Cluster compounds skeletal bond pairs

The relationship between boranes and metal-carbonyl clusters can be extended by considering the compound Fe5(CO)i5C, which has the square-based pyramidal structure shown in Fig. 13, with the carbide carbon atom just below the center of the Fe square, clearly contributing all its valence shell electrons to the cluster 24). The metal-carbonyl residue FeB(CO)i4 formally left by removal of this carbon as has the nido structure appropriate for a cluster with 5 skeletal atoms and seven skeletal bond pairs. 

A theory which shows greater applicability to bonding in cluster compounds is the Polyhedral Skeletal Electron Pair Theory (PSEPT) which allows the probable structure to be deduced from the total number of skeletal bond pairs (400). Molecular orbital calculations show that a closed polyhedron with n vertex atoms is held together by a total of (n + 1) skeletal bond pairs. A nido polyhedron, with one vertex vacant, is held together by (n + 2) skeletal bond pairs, and an arachno polyhedron, with two vacant vertices, by (n + 3) skeletal bond pairs. Further, more open structures are obtainable by adding additional pairs of electrons. This discussion of these polyhedral shapes is normally confined to metal atoms, but it is possible to consider an alkyne, RC=CR, either as an external ligand or as a source of two skeletal CR units. So that, for example, the cluster skeleton in the complex Co4(CO)10(RCCR), shown in Fig. 16, may be considered as a nido trigonal bipyramid (a butterfly cluster) with a coordinated alkyne or as a closo octahedron with two carbon atoms in the core. 

If the compounds shown in Figures 4.1 and 4.2 are regarded as mixed metal-carbon clusters, the pyramidal shapes of their MC skeletons will be seen to be those appropriate for nido systems in which (n -1-1) skeletal atoms are formally held together by (n -1- 3) skeletal bond pairs. This number includes the n pairs of electrons in the ring carbon-carbon o bonds as well as the three pairs of electrons in their K-systems. For example, in the cyclobutadiene complex (r " -C4H4)Fe(CO)3, 5 each CH unit can contribute three electrons, and the Fe(CO)3 unit can contribute two electrons for skeletal bonding, making... 

This last compound, treated as a mixed metal-carbon cluster Ic4(( ()) iT R (R = the one-electron carboxylate ligand COOMe) contains six skeletal bond pairs to hold together its five skeietai atoms (one carbon atom and four metal atoms). It is therefore a closo cluster, with the expected trigonal bipyramidal shape defined by its skeietai carbon and metai atoms. 

K. Wade, The Structural Significance of the Number of Skeletal Bonding Electron-pairs in Carboranes, the Higher Borane Anions, and Various Transition-metal Carbonyl Cluster Compounds, Chem. Comm. 1971, 792-793. 

Development of the concept of the chemical bond from hydrogen to cluster compounds Limitations of the polyhedral skeletal electron pair theory in organometallic cluster chemistry examples in tri- and tetrametallic systems Electron-count versus structural arrangement in clusters based on a cubic transition metal core with bridging main group elements... 

Wade, K. (1971) The stmctural significance of the number of skeletal bonding electron-pairs in carboranes, the higher boranes and borane anions, and various transition-metal carbonyl cluster compounds. J. Chem. Soc., D Chem. Commun., (15), 792-793. 

In a work by Kirchmann et al., a nickel complex with six stanna-c/oio-dodecaborate clusters as hgands was reported [57] (Fig. 18). A usual understanding on borane-Uke compounds is that they cannot be properly described in localized treatment. Indeed, there are not yet widely accepted localized model to treat the skeletal (tangential) bonding of such clusters. However, with respect to the radial valence pair, it can stUl be considered as localized on each unit (the radial valence pair serves as a B-H bonding pair in a BH unit and a lone pair in a Sn center). 

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