Octahedral fragments

Octahedral fragments of formula ML (where M has a configuration) are isolobal with square-planar fragments of formula ML 2 (where M has a d configuration and L = 2-electron donor) ... 

FIGURE 15-4 Comparison of Square-planar Fragments with Octahedral Fragments. 

CH2 " is two ligands and 3 electrons short of its parent compound (CH4). The corresponding octahedral fragment will therefore be a 15-electron species with the formula ML4 (two ligands and 3 electrons short of its parent M(CO)6). If L = CO, the four carbon monoxides contribute 8 electrons, requiring that the metal contribute the remaining 7. The first-row metal is Mn. The overall result is CH2 Mn(CO)4. 

Bs (octahedral fragment) yields the established BgHa structure if a fourfold axis is required. 

Fig. 41. Molecular orbital interaction diagram between a C atom and the (GdjIt Ie) octahedral fragment to form (Gdg I j g C) orbital occupation is appropriate for R7X12C.

Much of the structural chemistry of metal rich transition metal compounds containing p elements can be discussed in terms of fragments of the metal structures which are cut from the fee, hep, or bcc element structures and surrounded in a suitable way by nonmetal atoms. The octahedral fragment of the element structures, whether regular (fee, hep) or deformed (bcc), is the most abundant in these compounds. The following discussion will therefore focus on structures which are built from octahedral M units and will extend somewhat to other kinds of M units. 

The structures of large transition metal clusters can be derived by bringing together smaller triangular, tetrahedral and octahedral fragments. These con-... 

Figure 22. The octahedral fragment of the perovskite crystal structure ABO, with the transition metal atom B at the center and six oxygen atoms (shadowed) at the apexes of the octahedron. The letters a,b,c,. .. denote the off-center positions of the atom B in the eight wells of the APES induced by the PJTE.

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