Overlap, back-lobe

FIGURE 2.4 A comparison between overlap of a hydrogen Ir orbital with (a) an sp hybrid orbital and (b) an unhybridized 2p orbital. With the sp hybrid orbital, overlap is maximized because the non-overlapping back lobe is small. With the unhybridized 2p orbital, all of the non-overlapping back lobe is wasted. ... 

FIGURE 2 9 Each half filled sp orbital overlaps with a half filled hydrogen Is or bital along a line between them giving a tetrahedral arrangement of four ct bonds Only the major lobe of each sp orbital is shown Each orbital contains a smaller back lobe which has been omitted for clarity... 

The observed ya-SCS(X) values in 7-exo-substituted norcaranes 97 (225) and those of M(CH3)3 (M = Si, Ge, Sn, or Pb) in cyclohexyl and bicy-clo[2.2. l]heptane derivatives (133) were later interpreted on the same basis. The back-lobe-overlap treatment was further supported by interpretations of H and 13C contact shifts of aliphatic amine signals in the presence of nickel acetyl-acetonate and by INDO calculations (226,227). Additional support came from extensive investigations of the structure dependence of three-bond coupling constants 3JCX (X = H, C, or F) (228,229), although the interpretation of these data has been subjected to criticism (230). 

A picture of the electron distribution in the a orbitals between carbon and chlorine is revealed in the wire-mesh diagrams for methyl chloride in Fig. 1.47, which shows one contour of the major orbital crccl contributing to C—Cl bonding together with the LUMO, this book. Comparing these with the schematic version in Fig. 1.46, we can see how the back lobe on carbon in hydrogen atoms, and that the front lobe in cr ccl wraps back a little behind the carbon atom to include some overlap to the s orbitals of the hydrogen atoms. 

Next, let us consider the formation of the fluorine molecule, F2, from two fluorine atoms. As we can see from our table of electronic configurations (Table 1.1), a fluorine atom has two electrons in the s orbital, two electrons in the 2 orbital, and two electrons in each of two 2p orbitals. In the third Ip orbital there is a single electron which is unpaired and available for bond formation. Overlap of this p orbital with a similar p orbital of another fluorine atom permits electrons to pair and the bond to form (Fig. 1.4). The electronic charge is concentrated between the two nuclei, so that the back lobe of each of the overlapping orbitals... 

The magnitude of the splitting between Ha and Hb is greatest when 0° or 180° and is smallest when a= 90°. The side-side overlap of the two C—H bond orbitals is at a maximum at 0°, where the C—H bond orbitals are parallel, and at a minimimi at 90°, where they are perpendicular. At or= 180°, overlap with the back lobes of the orbitals occurs. 

Using valence bond theory, each of the four bonds in methane is represented by the overlap between an ry> -hybridized atomic orbital from the carbon atom and an s orbital from a hydrogen atom (Figure 1.23). For purposes of clarity the back lobes (blue) have been omitted from the images in Figure 1.23. 

Overlap between an sp orbital of one atom and an orbital of some other atom will be especially good if the fat lobe of the sp orbital is used (Fig. 2.4a). Contrast this overlap with the overlap when a Is orbital overlaps with one lobe of a 2p orbital of another atom (Fig. 2.4b). The better the overlap, the greater the stabilization and the stronger the bond. In a sense, overlap of a Ir orbital with an unhybridized atomic 2p orbital wastes the back lobe of the 2p orbital. Hybridization improves overlap ... 

A second problem with the model used in Figure 2.9 might he labeled inefficient overlap or wasted orbitals. Recall from Rgure 2.4 that a 2p atomic orbital is poorly designed for efficient end-on overlap because overlap can take place only with one lobe. The back lobes of 2p orbitals are unused and thus wasted (Rg. 2.10). Our unhybridized model for methane has two such overlapping pairs of orbitals. 

Bonds are formed through overlap between the carbon 2p atomic orbitals and hydrogen Ij- orbitals, thus wasting the back lobes of the 2p orbitals. In the hybridization model, overlap is much improved in the bonds formed using the fat lobes of sp orbitals and hydrogen F orbitals.Table 2.1 reviews the properties of sp, sp, and sp hybrid orbitals. 

As shown in Fig. 1, the big lobes of these hybrids point toward each other. Therefore, if the nucleophile approaches the substrate from the front side, its HOMO overlaps in phase with the big lobe of 0c and out-of-phase with the big lobe of 0x-Numerical calculations show that the unfavourable (nucleophile-leaving group) interaction usually overrides the favourable (nucleophile - reaction center) interaction in this front-side approach, so that back-side attack is finally preferred, leading to inversion of configuration. 

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