Hybridization bent bonds

The concept of hybrid orbitals is deeply ingrained in the thinking of organic chemists, as widely reflected in texts and the research literature. However, Pauling and others recognized that there was a different conceptual starting point in which multiple bonds can be represented as bent bonds. ... 

It has been shown that description of bonding based on the bent-bond concept can be just as successful in describing molecular structure as the hybridization concept. We will, however, use the hybridization terminology. 

This alternative description follows from classical ideas and from a VB description utilizing hybrid orbitals. According to this description, a double bond is described as consisting of two bent bonds, sometimes called r bonds or banana bonds, formed by the overlap of... 

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°. 

Problem Compare the OsH3X hybridizations and bond angles predicted by the simple Bent s rule estimates (4.59) and (4.60) with the actual values of Table 4.17. 

We have seen in Chapter 5 that the strength of a bond depends to a certain extent upon the hybridizations of the atoms forming the bond. We should therefore expect bond length to vary with hybridization. Bent has shown that this variation is quite regular C—C bond lengths are proportional to p character (Fig. 6.24) or, to say it... 

Pauling150 has provided an alternative, valence bond treatment of the quadruple bond involving spd hybrid orbitals and four equivalent bent bonds. His model also explains the experimental facts described above and provides a good estimate of the bond length. 

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 results of the Coulson-Moffitt model have been verified many times using different energy minimization criteria for obtaining the best hybridization ratios (n and m) and determining angles between bent bond orbitals. Some results are summarized in Table i7-23-31-45- 7... 

The molecular-orbital description of HF can be obtained if we note that the three equivalent lone pairs can be obtained from a a orbital and two n orbitals by a transformation similar to that used for obtaining the bent-bond description of N2. In the LCAO form the a lone pair will be another s p hybrid and the two lone pairs will be (2px)2 and (2py)z. It is generally found that n lone pairs are less firmly bound than a lone pairs so that the lowest ionisation potential would correspond to the removal of an electron from one of the last two orbitals. 

In the case of strained molecules such as cyclic molecules with less than six ring atoms the hybrids forming the bonds do not lie along the connection line of the nuclei any longer since the bond angles do not correspond with the hybridization of the atoms. This fact can be described by bent bonds. 

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. 

Angle strain in cyclopropane. The bond angles are compressed to 60° from the usual 109.5° bond angle of sp3 hybridized carbon atoms. This severe angle strain leads to nonlinear overlap of the sp3 orbitals and bent bonds. ... 

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. 

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