Hyperconjugative delocalizations

Cations are by no means the only species where the effects of hyperconjugative delocalization reveal themselves in such a striking manner. Similar effects exist in neutral systems or in anions. For instance, the normal propyl anion should tend to be eclipsed (E) since in this manner the molecule would optimize the 4-electron interactions between the ethyl group t orbital and the p orbital which carries the electron pair. In the bisected conformation, where ttchs and ttchs have both been raised in energy, the four-electron, destabilizing (see Section 1.7, rule 2) p ->7r interaction is stronger than in the eclipsed conformation. At the same time the two-electron, stabilizing p ->ir interaction is weaker than in the eclipsed conformation. Both effects favor the eclipsed conformation. 

Although certain formal parallels between transition-metal hydrides and the analogous hydrocarbon species (see, e.g., the discussion surrounding Fig. 4.23) have been noted, it is also important to recognize the profound differences between corresponding M Hm and C H, compounds. These differences are particularly apparent with reference to hyperconjugative delocalization effects, which have entirely different strengths and patterns (as well as increased basis sensitivity) in transition-metal compared with hydrocarbon species. A small selection of these effects will now be examined. 

The 29Si-NMR chemical shift of 12 (8 = 66.34 ppm) is considerably deshielded as compared to the progenitor alkene 13 (8 = 0.14 ppm), which is in accord with substantial P-Si-C-bond hyperconjugative delocalization of positive charge to silicon. 

Let us now consider the expected hyperconjugative delocalizations in the unique P structure 11b. This electronic pattern immediately suggests that a planar structure may be unfavorable, becanse the formal hole then has no possible donor-acceptor interactions with adjacent donors because of strict G-n separation in this geometry. Twisting about the C-C bond (to break a-n symmetry) is expected to turn on vicinal... 

Hyperconjugation Delocalization of a bonding electron pair, into an empty or a partly empty atomic orbital, stabilizing the molecule. 

Both carbocations contain an sp hybridized carbon, so both are trigonal planar with a vacant p orbital extending above and below the plane. There are no adjacent C-H a bonds with which the p orbital can overlap in CHj, but there are adjacent C-H a bonds in (CH3>2CH. This overlap (the hyperconjugation) delocalizes the positive charge on the carbocation, spreading it over a larger volume, and this stabilizes the carbocation. 

Other possible Lewis structures may be similarly tested. For example, the structures (5.24), (5.25) associated with the strong hyperconjugative delocalizations of the NBO (Fig. 5.4) could be specified with LONE/BOND lists given previously as... 

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