Let’s get things started with a discussion of some chemistry. A recent report (V. Lavallo et al. Angew. Chem. Int. Ed. 2008, 47, 5411) professes to have formed an allene within a 5-membered ring, a depiction of which appears below (far left structure). This structure is odd for an allene, which would prefer to be linear. As a result of crystal structure data the resonance structure third from the left (below) was offered. Quite ironically, the same crystal structure data show quite conclusively that the structure at the far left is not an accurate depiction.



This last fact was caught by M. Christl and B. Engels who wrote to Angewandte Chemie to recommend that the structure above not be considered an allene at all, but rather a zwitterion (as in the second structure from the left, above; DOI: 10.1002/anie.200803476). This is really a familiar debate: which drawing best depicts reality? Well, I do think it unfair to call this molecule an allene. So what does it come down to? Well, none of the resonance structures provided by Christl & Engels depicted above accurately depict the molecule in question. What seems most likely to me, is that the second structure from the left, along with resonance structures not shown here, together make up a good description of this interesting molecule.