Methyl cations complexation with benzene

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... 

When the methyl complex Pd(PPh3)2(I)(Me) is treated with Ag in CeDe at 85 °C, the only product is PPh3Me+. No benzene alkylation takes place. Heating the cationic Pd(PPh3)2(CD3CN)(Me)+ also produces only PPh3Me+. The relative instability of methyl radical probably accounts for the different reaction paths of the benzyl and methyl cationic complexes. When phosphonium salt formation is not possible, for example, in the neopentyl complex... 

Miklis PC., Ditchfield R., Spencer TA. Carbocation-n interaction computational study of complexation of methyl cation with benzene and comparisons with related systems. J Am Chem Soc. 1998 120 10482-89. 

The cationic methylplatinum(ii) complexes with 1,4-diaza-l,3-butadiene ligands cause G-H activation of methane and benzene under mild conditions. The complex having a coordinated water molecule and Bp4 counteranion 122 undergoes exchange of the methyl ligand with isotope-labeled methane at 45 °G in 2,2,2-trifluoroethanol (Scheme 19). " The reaction of benzene with the complex yields the cationic phenylplatinum(ii) complex 123. 

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