Orbitals hybrid, molecular geometry

Figure 2. Rearrangement in molecular orbital hybridization and geometry of the carbon atom undergoing a first-order nucleophilic substitution (or Sn1) reaction.

E More on Hybrid Bond Orbitals and Molecular Geometry... 

In Chapter 7, we used valence bond theory to explain bonding in molecules. It accounts, at least qualitatively, for the stability of the covalent bond in terms of the overlap of atomic orbitals. By invoking hybridization, valence bond theory can account for the molecular geometries predicted by electron-pair repulsion. Where Lewis structures are inadequate, as in S02, the concept of resonance allows us to explain the observed properties. 

We are now ready to account for the bonding in methane. In the promoted, hybridized atom each of the electrons in the four sp3 hybrid orbitals can pair with an electron in a hydrogen ls-orbital. Their overlapping orbitals form four o-bonds that point toward the corners of a tetrahedron (Fig. 3.14). The valence-bond description is now consistent with experimental data on molecular geometry. 

Using Valence Bond (VB) theory, the central atoms of the molecules with formulas AB2U2 and AB3U should undergo sp3 hybridized with predicted bond angles of 109.5°. If no hybridization occurs, bonds would be formed by the use of p orbitals. Since the p orbitals are oriented at 90° from each other, the bond angles would be 90°. Note that hybridization is only invoked if the actual molecular geometry data indicate that it is necessary. 

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. 

The Cl—F and Cl—Cl bonds in the cation are then formed by the overlap of the half-filled sp3 hybrid orbitals of the central chlorine atom with the half-filled p-orbitals of the terminal Cl and F atoms. Thus, by using sp3 hybridization, we end up with the same bent molecular geometry for the ion as that predicted by VSEPR theory (when the lone pairs on the central atom are ignored)... 

The GHO basis can therefore provide a localised, directional set of orbitals (hybrids) which do not have the principal qualitative disadvantage of the usual hybrid sets they can be mutually orientated in any directions. What is more the directions taken up by the GHOs can be decided variationally and not by the unitary properties of a hybridisation matrix . This conclusion means that the use of a GHO basis provides both a localised bonding picture and simultaneously a theoretical validation of the VSEPR rules. Thus, it is not necessary, for example, to contrast the hybrid method and the VSEPR method for molecular geometries (30) they are complementary. 

The VSEPR theory is only one way in which the molecular geometry of molecules may be determined. Another way involves the valence bond theory. The valence bond theory describes covalent bonding as the mixing of atomic orbitals to form a new kind of orbital, a hybrid orbital. Hybrid orbitals are atomic orbitals formed as a result of mixing the atomic orbitals of the atoms involved in the covalent bond. The number of hybrid orbitals formed is the same as the number of atomic orbitals mixed, and the type of hybrid orbital formed depends on the types of atomic orbital mixed. Figure 11.7 shows the hybrid orbitals resulting from the mixing of s, p, and d orbitals. 

The other electron-pair geometries that are listed in Table 9-2 are also related to specific hybrid molecular orbitals, but they are more complicated because they involve midp. In every case, the ami/ orbitals are of the same priflfap l( BMlSqis%mfeehySflffMftrofif feg... 

The concept of hybrid orbitals in relation to molecular geometry can be important for the description of excited molecules. Figure 3.6 shows the spatial structure of these orbitals. 

Hybridization. A satisfactory description of covalent bonding should also be able to account for molecular geometry, that is, for the mutual directions of bonds. Let us take for an example boron trifluoride, which is a trigonal planar molecule. Boron uses three orbitals to form three completely equivalent bonds to fluorine atoms. 

T. Vreven, B. Mennucci, C. O. da Silva, K. Morokuma and J. Tomasi, The ONIOM-PCM method Combining the hybrid molecular orbital method and the polarizable continuum model for solvation. Application to the geometry and properties of a merocyanine in solution, J. Chem. Phys., 115 (2001) 62-72. 

This chapter reviews molecular geometry and the two main theories of bonding. The model used to determine molecular geometry is the VSEPR (Valence Shell Electron Pair Repulsion) model. There are two theories of bonding the valence bond theory, which is based on VSEPR theory, and molecular orbital theory. A much greater amount of the chapter is based on valence bond theory, which uses hybridized orbitals, since this is the primary model addressed on the AP test. 

The predictions about the directions of hybrid orbitals based on minimization of repulsion are shown in Figure 2.14. Note that any pair of electrons requires an orbital. Note also that the molecular geometry is not the same as the hybrid orbital type. The oxygen atom in the... 

It is an angular or V -shaped molecule. The geometry of a molecule is predicted by the hybrid orbital type only if there are no unshared pairs of electrons. Hybrid orbital type is determined by the number of electron pairs on the central atom, but the molecular geometry is determined by where the atoms are located. 

The characterization of bonds in terms of sp, sp2 or sp3 orbital hybridization schemes is seen to consist of assuming either linear, trigonal or tetrahedral molecular geometry. Nothing is predicted or explained. 

True or False Hybridization of atomic orbitals is best used for rationalizing known molecular geometries rather than for predicting molecular geometries. 

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