions from the 2pin core level of the absorbing atom. In the silica-supported osmium-copper clusters, however, the osmium atoms appear to be less electron deficient than they are in the pure osmium clusters dispersed on silica. The presence of the copper thus appears to decrease the number of unfilled d states associated with the osmium atoms. This observation is the first that we have made regarding the electronic interaction between the components of a bimetallic cluster catalyst. Further studies of such interactions are currently in progress on other bimetallic catalysts. 

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