A quartet of articles has recently appeared on the topic of cyclobutadiene.[cite]10.1002/chem.201102942[/cite],[cite]10.1002/chem.201103017[/cite],[cite]10.1002/chem.201203234[/cite],[cite]10.1002/chem.201203235[/cite]. 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[cite]10.1126/science.1188002[/cite] 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)?
The two bonds to concentrate on are shown in gold; a O…C pair with a distance of 1.61Ã… as obtained from the x-ray refinement and a C…C pair with a distance of 1.5Ã… (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 3.22Ã… and a C…C contact 3.40Ã… and covalent values of respectively 1.38Ã… and 1.5Ã….
| Element | Covalent radius, Ã… | van der Waals radius |
| C | 0.75 | 1.70 |
| O | 0.63 | 1.52 |
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.Â
Oh, if the observed O…C pair with a distance of 1.61Ã… does not seem to perfectly fit either category above, one of the quartet of articles above[cite]10.1002/chem.201102942[/cite] 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.