Polyhedral skeleton electron pair

C NMR spectra, 29 188-189 structure, 29 207-208, 218 photoelectron spectra, 29 193 polyhedral skeleton electron pair theory, 29 200... 

This cluster has a monocapped trigonal bipyramidal structure in which the 18-electron rule is obeyed for only two of the six Os atoms. The situation is worse for the localized electron count and for the 18-electron rale that do not work at all in the very stable octahedral 86-electron dianion (see below). On the other hand, Wade s theory that uses the polyhedral skeleton electron pairs very well rationalizes the electron counts in large organometallic clusters. - ... 

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. 

As the cluster nuclearity increases, the structures and electron counting of saturated clusters conform to the polyhedral skeletal electron pair (SEP) theory. For organometallic clusters the carbon fragments of the hydrocarbyl ligands are frequently part of the cluster skeleton. Thus, Fc3(PhCCPh)(CO)9 (1) would be considered to be unsaturated according to the 18-electron rale, as the alkyne ligand is a four-electron donor to a Fc3(CO)9 cluster, which requires... 

If the square pyramidal metal carbonyl carbides Fe5(CO)i5C ° and Os5(CO)i5C are treated in a similar manner to I xyi ( ()) i T that is, as clusters in which all four of the core carbon atom s valence shell electrons are used for skeletal bonding, then they are seen to have the expected nido shapes of systems with five skeletal atoms (the metal atoms) held together by seven skeletal bond pairs. By contrast, if these carbide carbon atoms had occupied polyhedral vertex sites, with a lone pair of electrons in an exo-oriented sp hybrid orbital, then the number of skeletal bond pairs would have been reduced by one and the number of skeletal atoms would have increased by one. The five metal atoms and the carbide carbon atom would have had to be accommodated in some way on a trigonal bipyramidal skeleton. Clearly, the assumption that all four valence shell electrons from the carbide carbon atom are involved in the skeletal bonding is vindicated. 

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... 

The method does not yield accurate results in the description of the compounds that contain neighboring heteroatoms with electron lone pairs, the organic boron compounds etc. Since organic compounds with fixed valence of the atoms that form them are used for the parametrization of this method, it underestimates, as a rule, the stability of nonclassical skeleton and polyhedral structures. 

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