Equatorial hybrid orbitals

Although it will not be important for our purposes, the dsp3 hybrid orbital set is different from the hybrids we have considered so far in that the hybrid orbitals pointing to the vertices of the triangle (often called the three equatorial hybrid orbitals) are slightly different in shape than the other two (the axial orbitals). This situation stands in contrast to the sp, sp1, and sp3 hybrid sets in which each orbital in a particular set is identical in shape to the others. 

The situation is different in the PHi case [Eq. (39)], since one can think now of a third and low-energy curve, able to mix efficiently with the two Lewis curves, as in Fig. 8 and 7 b. Indeed, the P atom bears a lone pair in an equatorial hybrid orbital, and it is not very costly in energy to promote one electron from this lone pair to... 

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. 

Note that in this case the unshared pairs of electrons are in equatorial positions, which results in a linear structure for IF2 even though the hybrid orbital type is sp3d. It is the arrangement of atoms, not electrons, that determines the structure for a molecule or ion. It is apparent that the simple procedures described in this section are adequate for determining the structures of many molecules and ions in which there are only single bonds and unshared pairs of electrons. 

However, there is no reason at all to assume ha and hb are made up of only pz and dz2 orbitals The dz2 orbital can also contribute to the equatorial hybrids, and the s orbital can also contribute to the axial hybrids. In fact, if we use only the s and pz orbitals for the axial hybrids, and the dz2, px, and p-y for the equatorial hybrids, we then have... 

Most sets of hybrid orbitals are equivalent and symmetric, that is, four sp orbitals directed to the comers of a regular tetrahedron, six tPsp orbitals to the corners of an octahedron, etc. (n the case of hybrids the resulting orbitals are not equivalent. In the trigonal bipyramidal arrangement three orbitals directed trigonally form one set of equivalent orbitals (these may be considered sp hybrids) and two orbitals directed linearly (and perpendicular to the plane of the first three) form a second set of two (these may be considered dp hybrids). The former set is known as the equatorial orbitals and the latter as the axial orbitals. Because of the nature of the different orbitals involved, bonds formed from the two are intrinsically different and will have different properties even when bonded to irJenlica] atoms. For example, in molecules like PFj bond lengths differ for axial and equatorial bonds (see Chapter 6). 

FIGURE 8.30 (a) Hybrid orbitals formed from linear combinations of Pm P>r and orbitals. The pair of orbitals that point along the positive and negative z-axes are the same except for their orientation in space they are called axial orbitals. The set of three orbitals in the x-y plane are equivalent to one another and are called equatorial orbitals, (b) This set of hybrid orbitals can be used to describe the bonding in PF5, for example. 

As shown in Figure 8.30a, these orbitals point to the vertices of a trigonal bipyramid there are three equivalent equatorial hybrids and two eqnivalent axial hybrids. Examples of molecules whose shapes are described by dsp hybridization inclnde PF5, which you have seen in Section 3.9, and CnCl C PF5 is shown in Fig-nre 8.30b for comparison with the set of dsp hybrid orbitals. 

BiXs has three equatorial bonds, made from the usual sp hybrid orbital, and two axial bonds which use the three-center four-electron bonding, two electrons from the p orbital of bismuth and two electrons from the X ligands [94B-N]. In the three-center four-electron bonding, the central atom should be positively charged. This has been confirmed for MH5 (M = P, As, Sb, Bi) with recent ab initio calculation [95JA11790]. 

But, mixing with the 4s level produces a dzi-s hybrid orbital with reduced density along the axis and increased density in the doughnut-shaped lobe in the equatorial plane that is devoid of ligands. The low coordination number in the molecule leads to z2-s hybridization, allowing strong NiNR ji bonding. 

An sp d hybrid orbital set may be considered to be a combination of p d hybrids and spj>y hybrids. The former make two linear hybrid orbitals bonding axially and the latter form the trigonal, equatorial bonds. The sp2 hybrid orbitals are capable of forming stronger bonds, and they are shorter than the weaker axial bonds. When the electronegativities of the substituents on the phosphorus atom differ, as in the mixed chlorofluorides, and the alkylphosphorus fluorides, R,PFs r it is experi-... 

The mixing of 5, p, Py, p and d i atomic orbitals gives a set of five sp d hybrid orbitals, the mutual orientations of which correspond to a trigonal bipyramidal arrangement (Figure 4.7a). The five sp d hybrid orbitals are not equivalent and divide into sets of two axial and three equatorial orbitals the axial orbital lobes lie along the z axis. The model of sp d hybridization can be used to... 

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