<P> In 1951, it was first suggested that the 2 - norbornyl cation could actually be better described when viewed as a nortricyclonium ion . It has been shown that the major product formed from an elimination reaction of the 2 - norbornyl cation is nortricyclene (not norbornene), but this has been claimed to support both non-classical ion postulates . </P> <P> Herbert C. Brown did not believe that it was necessary to invoke a new type of bonding in stable intermediates to explain the interesting reactivity of the 2 - norbornyl cation . Criticizing many chemists for disregarding past explanations of reactivity, Brown argued that all of the aforementioned information about the 2 - norbornyl cation could be explained using simple steric effects present in the norbornyl system . Given that an alternative explanation using a rapidly equilibrating pair of ions for describing the 2 - norbornyl cation was valid, he saw no need to invoke a stable, non-classical depiction of bonding . Invoking stable non-classical ions was becoming commonplace; Brown felt that this was not only unwarranted but also counterproductive for the field of chemistry as a whole . Indeed, many papers reporting stable non-classical ions were later retracted for being unrealistic or incorrect . After publishing this controversial view in 1962, Brown began a quest to find experimental evidence incompatible with the delocalized picture of bonding in the 2 - norbornyl cation . </P> <P> Brown also worked to prove the instability of a delocalized electronic structure for the 2 - norbornyl cation . If the non-classical ion could be proven to be higher in energy than the corresponding classical ion pair, the non-classical ion would only be seen as a transition state between the two asymmetric cations . Though he did not rule out the possibility of a delocalized transition state Brown continued to reject the proposed reflectional symmetry of the 2 - norbornyl cation, even late in his career . </P> <P> The introduction of the three - centered two - electron delocalized bond invoked in the non-classical picture of the 2 - norbornyl cation allowed chemists to explore a whole new realm of chemical bonds . Chemists were eager to apply the characteristics of hypovalent electronic states to new and old systems alike (though several got too carried away). One of the most fundamentally important concepts that emerged from the intense research focused around non-classical ions was the idea that electrons already involved in sigma bonds could be involved with reactivity . Though filled pi orbitals were known to be electron donors, chemists had doubted that sigma orbitals could function in the same capacity . The non-classical description of the 2 - norbornyl cation can be seen as the donation of an electron pair from a carbon - carbon sigma bond into an empty p - orbital of carbon 2 . Thus this carbocation showed that sigma - bond electron donation is as plausible as pi - bond electron donation . </P>

What type of bond is represented in figure 2-2