Boron trifluoride hybrid orbitals

Figure 14.30 Atomic orbitals in boron (ground state), boron (excited state) and boron trifluoride (hybridized state)... 

In this case, there are three equivalent hybrid orbitals, each called sp (trigonal hybridization). This method of designating hybrid orbitals is perhaps unfortunate since nonhybrid orbitals are designated by single letters, but it must be kept in mind that each of the three orbitals is called sp. These orbitals are shown in Figure 1.4. The three axes are all in one plane and point to the comers of an equilateral triangle. This accords with the known structure of boron trifluoride (BF3), a planar molecule with angles of 120°. 

Boron trifluoride has a trigonal-planar structure. Formulate the bonding in terms of molecular orbitals for the Dsjj symmetry. In addition, construct wave functions for three equivalent sp2 hybrid orbitals, using the 2px, 2p, and 2s boron valence orbitals, which may be used to form three localized bonds with the three fluorines. Compare and contrast the molecular-orbital and the hybrid-orbital descriptions. 

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. 

Boron trifluoride and ethylene are but two of the many instances where the directional properties of covalent bonds are better described in terms of overlap of hybrid orbitals than in terms of simple atomic orbitals. 

Examples of neutral Lewis acids are halides of group 3A elements, such as BF3. Boron trifluoride, a colorless gas, is an excellent Lewis acid because the boron atom in the trigonal planar BF3 molecule is surrounded by only six valence electrons (Figure 15.12). The boron atom uses three sp2 hybrid orbitals to bond to the three F atoms and has a vacant 2p valence orbital that can accept a share in a pair of electrons from a Lewis base, such as NH3. The Lewis acid and base sites are evident in electrostatic potential maps, which show the electron poor B atom (blue) and the electron rich N atom (red). In the product, called an acid-base adduct, the boron atom has acquired a stable octet of electrons. 

We can now go on to apply the same ideas to some other simple molecules. In boron trifluoride, for example, we start with the boron atom, which has three outer-shell electrons in its normal or ground state, and three fluorine atoms, each with seven outer electrons. As shown in the upper diagram, one of the three boron electrons is unpaired in the ground state. In order to explain the trivalent bonding of boron, we postulate that the atomic s- and p orbitals in the outer shell of boron mix to form three equivalent hybrid orbitals. These particular orbitals are called sp2 hybrids, meaning that this set of orbitals is derived from one s-orbital and two p-orbitals of the free atom. 

Boron trifluoride has a plane trigonal shape a 2p orbital on each fluorine atom overlaps with a boron sp2 hybrid. In general, we can expect that all molecules in which a central atom uses three equivalent sp2 hybrid orbitals will exhibit plane trigonal geometry, since this represents the most symmetrical, and hence equivalent , arrangement of the three bonds. 

Like boron in boron trifluoride (Sec. 1.10), carbon here is bonded to three other atoms. Hybridization of the 2s orbital and two of the p orbitals provides the... 

Boron is a group IIIA element. Open the molecular model for boron trifluoride from the 3D Molecular Models section of the book s website. Near the boron atom, above and below the plane of the atoms in BF3, are two relatively large lobes. Considering the position of boron in the periodic table and the three-dimensional and electronic structure of BF3, what type of orbital does this lobe represent Is it a hybridized orbital or not ... 

Figure 14.29 sp hybrid orbitals in the boron trifluoride molecule... 

Boron in the boron trifluoride molecule, BF3, provides an example of sp hybridization (Figure 14.29). The boron atom has the electron configuration ls 2s 2pb After hybridization the electrons in the outer shell of the boron atom are in three spr hybrid orbitals. Single bonds between the boron and fluorine atoms involve the sharing of the unpaired electrons in the sp hybrid orbitals of boron and the sp orbitals of the fluorine atoms (Figure 14.30). 

Figure 18.2 shows the reaction between boron trifluoride and ammonia to form a stable adduct. The donor orbital is a lone pair on the nitrogen atom of ammonia and the acceptor is the empty p orbital left over from the sp hybridization at boron in boron trifluoride. 

Figure 4.15 Boron trifluoride is a classic Lewis acid. The hybridization of boron is sp and boron, therefore, also has a p-orbital that is vacant, and can accept a pair of electrons.

---