Hybrid orbitals electron-group geometry

An inner atom with a steric number of 4 has tetrahedral electron group geometry and can be described using S p hybrid orbitals. 

Both inner atoms have steric numbers of 4 and tetrahedral electron group geometry, so both can be described using s p hybrid orbitals. All four hydrogen atoms occupy outer positions, and these form bonds to the inner atoms through 1 s-s p overlap. The oxygen atom has two lone pairs, one in each of the two hybrid orbitals not used to form O—H bonds. 

We generate hybrid orbitals on inner atoms whose bond angles are not readily reproduced using direct orbital overlap with standard atomic orbitals. Consequently, each of the electron group geometries described in Chapter 9 is associated with its own specific set of hybrid orbitals. Each type of hybrid orbital scheme shares the characteristics described in our discussion of methane ... 

With a steric number of 6, xenon has octahedral electron group geometry. This means the inner atom requires six directional orbitals, which are provided by an. s p d hybrid set. Fluorine uses its valence 2 p orbitals to form bonds by overlapping with the hybrid orbitals on the xenon atom. The two lone pairs are on opposite sides of a square plane, to minimize electron-electron repulsion. See the orbital overlap view on the next page. 

For each of the following Lewis structures, name the electron group geometry and the hybrid orbitals used by the inner atoms. 

The steric number of an inner atom determines the electron group geometry, each of which is associated with one specific type of hybrid orbital. 

The sp hybridization scheme corresponds to trigonal-planar electron-group geometry and 120° bond, as in BF3. Note again that in the hybridization schemes of valence bond theory, the number of orbitals is conserved that is, in an sp hybridized atom there are still four orbitals three sp hybrids and an unhybridized p orbital. 

To describe hybridization schemes that correspond to the 5- and 6-electron-group geometries of VSEPR theory, we need to go beyond the s and p subshells of the valence shell, and traditionally this has meant including d-orbital contributions. We can achieve the five half-filled orbitals of phosphorus to account for the five P—Cl bonds in PCI5 and its trigonal-bipyramidal molecular geometry through the hybridization of the s, three p, and one d orbital of the valence shell into five sp d hybrid orbitals. 

A FIGURE 11-14 Using electron-group geometry to determine hybrid orbitals... 

Note that the number of electron pairs in the electron-group geometry dictates how many orbitals are used in the hybridization scheme. We see that a combination of VSEPR and hybridization theory is an appealing way to describe the shape of and bonding in a molecule. However, we should emphasize a point made earlier on page 478 The inclusion of d orbitals in hybridization schemes is questionable. In other words, the use of the sp d hybridization scheme for xenon is probably not the best way to describe the bonding in XeF. ... 

Identify the hybridization scheme that conforms to the electron-group geometry. A trigonal-planar orientation of orbitals is associated with sp hybrid orbitals. 

With VSEPR theory, we predict a trigonal-planar electron-group geometry (the measured bond angle is 117°). The hybridization scheme chosen for the central O atom is sjp-, and although we normally do not need to invoke hybridization for terminal atoms, this case is simplified if we assume sjP hybridization for the terminal O atoms as well. Thus, each O atom uses the orbital set + p. 

The VSEPR notation for the Cl2F+ ion is AX2E3. According to Table 11.1, molecules of this type exhibit an angular molecular geometry. Our next task is to select a hybridization scheme that is consistent with the predicted shape. It turns out that the only way we can end up with a tetrahedral array of electron groups is if the central chlorine atom is sp3 hybridized. In this scheme, two of the sp3 hybrid orbitals are filled, while the remaining two are half occupied. 

To determine the hybridization of an atom in a molecule, we count groups around the atom, just as we did in determining geometry. The number of groups (atoms and nonbonded electron pairs) corresponds to the number of atomic orbitals that must be hybridized to form the hybrid orbitals. 

---