Absolutely Small - Michael D. Fayer [94]
In total the BeH2 molecule has six electrons counting the two pairs of bonding electrons and the two electrons in the Be 1s orbital. It is important to note that all of these electrons are probability amplitude waves that span the entire molecule. When we say that the 1sa electron makes an electron pair bond with the spz- electron, this is bookkeeping. All of the electrons are free to roam over the entire molecule. However, the probability distributions for the electrons are such that at any given instant there is electron density associated with the Be and one of the H atoms that corresponds to one bond and electron density associated with the Be and the other H atom that corresponds to the other bond.
HYBRID ATOMIC ORBITALS—TRIGONAL MOLECULES
As discussed in connection with Figure 14.3, BH3 is a trigonal molecule with 120° between bonds. Atomic boron has three valence electrons, two in the 2s and one in a 2p orbital. To form three electron pair sharing bonds to three hydrogen atoms, the boron atom needs three unpaired electrons. As shown at the top of Figure 14.7, B will promote one electron from a 2s orbital to a 2p orbital to have the three unpaired electrons. If the molecule lies in the xy plane, then the 2p orbitals involved in bonding will be the 2px and 2py. To form the equilateral trigonal BH3 molecule, the three boron atomic orbit als will hybridize to yield the three hybrid atomic orbitals, , , and . The sp2 notation means that the hybrid orbital is composed of an s orbital and two different p orbitals. We start with three different orbitals, 2s, 2px, and 2py. Orbitals are never gained or lost, so we end up with three different hybrid orbitals. These are shown in the middle portion of Figure 14.7. Each orbital has a positive and a negative lobe. The adjacent lobes have an angle of 120° between them. Each of these contains one of the three unpaired B valence electrons.
The bottom portion of Figure 14.7 shows the bonding of B with three H atoms. Each H atom has a single 1s electron. An H 1s orbital combines with a B sp2 orbital to form a bonding molecular orbital. There are two electrons in this bonding MO, one from the H and one from the B. The result is an electron pair bond. Each of the bonds between B and H is a σ bond since there is electron density along the line connecting the nuclei. A model of BH3 is shown in Figure 14.3.
FIGURE 14.7. Top: B valence electrons with one promoted to a 2p orbital. Middle: the 2s, 2px, and 2pyorbitals of B combine in three combinations to form three hybrid atomic orbitals, , and . The angle between the lobes is 120. Bottom: The three B hybrid orbitals forming bonds with the three H atom 1s orbitals.
HYBRID ATOMIC ORBITALS—TETRAHEDRAL MOLECULES
In methane, carbon makes four bonds to four hydrogen atoms. As discussed above and shown in Figures 14.1 and 14.2, methane is tetrahedral. As illustrated in Figure 14.4, to make four electron pair sharing covalent bonds, carbon promotes one of its 2s electrons to a 2p orbital. It then has four unpaired electrons in the 2s, 2px, 2py, and 2pz orbitals. As discussed in some detail following Figure 14.4, these four carbon atomic orbitals will not yield four identical bonds to four hydrogens and produce a tetrahedral molecule. Therefore, the 2s, 2px, 2py, and 2pz orbitals combine in four different combina tions to form four atomic hybrid orbitals, , , , and . The sp3 designation means that each of the four hybrid atomic orbitals is a combination of an s orbital and the three different p orbitals. Figure 14.8 shows the four sp3 orbitals overlapping with the four hydrogen 1s orbitals. Only the positive lobes of the sp3 orbitals are shown. Each of them has a small negative lobe pointing in the opposite direction from the positive lobe in a manner analogous to that shown for the sp2 orbitals in the middle portion of Figure 14.7. The orbitals represented by dashed curves are in the plane of the page. The orbital shown as a solid curve is coming out of the plane of the page at an angle,