Pentachlorocyclopentadienyl cation

It was Breslow (Breslow, 1982 Breslow et ai., 1982) who first paid attention to this theory. Knowing that the pentachlorocyclopentadienyl cation (Breslow et ai, 1964 Saunders et ai, 1973), the hexachlorobenzene dication (Wasserman et al., 1974) and the 2,3,6,7,10,11-hexamethoxy-triphenylene (HMT, [36]) dication are all ground-state triplets, in good agreement with theory, Breslow and coworkers set out on the synthesis of analogues that should have lower oxidation potentials, be chemically more stable and therefore form CT complexes more readily (Fig. 21c Breslow et al., 1982, 1984 Breslow, 1985, 1989 LePage and Breslow, 1987). 

In contrast, the less strained four-7r-electron cyclopentadienyl cation is very unstable. Its p r+ has been estimated as --40, using an electrochemical cycle. The heterolytic bond dissociation energy to form the cation from cyclopentadiene is 258 kcal/mol, which is substantially more than for formation of an allylic cation from cyclopentene but only slightly more than the 252 kcal/mol for formation of an unstabilized secondary carbocation. " Solvolysis of cyclopentadienyl halides assisted by silver ion is extremely slow, even though the halide is doubly allylic. When the bromide and antimony pentafluoride react at -78°C, the EPR spectrum observed indicates that the cyclopentadienyl cation is a triplet. Similar studies indicate that pentachlorocyclopentadienyl cation is also a triplet, but the ground state of the pentaphenyl derivative is a singlet. 

Concerning the structure of the pentachlorocyclopentadienyl-substituted thallium(I) compound LHIm it has been suggested that in the solid state the thallium cation nestles in the C5CI5 anion and that some charge is transferred from the anion to the empty thallium orbital (177), a situation typical for an ion pair with a low degree of covalent interaction. 

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