Pentagonal bipyramidal shape

Figure 4 The reaction of a nido-diruthenapentaborane with an aUsyne, ultimately generating two clusters of identical composition in which the geometric isomerism is associated with the type of imoccupied vertex in the pentagonal bipyramidal shape upon which both strucmres are based...

M FIGURE 22.11 c/oso-Borane Structures cteo-Borane structures form closed polyhedrons with triangular faces. In this figure, each sphere represents a BH unit. BgHg has an octahedral (square bipyramidal) shape. B7H7 has a pentagonal bipyramidal shape. 

Complexes of d- and /-block metals can be described in terms of hybridization schemes, each associated with a particular shape. Bearing in mind that the number of atomic orbitals hybridized must be the same as the number of hybrid orbitals produced, match the hybrid orbitals sp1d, sp fd , and sp d3f to the following shapes (a) pentagonal bipyramidal ... 

The XY3 molecule has a T-shaped structure axial (ax), equatorial (eq) (103)2 is a dimer bridging (b), terminal (t) XY5 forms a square-based pyramid apical (ap), basal (ba) XY7 has the shape of a pentagonal bipyramid equatorial (eq), axial (ax). 

The structures of these ions normally conform to those predicted by the VSEPR theory, as shown in Fig. 17.2.2. Since the anion XY has two more electrons than the cation XY+, they have very different shapes. The anion IFj- is planar with lone pairs occupying the axial positions of a pentagonal bipyramid. In [Me4N](IF6), IFg is a distorted octahedron (C3V symmetry) with a sterically active lone pair, whereas both BrFg and ClFg are octahedral. The anion IF " has the expected square antiprismatic structure. 

Sessler and coworkers prepared two similar uranyl complexes 6.48 and 6.49. A single crystal X-ray diffraction study of the former revealed a saddle-shaped penta-phyrin macrocycle with the uranyl cation centrally coordinated in a nearly pentagonal bipyramidal fashion (Figure 6.4.3). Distortion from planarity arises, presumably. 

Iodine heptafluoride, the only example of the AX type, can be made by heating IF5 with Fg at 250-270°. It is comparable with CIF3 and BrFg in its violent fluorinating action. In shape it is the unusual pentagonal bipyramid. 

It is evident that the geometries of these 7-coordinated complexes are very much dependent on the constraints due to the shapes of the polydentate ligands. A more extreme example is the pentagonal bipyramidal coordination in complexes formed by the pentadentate ligand L, such as the cations in the salts [FeL(NCS)2]C104, (where the NCS groups occupy the apical positions with Fe—N, 2-01 A as compared... 

The way that cyclobutane can be classified as a hypho species with a shape that can be regarded as based on the pentagonal bipyramid was illustrated in Section 3.2 and Figure 3.6, which also showed other hydrocarbon ring systems whose shapes can be related to the same deltahedra as carboranes. 

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