Polyhedra, isolobal fragments

As Figure 15-2 shows, each of these fragments has a single electron in a hybrid orbital at the vacant site of the parent polyhedron. These orbitals are sufficiently similar to meet Hoffmann s isolobal definition. Using Hoffmann s symbol to designate... 

All of these examples are one ligand short of the parent complex. Fe(CO)5 is isolobal with CH3, for example, because both have filled electron shells and both are one vertex short of the parent polyhedron. By contrast, Fe(CO)5 and CH4 are not isolobal. Both have filled electron shells (18 and 8 electrons, respectively), but CH4 has aU vertices of the tetrahedron occupied, whereas the Fe(CO)5 fragment has an empty vertex on the basis of the parent octahedron. ... 

The frontier orbitals of Fe(CO)3 and of CoCp are isolobal with those of B—H (p. 207). They consist of a radial hybrid directed to the centre of the polyhedron and two tangential orbitals at right angles to each other. In the Fe(CO)3 fragment (or CoCp fragment) these orbitals, as in B—H, contain two valence electrons. Fe(CO)3 can therefore replace B—H in deltahedral clusters. Mn(CO)3 provides one less valence electron than Fe(CO)3, so that one extra electron is required from the remaining cluster atoms. With Co(CO)3 one electron less is needed for an isoelectronic arrangement. 

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