B-F bond

Boron trifluoride is a trigonal planar molecule There are six electrons two for each B—F bond associated with the valence shell of boron These three bonded pairs are farthest apart when they are coplanar with F—B—F bond angles of 120°... 

In the planar molecule BF3, in Figure 4.3(b), the C3 axis through B and perpendicular to the figure is the highest-fold axis and, therefore, the three planes of symmetry, perpendicular to the figure and through each of the B-F bonds, are labelled (t . The plane of the molecule is also a plane of symmetry and is labelled u , where /z stands for horizontal with respect to C3. 

The BF3 molecule, shown in Figure 4.18(i), is now seen to have /r = 0 because it belongs to the point group for which none of the translational symmetry species is totally symmetric. Alternatively, we can show that /r = 0 by using the concept of bond moments. If the B-F bond moment is /Tgp and we resolve the three bond moments along, say, the direction of one of the B-F bonds we get... 

Table 1 Hsts some of the physical properties of duoroboric acid. It is a strong acid in water, equal to most mineral acids in strength and has a p p o of —4.9 as compared to —4.3 for nitric acid (9). The duoroborate ion contains a neady tetrahedral boron atom with almost equidistant B—F bonds in the sohd state. Although lattice effects and hydrogen bonding distort the ion, the average B—F distance is 0.138 nm the F—B—F angles are neady the theoretical 109° (10,11). Raman spectra on molten, ie, Hquid NaBF agree with the symmetrical tetrahedral stmcture (12).

The designation of hard acids is not restricted to metal cations. For example, in BF3 the small boron atom in its +3 oxidation state is bonded to three highly electronegative fluorine atoms. All the B—F bonds are polarized away from a boron center that is already electron-deficient. Boron trifiuoride is a hard Lewis acid. 

In addition to the C3 axis (the z-axis), there are also three vertical planes of symmetry which can be seen clearly in Figure 5.1. They are perpendicular to the plane of the molecule and bisect the molecule along each B-F bond. Because the molecule is planar, there is also a horizontal plane of symmetry (crj,) that bisects all four of the atoms. Each B-F bond is also a C2 axis because rotation around the bond would give the same orientation of the molecule except for changing the positions of the fluorine atoms. One of the C2 axes is shown above, and rotation by 180° around that axis would produce the orientation... 

Note that each of the C2 axes not only is coincident with a B-F bond but also is the line of intersection of the horizontal plane with one of the vertical planes. It is generally true that the intersection of a vertical plane of symmetry with a horizontal plane generates a C2 axis. The list of symmetry elements that we have found for the BF3 molecule includes one C3 axis, three vertical planes (horizontal plane (ah). A molecule possessing these symmetry elements, such as BF3, S03, C03 , and N03 , is said to have l)ih symmetry. In the cases of H20, C1F3, H2CO, and NH3, the symmetry elements included only a C axis and n vertical planes. These molecules belong to the general symmetry type known as C . Molecules that have a Cn axis and also have n C2 axes perpendicular to the C axis are known as Dn molecules. 

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. 

It is readily apparent that the three a bonds are capable of holding the six bonding electrons in the a t and e molecular orbitals. The possibility of some 7r-bonding is seen in the molecular orbital diagram as a result of the availability of the a2" orbital, and in fact there is some experimental evidence for this type of interaction. The sum of the covalent radii of boron and fluorine atoms is about 152 pm (1.52 A), but the experimental B-F bond distance in BF3 is about 129.5 pm (1.295 A). Part of this "bond shortening" may be due to partial double bonds resulting from the 7r-bonding. A way to show this is by means of the three resonance structures of the valence bond type that can be shown as... 

From these resonance structures, we determine a bond order of 1.33 for the B-F bonds, which would predict the observed bond shortening. However, another explanation of the "short" B-F bonds is based... 

The strength of a Lewis acid is a measure of its ability to attract a pair of electrons on a molecule that is behaving as a Lewis base. Fluorine is more electronegative than chlorine, so it appears that three fluorine atoms should withdraw electron density from the boron atom, leaving it more positive. This would also happen to some extent when the peripheral atoms are chlorine, but chlorine is less electronegative than fluorine. On this basis, we would expect BF3 to be a stronger Lewis acid. However, in the BF3 molecule, the boron atom uses sp2 hybrid orbitals, which leaves one empty 2p orbital that is perpendicular to the plane of the molecule. The fluorine atoms have filled 2p orbitals that can overlap with the empty 2p orbital on the boron atom to give some double bond character to the B-F bonds. 

The data given in Table 13.3 show that the extent of bond shortening is greatest for B-F bonds. This is to be expected because back donation of electron density from F to B is more effective when the donor and acceptor atoms are of comparable size. The following resonance structures are used to represent the multiple bonding between B and F ... 

This molecule (type AB3) has a trigonal planar electronic and molecular geometry. The B-F bonds are polar. Since the molecule is symmetric, the bond dipoles cancel to give a nonpolar molecule (Section 8-6). 

The ground-state electron configuration of B suggests an ability to form one B—F bond, rather than three., JB JB -Jil I I Again, excitation, or promotion of an electron to a higher energy orbital, followed by hybridization is required to form three equivalent half-filled B orbitals. 

Addition of B—F bonds across C=N groups has never been achieved. Therefore, the only B-fluoro substituted iminoborane so far known was obtained by the reaction of diphenylketimine-lithium with trifluoroborane 13> according to the general Equation (25). 

There are three sigma bonds in BF, and no unshared pairs therefore, three HO s are needed. Hence. B uses sp HO s. and the shape is trigonal planar. Each F—B—F bond angle is 120°. 

Owing to the strength of the B—F bond, die BF3 complexes are of widespread use as model compounds, for investigating Lewis acid-base interactions and the nature of the donor-acceptor bond. BF3 is frequently employed as a standard Lewis acid, for the quantitative characterization of the Lewis basicity of donor mojecules.62,63 The gas-phase equilibrium constants for some BF3 complexes are shown in Table 5. 

The Third-Group Elements.—The B—F bond has about 63 percent ionic character, B—O 44 percent, B—Cl 22 percent, and so forth. Bor,on forms normal covalent bonds with hydrogen. The aluminum bonds are similar to those of beryllium in ionic character. 

The observed B—F bond length1 is 1.38 + 0.01 A, in agreement with the calculated value 1.37 A. The values 1.39 + 0.01 A for trimethyl-amine-boron trifluoride1 and 1.43 + 0.03 A for dimethylether-boron trifluoride14 have also been reported. In trichloroborazole, to which we may assign the structure... 

The short B-F distance in BF3 (130 pm compared with 143 pm in BFj") and the large B-F bond energy are suggestive of partial double bond character. BF3 is a weaker Lewis acid than the other boron trihalides, which may indicate that the four boron valence orbitals in BF3 are more fully engaged than in BC13 etc. Molecules of the type R2BNR2 are believed to have substantial p -p bonding ... 

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