Bent-bond models

Apply the bent-bond model to the preferred conformations of acetaldehyde and propene. Do bent-bonds maintain or remove eclipsing interactions in the equilibrium structures of the two molecules Formulate a simple rule based on the bent-bond model for predicting conformational preferences in systems containing trigonal atoms. 

The bent-bond model can be expressed in orbital terms by assuming that the two components of the double bond are formed from sp3 hybrids on the carbon atoms (Figure 3.19) That this model and the ct-tt model are alternative and approximate, but equivalent, descriptions of the same total electron density distribution can be shown by converting one into the other by taking linear combinations of the orbitals, as shown in Figure 3.20. But neither form of the orbital model can predict the observed deviations from the ideal angles of 109° and 120°. 

Figure 9.16 (a) The geometry of OSF4, HNSF4, and H2CSF4. (b) Bent-bond models of OSF4... 

The bent-bond model of the triple bond gives this bond a threefold symmetry axis and leads to the prediction that the two methyl groups in dimethylacetjdeno should be somewhat restricted in their mutual rotation, with the eclipsed configuration stable. Restriction of rotation about the single bonds in conjugated systems is also expected, and the nature of the stable configurations can be predicted from the theory described above. These systems are discussed in Chapters 6 and 8. 

Coulson and Moffitt23 established a bent bond model of 1 by elaborating ideas first proposed by Forster24. They determined sp" (CC) and sp" (CH) hybrid orbitals with optimal hybridization ratios n and m to describe bonding in 1. Calculations showed that, for 1, the p-character of the CC hybrid orbitals has to be increased from sp3 to sp4 while the s-... 

The bonding between phosphorous atoms in the P4 molecule can be described by a simple bent bond model, which is formed by the overlap of sp3 hybrids of the P atoms. Maximum overlap of each pair of sp3 orbitals does not occur along an edge of the tetrahedron. Instead, the P-P bonds are bent, as shown in Fig. 15.3.1(b). In a more elaborate model, the P4 molecule is further stabilized by the d orbitals of P atoms which also participate in the bonding. 

It turns out, in fact, that the electron distribution and bonding in ethylene can be equally well described by assuming no hybridization at all. The "bent bond" model depicted at the right requires only that the directions of some of the atomic-p orbitals be distorted sufficiently to provide the overlap needed for bonding. So one could well argue that hybrid orbitals are not real they do turn out to be convenient for understanding the bonding of simple molecules at the elementary level, and this is why we use them. 

Note that this is true whether the [Pg.396]

In the notation of Fig. 7, the straight bond model requires an electronic configuration of type (I)2(II)2(IV)2 while the bent bond model configuration is (1)2(11)2(111)2. While there is no difference in symmetry type orbitals III and IV, the electron density distributions on these orbitals are quite different. It follows that the distinction between the two extremes is meaningful, although intermediate situations are possible in the low-symmetry cases under dispute. 

An important illustration of the importance of the Pauli exclusion principle is seen in the Oj molecule. If we were to describe Oj using either the sp hybridization or bent bond model, we would expect a double bond with all the electrons paired. In fact, O2 is paramagnetic, with two unpaired electrons, and yet it does have a double bond. If we ask how electrons would be distributed to maintain maximum separation, we arrive at two tetrahedral arrays, with the tetrahedra offset by the maximum amount. Electronic spin can be represented as x and 0. The structure still has four bonding electrons between the oxygen atoms, that is, a double bond. It also obeys the octet rule for each oxygen and correctly predicts that two of the electrons are unpaired. 

Figure 1-1 The o-7t and bent bond models for bonding in acetylene.

Two models have been advanced to describe the bonding in the cyclopropane ring the bent bond model advocated by Coulson, Moffitt and Forster and the model proposed by Walsh Both descriptions are natural extensions to three centers of theoretical descriptions of the ethylenic two center double bond. 

If we depict the same Newman projections with bent bonds, as shown in Figure 1.34, we see that conformer II now represents essentially an all-eclipsed conformation. Thus, it is easily predicted to be less stable than I, which is an all-staggered arrangement. If utility is the main criterion for adopting conceptual models, then this result would seem to argue persuasively for using the bent bond formulation. The bent bond model leads directly to a correct prediction of conformational stability, but the G,n model does not. ... 

The popularity of HMO theory has helped the cr,7t formulation become the standard pictorial representation of double bonds. The applicability of the bent bond model to benzene was discussed by Schultz, P. A. Messmer, R, P. /. Am. Chem. Soc. 1993, 125,10943. 

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