Shapes with Five Electron Groups Trigonal Bipyramidal Arrangement

Molecular Shapes with Five Electron Groups (Trigonal Bipyramidal Arrangement) 

All molecules with five or six electron groups have a central atom from Period 3 or higher because only these atoms have the d orbitals available to expand the valence shell beyond eight electrons. 

With all five positions occupied by bonded atoms, the molecule has the trigonal bipyramidal shape (AXj), as in phosphorus pentachloride (PCI5)  

The tendency of lone pairs to occupy equatorial positions causes molecules with three bonding groups and two lone pairs to have a T shape (AX3E2). Bromine trifluoride (BrF3), one of many compounds with fluorine bonded to a larger halogen, has this shape. Note the predicted decrease from the ideal 90 F—Br—F bond angle  

Molecules with three lone pairs in equatorial positions must have the two bonding groups in axial positions, which gives the molecule a linear shape (AX2E3) and 

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For the trigonal bipyramidal shape of the PCI5 molecule, for example, the VB model proposes that the one 35, the three 3p, and one of the five 3d orbitals of the central P atom mix and form five sp d hybrid orbitals, which point to the vertices of a trigonal bipyramid (Figure 11.6). Seesaw, T-shaped, and linear molecules have this electron-group arrangement with lone pairs in one, two, or three of the central atom s sp d orbitals, respectively. 

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