Valence electrons trigonal bipyramidal

In many respects, the successes of this model are remarkable. Iron(O) possesses a total of eight electrons in its valence shell. To satisfy the eighteen-electron rule, five two-electron donors are needed, and compounds such as [Fe(CO)5] are formed. These molecules also obey simple VSEPR precepts, and [Fe(CO)s] adopts a trigonal bipyramidal geometry. Conversely, the use of two five-electron donor ligands such as the strong r-acceptor cyclopentadienyl, Cp, gives the well-known compound ferrocene (9.3). 

Nucleophilic substitution at RSO2X is similar to attack at RCOX. Many of the reactions are essentially the same, though sulfonyl halides are less reactive than halides of carboxylic acids. The mechanisms are not identical, because a tetrahedral intermediate in this case (148) would have five groups on the central atom. Though this is possible (since sulfur can accommodate up to 12 electrons in its valence shell) it seems more likely that these mechanisms more closely resemble the Sn2 mechanism, with a trigonal bipyramidal transition state (148). There are two major experimental results leading to this conclusion. 

With a steric number of 5, chlorine has trigonal bipyramidal electron group geomehy. This means the inner atom requires five directional orbitals, which are provided hymsp d hybrid set. Fluorine uses its valence 2 p orbitals to form bonds by overlapping with the hybrid orbitals on the chlorine atom. Remember that the trigonal bipyramid has nonequivalent axial and equatorial sites. As we describe in Chapter 9, lone pairs always occupy equatorial positions. See the orbital overlap view on the next page. 

Table 8.3 Axial and equatorial bond lengths /pm for trigonal-bipyramidal distribution of the valence electrons...

We have so far considered valence shells containing four pairs of electrons, but we can extend the same arguments to other numbers of valence shell electron pairs. The most probable arrangements of pairs of opposite spin electrons in the valence shell of an atom in a molecule are two pairs, collinear three pairs, equilateral triangular four pairs, tetrahedral five pairs, trigonal bipyramidal six pairs, octahedral. This is because, as we will now see, these are the arrangements that keep the electron pairs as far apart as possible. We discuss valence shells with more than six electron pairs in Chapter 9. 

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. 

For AX molecules with no lone pairs in the valence shell of A, both the VSEPR model and the LCP model predict the same geometries, namely AX2 linear, AX3 equilateral triangular, AX4 tetrahedral, AX5 trigonal bipyramidal, and AX octahedral. Indeed Bent s tangent sphere model can be used equally as a model of the packing of spherical electron pair domains and as a model of the close packing of spherical ligands around the core of the central atom. 

According to the VSEPR model the T-shaped CIF3 molecule has a trigonal bipyramidal arrangement of three bonding electron pairs and two nonbonding electron pairs in the valence shell of the central Cl atom. 

An interesting application of VSEPR is illustrated by the structure of SF4. The sulfur atom has 10 electrons around it (six valence shell electrons from S and one from each of the four F atoms). We predict that the structure will be based on a trigonal bipyramid, but there are two possible structures ... 

The dianion [Os5(CO)i5]2- (72 valence electrons) has a trigonal-bipyramidal structure similar to that of the monoanion [Rh5(CO)15] (76 valence electrons) illustrated in Figure 27.ld... 

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