Bond polarity, electronegativity, and Bents rule

One can see from Equation (4.10) that hybrid p-character (and associated X hybridization parameter) strongly affects hybrid direction and molecular shape. But what affects hybrid p-character The answer to this question gives one of the deepest insights into molecular shape, and is expressed in simple and intuitive terms by Bent s rule (cf. FcfeB, p. 13 8ff), the deeper principle that underlies success of the valence shell electron pair repulsions (VSEPR) model (Sidebar 4.3). 

Although the strange steric demand concepts of VSEPR models, skillfiilly deployed, can give the right answer in a surprising number of cases, the same steric-style VSEPR reasoning proves unreliable in wbat might be considered reasonable extensions to more complex molecules. (Quantitation of steric size is discussed in Chapter 6.) 

For example, if one considers ethane-bke molecules in which one of the hydride bonds on each end is replaced by a fat lone pair (e.g., in hydrazine, H2N—NH2) versus a skinny polar bond (e.g., in difluoroethane, H2FC—CH2F), one might reasonably expect that the fat lone pairs would repel as far as possible, to the anti conformation, whereas the skinny polar bonds would prefer being adjacent to one another in a tilted syn conformation, namely. 

However, neither expectation is correct both species adopt twisted gauche conformers by signihcant energetic margins. If instead one puts a lone pair at one end and polar bond at the other, as in H2N—CH2F, the preferred anti conformer separates the lone pair and polar bond as, far as possible, contrary to what seems reasonable from VSEPR assumptions. 

For those who continue chemistry studies beyond freshman level, it is not surprising that VSEPR-type concepts play no significant role in more advanced quantum mechanical theories of organic or inorganic molecular structure. Judicious replacement of VSEPR concepts with equivalent Bent s rehybridization concepts (e.g., replacing fat by more s-like, and skinny by more p-like ) could significantly improve the accuracy of current freshman-level pedagogy. 

---