An interesting discussion has been taking place between Guy Bertrand & group (University of California) in one corner and Christl & Engels (University of Würzburg) in the other on a certain cyclic allene. In this type of compound a central carbon atom is connected to two other carbon atoms through a double bond. This unit is linear and forcing it in a cycle is expected to be tricky. Yet, the Bertrand group in 2008 reported one such cyclic allene with an unusual CCC bond angle of 97.5°, and stable (Lavallo et al. DOI).

In terms of resonance structures Bertrand not only considers allene A but also 4-electron donor B which can be considered a carbodicarbene with a divalent carbon(0) center.

Not so, say Christl & Engels, this new compound is not an allene but a zwitterion (C) (DOI). Their evidence: no experimental evidence on five-membered cyclic allenes exists in the literature, six-membered ring allenes found are also zwitterions (DOI) and the finding is not supported by in silico experimentation. Importantly, in the X-ray structure the OCCCO unit is found in a single plane whereas for an allene an large angle is expected.

In the Bertrand response (Lavallo et al. DOI) the semantic and scientific arguments raised by Christl & Engels are summarized as erroneous: first of all, depicting a molecule as an allene and calling it an allene does not imply that the ground state is an actual allene. Jokingly, structure D is proposed with all electron pairs omitted: readers are invited to place them back using their own judgement. A strong argument in favor of the allenic structure according to Bertrand is pyramidalization of the nitrogen atoms which would disrupt aromatic stabilization of the zwitterion.

Perhaps both teams can agree on structure B. Christl & Engels dismiss it as such because the C4 atom carries a double negative charge but nevertheless see possibilities for it as a ligand to a strongly electron-withdrawing transition metal. More experimental evidence would be welcome. Apparantly an X-ray structure is not enough and any spectroscopic method fails. Polarity, usually a telltale zwitterion signature was not part of any discussion.

Update June 2009: Here

Vincent Lavallo, C.?Adam Dyker, Bruno Donnadieu, Guy Bertrand (2008). Synthesis and Ligand Properties of Stable Five?Membered?Ring Allenes Containing Only Second?Row Elements Angewandte Chemie International Edition, 47 (29), 5411-5414 DOI: 10.1002/anie.200801176