What is the correct IUPAC name (or least incorrect one, if the current rules can't precisely describe that) of the interesting species $\ce{[C6(CH3)6]^{2+}}$ (which is extensively discussed here), containing exotic hexacoordinate pyramidal carbocation* ?

The structure can be drawn as

structure formula


structure formula 2

My guess is methyl(pentamethyl-η5-cyclopentadienyl)carbon(2+).

*) Some structures in the Wikipedia article have the two positive charges placed into the cyclopentadienyl (aromatic) ring, which I think is incorrect.

  • $\begingroup$ Rather preferred (PIN) or one of various others (general etc.) then (in)correct. $\endgroup$
    – Mithoron
    Jul 29, 2018 at 17:59
  • $\begingroup$ This article on Wikipedia is based on stuff that was published 40 or more years ago :( There has to be 2+ formal charge on apical carbon, but not real charge. $\endgroup$
    – Mithoron
    Jul 29, 2018 at 18:13
  • $\begingroup$ @Mithoron .. and the "least incorrect" variant request was for the case if the current rules are incapable of that.. my question rewording edit covers that as well, sort of. I think organic nomenclature cannot handle this, and inorganic nomenclature does not have PINs. $\endgroup$
    – mykhal
    Jul 29, 2018 at 20:24

1 Answer 1


From the IUPAC Gold Book:

Carbocations may be named by adding the word 'cation' to the name of the corresponding radical. Such names do not imply structure (e.g. whether three-coordinated or five-coordinated carbon atoms are present).

So since this is the dication of hexamethylbenzene, it would probably least incorrectly be called hexamethylbenzene dication. At the very least, this seems to be how a lot of people refer to it (or more verbosely, pentagonal-pyramidal hexamethylbenzene dication).


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