To be cyclobutadiene, or not to be, that is the question? You decide.

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)?

Click for 3D. The unconnected atoms are the result of disordered partial occupancy.

The two bonds to concentrate on are shown in gold; a O…C pair with a distance of 1.61Å as obtained from the x-ray refinement and a C…C pair with a distance of 1.5Å (and if you want to go further, the O=C=O bond angle). I list below values obtained from the wonderful Webelements site. Using these values, this makes a van der Waals O…C contact 3.22Å and a C…C contact 3.40Å and covalent values of respectively 1.38Å and 1.5Å.

According to chemistry convention, we classify the interaction between a pair of atoms according to which category best fits the observed distance. So this should allow you to decide if the molecule is a van der Waals complex of 1,3-dimethyl cyclobutadiene and carbon dioxide or the covalent system 2-oxabicyclo[2.2.0]hex-5-en-3-one. 

Oh, if the observed O…C pair with a distance of 1.61Å does not seem to perfectly fit either category above, one of the quartet of articles above[1] offers the explanation of an unusual π-anomeric effect lengthening the C…O bond in 2-oxabicyclo[2.2.0]hex-5-en-3-one slightly beyond the standard covalent distance. Of course, if the system were to be a van der Waals complex, that explanation cannot apply.

References

1. 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. https://doi.org/10.1002/chem.201102942
2. 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. https://doi.org/10.1002/chem.201103017
3. 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. https://doi.org/10.1002/chem.201203234
4. 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. https://doi.org/10.1002/chem.201203235
5. 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. https://doi.org/10.1126/science.1188002