Some time ago Shaik, Rzepa and Hoffmann (SR&H) conducted a so-called trialogue on the topic of diatomic carbon titled "One Molecule, Two Atoms, Three Views, Four Bonds?" (DOI). In it they expressed their fascination with C2 as a quadruple bonded molecule that goes against the mainstream double bonded model. They also managed to step on some toes with the predictable result that the trialogue has now received complaints.
In a letter to the Angewandte, Gernot Frenking and Markus Hermann (F&H) write they found the trialogue "more confusing than enlightening" and although they do not express a favoured model themselves they certainly do not buy into the quadrupled one (DOI). The SR&H rebuttal is also out (DOI) so let's see what is the disagreement all about.
According to SR&H the C2 molecule not only has 4 bonds, the C-C bond in C2 is also stronger than that of acetylene. Not true say F&H: the force constants can be measured experimentally and that of C2 is smaller. The Badger rule applies. To this SR&H counter that many exceptions exist to this rule and that calculated C2 force constants vary wildly depending on the degree of mixing in of excited states, something F&H again strongly oppose.
Moving on to argument number two, SR&H offered experimental evidence for the existence of the fourth bond: it's bond strength is 16.8 kcal/mole as the difference in bond dissociation energy of removing the first hydrogen atom from acetylene and then the other. What both parties agree to that this is an experimental value but if this article is the source, does it count as such? In the SR&H rebuttal "using the experimental and theoretical data (...) leads to (...) as the experimental bond energy of the 4th bond" which does not sound right. The disagreement starts when F&H argue that BDE's cannot be compared if properties of other bonds vary over the different species. If there is a stabilisation on going from the HCC radical to the CC diradical, it originates from the electronic structure of the carbon atoms themselves. In response SR&H have calculated the carbon energies as function of C-C length and found no differences.
And then there are the insults. SR&H in their trialogue state in so many words that any attempt to measure stability in molecules that cannot be stored in a bottle because of their instability, is "silly". F&H are not amused. How about astrochemists! C2 may be pretty obscure on planet Earth but is abundant in space. The point is made and in the rebuttal SR&H concede.
The Hoffmann remark on QTAIM studies is also revisited: "I have yet to see one that makes a chemical prediction, or suggests an experiment". F&H go out of their way to defend the theory and laud its utility but SR&H appear unimpressed. No apologies here.
And what are we supposed to take from all of this? One conclusion could be that the theorists are incapable on distinguishing experiment from calculation and when it comes to calculations anything can happen.