Transition metal clusters bonding

Recent results in the chemistry of transition metal clusters with organic ligands. H. Vahrenkamp, Struct. Bonding (Berlin), 1977, 32, 2-56 (408). 

Developments in transition metal cluster chemistry — the way to large clusters. G, Schmid, Struct. Bonding (Berlin), 1985,62, 51 (132). 

Pauling, L. Structure of Transition-Metal Cluster Compounds Use of an Additional Orbital Resulting from the f,g Character of spd Bond Orbitals Proc. Natl. Acad. Sci. (USA) 1977, 74, 5235-5238. 

Wade also extended the application of his rules to transition metal clusters the further extension by D. M. P. Mingos mainly concerns the bonding in metal carbonyl and metal phosphane clusters, i.e. organometallic compounds (Wade-Mingos rules) these are beyond the scope of this book. 

Although there are a lot of publications on the chemistry of technetium [2-4] and transition-metal clusters [1,5-8], the chemistry of technetium clusters was insufficiently studied until the early eighties [1,2]. Nevertheless, the available scanty data on the compounds with Tc-Tc bonds inspired hope that interesting results would be obtained in the chemistry of technetium in general, in radiochemistry, and in the chemistry of transition-metal cluster compounds. The anticipated results were actually obtained [9-15] and the conclusion was drawn that technetium had a number of anomalous cluster-forming properties [9]. This review looks at the detailed studies of these properties and their interpretation in terms of electronic structure theory. 

Gas phase transition metal cluster chemistry lies along critical connecting paths between different fields of chemistry and physics. For example, from the physicist s point of view, studies of clusters as they grow into metals will present new tests of the theory of metals. Questions like How itinerant are the bonding electrons in these systems and Is there a metal to non-metal phase transition as a function of size are frequently addressed. On the other hand from a chemist point of view very similar questions are asked but using different terminology How localized is the surface chemical bond and What is the difference between surface chemistry and small cluster chemistry Cluster science is filling the void between these different perspectives with a new set of materials and measurements of physical and chemical properties. 

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

The efforts to rationalize the formulas and structures of Zintl ions and related species predated extensive definitive structural information on anionic post-transition metal clusters obtained by Corbett and his group in the 1970s [8, 9]. After enough such structural information on the bare post-transition metal clusters became available, the resemblance of their polyhedra to the known polyhedral boranes became apparent. For this reason, the simple Zintl-Klemm concept has been largely superseded by newer, more advanced descriptions of chemical bonding in such clusters, initially those applied to the polyhedral boranes. 

Some examples of bare ionic post-transition metal clusters are depicted in Fig. 3. Their chemical bonding topologies can be analyzed as follows ... 

It has been long known that for a given transition-metal cluster the open-shell and the closed-shell species may differ. Typically, metal-ligand bond lengths are elongated for the open-shell structures, where the metal centers carry local spins and electrons occupy antibonding orbitals, in comparison to their closed-shell... 

The topic of molecular transition metal-gold bond-containing compounds, especially of cluster compounds, has developed considerably in recent years (Table 4.6). 

Interest in the chemistry of transition metal clusters is expanding rapidly in various directions, as indicated by three recent books1-3 and a selection of recent review articles.4-6 The concept of a cluster is being broadened to signify a species with properties between a mononuclear complex and a bulk solid material, irrespective of direct metal-metal bonding. Thus, a cluster is a... 

The political justification for transition metal cluster chemistry is the assumption that clusters are models in which metallic properties may be more easily studied than in the metals themselves. These properties include electronic phenomena such as color and conductivities as well as surface phenomena, such as atom arrangements and catalytic activities. Thus, there are two main lines of cluster research. The more academic line leads to the search for new types of clusters and their structure and bonding, whereas the more practical line leads to the investigation of reactivities with the hope that clusters may open catalytic pathways that neither plain metals nor mononuclear catalysts can provide. The interdependence of both lines is obvious. 

Considerable advances in the field of transition metal cluster chemistry have been made during the last five years. They have confirmed that in many respects a cluster complex is comparable to a metallic surface. They have also shown that clusters allow reactions which are not observed with simple metal complexes. And they have finally demonstrated that structural and bonding properties of clusters require new concepts for their description. 

The bonding capabilities of transition metal clusters (no nonmetals in the framework), based on molecular orbital calculations, has been nicely summarized by Lauher14 (Table 16.3). Within this table we see three structures (tetrahedron, butterfly, and square plane) for tetranuclear metal clusters. The tetrahedron is a 60-electron cluster, while the butterfly and square plane clusters have 62 and 64 electrons. respectively. When we go from a tetrahedron to a butterfly, one of the edges of the tetrahedron is lengthened corresponding to bond breaking. 

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