Metal cluster, multicenter bonding

The structures and skeletal bond valences of Oss(CO)i6 and B5H52- are similar as a pair, as are also Fe5C(CO)is and B5H9. But the bonding types in the boranes and the metal clusters are not the same. Since every B atom in a polyhedral borane has three AO s for bonding of the BM skeleton, any vertex more than three-connected must involve multicenter bonds. In the transition-metal skeleton, the Mm atoms form either 2c-2e single bonds or 3c-2e multicenter bonds. 

Although the Mulliken population analysis is useful for the discussion of two-center bonding, it is not sufficient for the description of the three- or four-center bonds that may exist in metal clusters. In order to elucidate the multicenter bonds, we examine contour maps of the charge densities of Na4 and Mg4 as typical tetramers of AM and AE, respectively. We use the differential charge density, Ap defined by eq. (3)., to express the charge redistribution on the formation of chemical bonds. 

The development of cluster chemistry has been influenced by possible, practical application of clusters in catalysis and organic synthesis. This development also comes about because of interest in metal-metal bonds and the theory of multicenter bonds, relationships between the structural theories of complexes and clusters, as well as analogies between clusters and metal surfaces. In clusters there are types of bonds which are not encountered in any other compounds, for example, bridges ... 

Many people make the mistake of equating the study of three-center bonds to the study of the boron hydrides. Certainly, the Colonel was very successful in applying 3-center bond concepts to boron hydrides. However, we should see the larger applicability of the general concept to noble gas compounds, localized orbitals for species such as COa, metals, metal- cluster compounds, etc. Someplace between the localized 2e-2c bonds beginning students (and some elderly chemists) love and cherish, and the fully-delocalized molecular orbitals we so often find useful, lies a localized multicenter bond. This approach to localized bonds looks like it will be very useful to chemists when it is more fiilly developed. They have some resemblance to the intuitive "best" valance structure selected from resonance hybrids. One should be able to do chemistry with these orbitals. Fractional bonds may be uncomfortable at first, but note that the octet rule applies to electrons, not orbitals. One of the most inqjortant contributions... 

Molecular metal clusters represent a very large family of compounds in which some elements form a framework structure that has a polyhedral form or can be considered a fragment of a polyhedron. In these polyhedra or polyhedral fragments, the framework elements are generally bonded to each other in a multicenter form, that is, the molecular orbitals that contribute largely to the framework bonding, typically involve orbitals from more than two framework atoms. 

Cluster modeling of possible chemisorption states and of possible intermediate states in surface reactions can to a first approximation be useful in guiding experiments or interpretations of experimental data for surface reactions (23-25). One important and enlightening result (6, 26, 27) in metal carbide cluster chemistry will be used here to illustrate this particular point because it bears directly on the importance of multicenter C-H-M bonding for hydrocarbon fragments in metal chemistry. 

Reaction of alloys such as NaSnx with macrocyclic ligands in amine solvents gives compounds containing anionic clusters such as [Sn5]2-, [Sn9]4- and [Pb5]2-. These have multicenter metal-metal bonding, which can often be rationalized by Wade s rules (see Topic C7. Structures 4 and 7). 

Metallaborane and borylene complexes are related to some extent because both types possess direct metal-boron bonds however, the nature of these interactions is varied. In metallaborane clusters, the framework is made up by nonclassical, electron-deficient, multicenter two-electron bonds, while the borylene ligand (BR) is related to one or more transition metal centers by classicrJ, electron-precise, two-center two-electron (2c-2e) bonds [280 -283]. Since the discovery of various bridged and terminal borylene complexes by Braunschweig and Aldrige, the chemistry of this subarea of transition metal complexes of boron has received significant attention from both structure/bonding and... 

---