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CHAPTER 13
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Figure 13.2. Examples of nitrenium ions.
of benzidine diimine. Yet Dicks et al.2 found that it reacts in the same way as 3–5, albeit with highly attenuated rate constants. The triazolium ions (e.g., 7) have been known for decades to be stable isolable salts.3 Yet Boche and co-workers4,5 argued that these species are nitrenium ions by virtue of the resonance form that assigns a positive charge to the central nitrogen. In fact, Boche’s study of these species was undoubtedly inspired by Wanzlik and Schikora6 and the work of Arduengo et al.7 on stable carbenes possessing analogous structures. However, many would argue that these ions are not nitrenium ions. They are readily isolable and are no more reactive toward nucleophiles than are pyridinium ions or any other quaternary nitrogenous heterocycle. Furthermore, they are ground-state singlets with very high energy triplet states.
This raises the question: When is a nitrenium ion not a nitrenium ion? Or to state it another way: When is the degree of charge delocalization so great that the species in question is no longer suitably considered a nitrenium ion. In principle, one could propose either a structureor reactivity-based definition of nitrenium ion. As seen in the discussion above any definition based on qualitative valence bond structures faces the problem of judging which canonical structure is ‘‘better.’’ One might do better with ab initio quantum chemistry calculations. However, the parameters that can recovered from such calculations (bond angles, charge distributions, orbital energies, etc.) are likely to vary in a more or less continuous fashion throughout the series. Thus a quantitative structural definition would have to rely on an arbitrary, and ultimately subjective definition of charge distribution or other structural features.
N 
C 
N