A quartet of articles has recently appeared on the topic of cyclobutadiene.[1],[2],[3],[4]. You will find a great deal discussed there, but I can boil it down to this essence. Do the following coordinates (obtained from a (disordered) previously published[5] x-ray refinement) correspond to a van der Waals complex of 1,3-dimethyl cyclobutadiene and carbon dioxide, or do they instead represent a covalent interaction between these two components resulting in a compound with the chemical name 2-oxabicyclo[2.2.0]hex-5-en-3-one (i.e. not a cyclobutadiene)?
References
- H.S. Rzepa, "A Computational Evaluation of the Evidence for the Synthesis of 1,3‐Dimethylcyclobutadiene in the Solid State and Aqueous Solution", Chemistry – A European Journal, vol. 19, pp. 4932-4937, 2013. http://dx.doi.org/10.1002/chem.201102942
- M. Shatruk, and I.V. Alabugin, "Reinvestigation of “Single‐Crystal X‐ray Structure of 1,3‐dimethylcyclobutadiene”", Chemistry – A European Journal, vol. 19, pp. 4942-4945, 2013. http://dx.doi.org/10.1002/chem.201103017
- Y. Legrand, D. Dumitrescu, A. Gilles, E. Petit, A. van der Lee, and M. Barboiu, "A Constrained Disorder Refinement: “Reinvestigation of “Single‐Crystal X‐ray Structure of 1,3‐Dimethylcyclobutadiene” by M. Shatruk and I. V. Alabugin”", Chemistry – A European Journal, vol. 19, pp. 4946-4950, 2013. http://dx.doi.org/10.1002/chem.201203234
- Y. Legrand, D. Dumitrescu, A. Gilles, E. Petit, A. van der Lee, and M. Barboiu, "Reply to A Computational Evaluation of the Evidence for the Synthesis of 1,3‐Dimethylcyclobutadiene in Solid State and Aqueous Solution—Beyond the Experimental Reality", Chemistry – A European Journal, vol. 19, pp. 4938-4941, 2013. http://dx.doi.org/10.1002/chem.201203235
- Y. Legrand, A. van der Lee, and M. Barboiu, "Single-Crystal X-ray Structure of 1,3-Dimethylcyclobutadiene by Confinement in a Crystalline Matrix", Science, vol. 329, pp. 299-302, 2010. http://dx.doi.org/10.1126/science.1188002