N-Complex characters

While the methyl and silyl cations behave like the proton in only forming a complexes with benzene [279], there is considerable experimental and quantum chemical evidence that alkali metal ions, SF) and NO+ only form n complexes [280-284]. It has been demonstrated by X-ray crystallography that there is a continuous transition in going between these extremes (Fig. 8) [285]. It seems that the more electrophilic the cation is, the closer is the complex to the a form, while the more electron deficient the ring is, the more it seems to tend towards the limiting n complex character. The latter has also been inferred from the appearance of the collisional mass spectra of a variety of aromatic complexes [286]. It is quite surprising that so far we have not discovered any documented examples for which an ion may form both a o complex and a n complex. This... 

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