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We have prepared the dipositive ion of hexachlorobenzene (I) by a two-step oxidation of the parent HCB.This ion is the first example of a benzene system having four -electrons (A).1 As predicted by simple Hiickel theory, I is a ground-state triplet. The two unpaired electrons occupy degenerate orbitals and the molecule retains the sixfold symmetry of the parent. In contrast to I a Jahn-Teller distortion to lower symmetry can occur in the monopositive2 and mononegative3 ions as well as the excited states4 5of benzene. The -electron configuration of I parallels that of the cyclopentadienyl cations6 (II) which also have triplet ground states (B).

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The radical cations 4 and 6 of 1,2:9,10-bismethano[2.2]paracyclophane (3) and of 10,l l-methano[3.2]paracyclophane (5) have been generated by electron transfer to photoexcited (triplet) chloranil. 'H and "C CIDNP effects observed during these reactions allow an insight into the structure of these species. The polarization patterns indicate that charge and spin are delocalized into the benzylic cyclopropane positions, Le., that the tertiary-tertiary cyclopropane bonds are broken or weakened. The different enhancement of the secondary cyclopropane protons is ascribed to different dihedral angles with the singly occupied orbital at the benzylic carbon. The findings that only the benzylic cyclopropane carbons show I3C polarization and that the 'H polarization in this position is relatively weak are interpreted as evidence that these carbon atoms are pyramidal in the radical ion.