Octahedral molecular shape table

Three of the five basic molecular shapes are linear, trigonal planar and tetrahedral. Table 4.9 shows the arrangement of the electron pairs (charge centres) that results in minimum repulsion and the basic shapes of the molecules. The two other basic shapes adopted by molecules, trigonal bipyramidal and octahedral, are discussed in Chapter 14. 

A molecule can have only one center of symmetry, which may or may not coincide with an atom. For a molecule to have a center of symmetry, all atoms, with the exception of the one which may coincide with the center, must exist in paired sets. In carbon dioxide, the C atom is the center of symmetry. Of the typical molecular shapes mentioned in Tables 19.4 and 19.5, only the linear, square-planar, and the octahedral forms, as exemplified by BeCl2, XeF4, and SFe, respectively, have centers of symmetry. 

Answer. The metal count is 59 (9 x 9 (Co) — 11x2 ([Se]2—) = 59). This is 25 more than for [Mo9Sen]2 Indeed a large gamut of electron counts is possible for these condensed octahedral clusters, either molecular or part of extended solids. They are nice examples of compounds which can display variable electron count with constant shape. This situation is somewhat reminiscent of the transition-metal elements which exhibit a metallic d band which is gradually filled as we go from the left to the right of the periodic table without substantial change in their structure. Also review the discussion of cubic clusters in Section 5.2.5. 

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