Pentagonal-bipyramidal molecules

A good example of a pentagonal-bipyramidal molecule is IF7, where P = All the pairs are bond pairs. Figure 9-15 shows the structure. 

The presence of axial and equatorial sites in a pentagonal bipyramidal molecule leads to stereoisomerism in a similar manner to that in a trigonal bipyramidal species. In a square antiprismatic molecule EXg, each X atom is identical (Figure 2.15). Once two or more different atoms or groups are present, e.g. EXfiY2, stereoisomers are possible. As an exercise, draw out the four possibihties for square antiprismatic EX6Y2. 

C09-0133. Among the halogens, only one known molecule has the formula X 7. It has pentagonal bipyramidal geometry, with five Y atoms in a pentagon around the central atom X. The other two Y atoms are in axial positions. Draw a ball-and-stick model of this compound. Based on electron-electron repulsion and atomic size, determine the identities of atoms X and Y. Explain your reasoning. (Astatine is not involved. This element is radioactive and highly unstable.)... 

Figure 9.24 The pentagonal bipyramidal geometry of some AX molecules.

The molecule XeF52 is the only known example of the AX5E2 type of molecule. It has the expected planar pentagonal geometry based on the pentagonal bipyramid with a lone pair in each of the less-crowded axial positions and an Xe—F bond length of 201 pm (Figure 9.27). 

In certain complexes, even though the coordination number is the same, two molecules in the same unit cell exhibit different conformations. Thus, in Eu(thd)3 DMSO, the gross geometry around Eu(III) is a distorted pentagonal bipyramid with the oxygen of the DMSO occupying one of the apices 283, 284), but the structure of the two molecules in the unit cell differs in detail. A similar situation also exists in Eu(thd)3(DMF)2 283, 284). 

An interesting structural feature of [Fe(dapsox)(H20)2l is its pentagonal-bipyramidal (PBP) geometry with kinetically labile solvent molecules coordinated in the axial positions and the completely planar pentadentate chelate in the equatorial plane, which generally facilitates easy access of nucleophiles to the metal center without any steric hindrance above and below the pentadentate plane. 

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