Classical-nonclassical ion controversy

The classical-nonclassical ion controversy also frequently included the question of the so-called ethylenephenonium ions. 

The isotopic perturbation method is very useful for studying fractional bonding and hypercoordination in coordinatively unsaturated and electron deficient compounds such as transition metal complexes or carbocations. The 2-norbornyl cation and the bicyclobutonium cation are the most prominent examples of carbocations whose structures have led to much controversial discussion (the so-called classical-nonclassical ion controversy). The application of the isotopic perturbation methed is likely to be the most decisive piece of nmr evidence for the hypercoordinated structure of these two cations in solution. 

In contrast to the rather well-defined trivalent ( classical ) carbenium ions, nonclassical ions 26 have been more loosely defined. In recent years, a lively controversy centered on the classical-nonclassical ion problem.27-37 The extensive use of dotted lines in writing carbonium ion structures has been (rightly) criticized by Brown, 31 who carried, however, the criticism to question the existence of any o-delocalized (nonclassical) ion. For these ions, if they exist, he stated ... a new bonding concept not yet established in carbon structures is required. ... 

While only an ancillary topic in this discussion, the nonclassical ion controversy, extensively reviewed elsewhere,has a foundation in the historical context of the Wagner-Meerwein rearrangement. After completing a solvolytic study of structurally similar exo- and e cto-2-norbomyl brosylates, Winstein and Trifan suggested that the reaction s cationic intermediate was instead a o-delocalized, symmetrically bridged norbomyl ion 9. This concept deviated from the accepted classical cation structure proposed by Meerwein as the equilibrium between 7 and 8, where the positive charge was considered to be localized on a single atom. 

Consideration of the cyclopentyl cation is leading us to the discussion of one of the most controversial of all carbocations, the norbornyl cation, 163 around which the structure the much publicized classical-nonclassical controversy of carbonium ions mainly centered in recent years9). 

The norbornyl ion controversy """ centered on the question of whether the ion had the static bridged nonclassical structure (126) containing pentacoordinate hypercarbons or whether its structure was better depicted as a rapidly equilibrating pair of trivalent classical ions (127a and 127b). 

Perhaps the "classic" example of a nonclassical carbocation is the 2-norbornyl cation, which was at the center of what has been called "the most heated chemical controversy in our time." In Chapter 8 we will review the experimental evidence, largely based on solvolysis reactions, that led to the proposal of the nonclassical carbonium ion structure shown in Figure 5.48. However, this description was not accepted by all researchers, and an alternative model for the 2-norbomyl cation was a pair of rapidly equilibrating classical (carbenium) ions, as shown in Figure 5.49. Many papers relating to the development of contrasting ideas in this area were published in a reprint and commentary volume by Bartlett. ... 

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