Five electron pair valence shells

Five points can be arranged on the surface of a sphere such that they are all equivalent, only in a planar pentagonal arrangement, which does not maximize the distance between the points. In other words, there is no regular polyhedron with five equivalent vertices. There are two 

Unlike the tetrahedron and the octahedron, which are regular solids with four and six equivalent vertices, respectively, the trigonal bipyramid has two sets of vertices that are not equivalent. The axial vertices have three close neighbors, whereas the equatorial vertices have two close neighbors (the axial vertices) and two more that are farther away (the other two equatorial vertices). This nonequivalence of the vertices of a trigonal bipyramid has several important consequences 

The axial bonds in a trigonal bipyramidal molecule are longer than the equatorial bonds. 

Because the axial positions have more close neighbors than the equatorial positions, there is more space available to a ligand or its bonding domain in an equatorial position than in an axial position. Thus nonbonding domains and larger bonding domains preferentially occupy the equatorial positions. Consequently 

Because there is more space available in the equatorial than in the axial positions, 

---

We discuss molecules with a valence shell containing five electron pair domains in Section 4.6. The preferred arrangements of five valence shell domains, the trigonal bipyramid and the square pyramid, are not regular polyhedra and therefore exhibit special features not found in tetrahedral and octahedral molecules. Molecules with seven and more electron pair domains in the valence shell of a central atom are not common, although they are of considerable interest. They are restricted to the elements of period 4 and higher periods, with very small ligands such as fluorine, and are discussed in Chapter 9. 

Stereochemistry of Electron-Pair-Coordination-Number 5. Atoms that contain in their valence shells five electron-pairs present interesting stereochemical problems to which several of the ideas developed above may be applied. 

We thus obtain an immediate explanation for the angular shape of CI2O, which is based on a tetrahedral arrangement of four electron pairs in the valency shell of the oxygen atom and for the shape of SF4, which is based on a trigonal bipyramidal arrangement of five electron pairs. 

S2.3 The Lewis structures and molecular shapes for XcFi and ICb are shown below. The XeFi Lewis structure has an octet for the 4 F atoms and an expanded valence shell of 10 electrons for the Xe atom, with the 8 + (2 x 7) = 22 valence electrons provided by the three atoms. The five electron pairs around the central Xe atom will anange themselves at the comers of a trigonal bipyramid (as in PF5). The three lone pairs will be in the equatorial plane, to minimize lone pair-lone pair repulsions. The resulting shape of the molecule, shown at the right, is linear (i.e., the F-Xe-F bond angle is 180°). 

Figure 4.6 Styrofoam sphere models representing the arrangements of two, three, four, five, and six valence shell electron pair domains.

In this section, we consider the structures of species such as CIF3 and SF4 which have five electron pairs in the valence shell of the central atom. The experimentally determined structure of CIF3 is shown in Figure 1.30, and VSEPR... 

The A1 atom is surrounded by three covalent bond electron pairs and two dative bond electron pairs. According to the VSEPR model the most stable arrangement of five electron pairs in the valence shell of the central atom is trigonal bipyramidal, and this is indeed the structure observed. The VSEPR model may also be used to rationalize the observation that the donor atoms occupy axial positions an axial bond electron pair is repelled by three (equatorial) bond pairs, while an equatorial bond electron is repelled by two axial bond pairs across the same angle. Since a covalent bond electron pair requires more space at the aluminum atom than a dative bond electron pair, the covalently bonded atoms occupy the equatorial positions. 

The simplest Lewis formula for SO2 would place two electron pairs between the S and each of the two O atonos, one electron lone pair at the S and two electron lone pairs at each O atom S( 0 )2. This Lewis formula implies that the sulfur atom in SO2 accomodates five electron pairs in the valence shell (as in SF4). Similarly the simplest Lewis formula of the trioxide would place two electron pairs between the S and each O and two nonbonding electron pairs at each of the latter, S( 0 )3, indicating that the S atom in SO3 accomodates six electron pairs in its valence shell (as in SFe) [3]. Note that both the angular structure of SO2 and the trigonal planar structure of SO3 are in agreement with the VSEPR model. 

Bromine triflouiide, BrF The central atom bromine has seven electrons in its valence shell. Three electrons form bonds widi three fluorine atoms while four of them do not participate in bonding. In bromine trifluoride molecule there are a total of five electron pairs in the outer shell. The structure is a trigonal bipyramid. There are three bond pairs and two lone pairs. 

Central Atom with Five or Six Valence-Shell Electron Pairs... 

Central atom with five or six valence-shell electron pairs. 

The A1 atom in the 2 1 complex is surrounded by five electron pairs in the valence shell, and the trigonal bipyramidal structure is in accordance with the... 

---