Hybrids and Bonds in Molecules

Age-old questions concerning the nature of the bonds between atoms in molecules culminated in the remarkable Lewis structure model of G. N. Lewis (1916). The notion that such bonds were formed from directed hybrids was subsequently developed by Linus Pauling (1932), shortly after the discovery of quantum mechanics. Although many theoretical advances have ensued, it is fair to say that the underlying concepts of valence-shell hybridization, shared-electron pair bonds, and Lewis structural dot diagrams continue to dominate chemical thinking and pedagogy to this day. 

In this chapter, we illustrate how to obtain the optimal natural Lewis stmcture (NLS) formulation of the wavefunction in terms of optimal NBOs for shared pairs (bonds) and lone pairs of the conventional Lewis stmctural dot diagram. We also describe how to assess the accuracy of the NLS representation, comparing it with alternative Lewis stmctural formulations (alternative resonance stmctures ) that might be suggested. In Sections 4.1. 2, we first consider the relatively simple closed-shell molecules such as HF, CH3OH, or H2NCHO that conform to the octet mle. The 

Discovering Chemistry With Natural Bond Orbitals, First Edition. Frank Weinhold and Clark R. Landis. 2012 John Wiley Sons, Inc. Published 2012 by John Wiley Sons, Inc. 

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Produced by mixing (hybridizing) orbitals of a central atom A construct consistent with observed shapes and bonding in molecules... 

When resonance is involved, different resonance forms may suggest different hybridization and bond angles, but a real molecule can have only one set of bond angles, which must be compatible with all the important resonance forms. The bond angles imply the hybridization of the atoms, which also must be the same in all the resonance forms. 

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. 

On increasing the coordination number of silicon bonded to three and more fluorine atoms, the Jsip value decreases regularly (Table 15). The ratio between coupling constants in the spectra of SiF, SiF , and SiFg (1.6 1.35 1) corresponds to a regular decrease of s- and p-character of the bonding hybride silicon AOs in molecules of these compounds (1.5 1.2 1). 

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