“Each carbon atom can only bond to a maximum of four other carbon atoms.” This well known and seemingly incontrovertible axiom of organic chemistry has now been definitely shown not to be true. The idea that a single carbon atom can bond to more than four other carbon atoms is not new. In the 1970s Hogeveen and Kwant prepared a dipositive ion (dication) of hexamethylbenzene with the formula [C₆(CH₃)₆]²⁺. Based on NMR spectroscopy (see below) and reactivity studies they proposed that when the planar hexamethylbenzene lost two electrons the resulting dication rearranged itself into a pentagonal-pyramidal structure. This meant that one of the carbon atoms was attached to five other carbon atoms to form the base of the pyramid with the sixth carbon atom at the apex of the pyramid. This month (January 2017) two researchers, Malischewski and Seppelt, from the Free University Berlin, reported in Angewandte Chemie that they had made a solid salt of this compound. This has enabled them to determine the structure by X-ray crystallography. The salt they prepared was [C₆(CH₃)₆]²⁺(SbF₆⁻)₂.HSO₃F. They have conclusively shown that Hogeveen and Kwant’s original proposal was correct and the apex carbon atom is indeed bonded to six other carbons atoms.

The structure of the hexamethylbenzene dication, [C₆(CH₃)₆]²⁺

It is worth noting that the apex carbon atom still only contains four outer electrons which it shares with six not four other carbon atoms. As expected this results in weaker carbon to carbon bonds and the measured bond lengths are longer than the normal C−C single bond length. From sub-topic 11.3 Spectroscopic identification of organic compounds, IB chemistry students will know that the number of signals in the ¹H NMR spectrum of a compound indicates the number of different chemical environments where the protons are found . The ¹H NMR spectrum of the [C₆(CH₃)₆]²⁺ ion shows two peaks in the ratio of 5:1. The 15 protons in the five methyl groups forming the base of the pyramid give a signal with a chemical shift of 2.51 ppm (integration trace: 5) while the 3 protons in the apex methyl group gives a signal with a chemical shift of 1.85 ppm (integration trace: 1).

(I am grateful to Alec Ogilvie, a subscriber to this website and a chemistry teacher at TASIS - The American School in Switzerland who first brought this to my attention.)