Fluorine molecule, molecular orbitals

If we now consider a planar molecule like BF3 (D3f, symmetry), the z-axis is defined as the C3 axis. One of the B-F bonds lies along the x-axis as shown in Figure 5.9. The symmetry elements present for this molecule include the C3 axis, three C2 axes (coincident with the B-F bonds and perpendicular to the C3 axis), three mirror planes each containing a C2 axis and the C3 axis, and the identity. Thus, there are 12 symmetry operations that can be performed with this molecule. It can be shown that the px and py orbitals both transform as E and the pz orbital transforms as A, ". The s orbital is A/ (the prime indicating symmetry with respect to ah). Similarly, we could find that the fluorine pz orbitals are Av Ev and E1. The qualitative molecular orbital diagram can then be constructed as shown in Figure 5.10. 

Oxygen, fluorine, and man. These three molecules can be treated with the same energy diagram that we have been using for other diatomic molecules of the second-row elements. As we shall see shortly, the intervening molecules, B, C-. and N2. require additional considerations, which lead to an alteration in (he relative energies of the molecular orbitals. 

TABLE 7.4 MOLECULAR ORBITAL CONFIGURATIONS AND OTHER DATA ABOUT PERIOD 2 ELEMENTS. IT IS UNDERSTOOD THAT EACH MOLECULE HAS A COMPLETE S1s AND s is ORBITAL. NOTE THAT OXYGEN, FLUORINE, AND NEON HAVE A SLIGHTLY DIFFERENT ORDER DUE TO INTERACTIONS BETWEEN 2s AND 2p ORBITALS. 

Although both molecules have threefold symmetry, the procedure for describing molecular orbitals of BF3 differs from NH3, because the fluorine atoms surrounding the central boron atom have 2p as well as 2s electrons to be considered. In this case, the Py axes of the fluorine atoms are chosen so that they are pointing toward the boron atom and the Px axes are in the plane of the molecule. The group orbitals and their symmetry in the point group are shown in Figure 5-32. The molecular orbitals are shown in... 

The strong electronegativity of fluorine renders its related molecular orbitals relatively lower-lying. For example, a C-F bond can readily accept electrons to its vacant c C-f orbital from a vicinal electron-donating orbital, while the electron-occupied cC-f orbital is reluctant to donate electrons. Such characteristics often influence the three-dimensional shape of a molecule in a distinct manner. Figure 1.7 illustrates basic examples. 

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