Carbon is well known to form single, double, and triple $\ce{C-C}$ bonds in compounds. There is a recent report (2012) that carbon forms a quadruple bond in diatomic carbon, $\ce{C2}$. The excerpt below is taken from that report. The fourth bond seems pretty odd to me.

$\ce{C2}$ and its isoelectronic molecules $\ce{CN+}$, BN and $\ce{CB-}$ (each having eight valence electrons) are bound by a quadruple bond. The bonding comprises not only one σ- and two π-bonds, but also one weak ‘inverted’ bond, which can be characterized by the interaction of electrons in two outwardly pointing sp hybrid orbitals.

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According to Shaik, the existence of the fourth bond in $\ce{C2}$ suggests that it is not really diradical...
If $\ce{C2}$ were a diradical it would immediately form higher clusters. I think the fact that you can isolate $\ce{C2}$ tells you it has a barrier, small as it may be, to prevent that.

Molecular orbital theory for dicarbon, on the other hand, predicts a C-C double bond in $\ce{C2}$ with 2 pairs of electrons in $\pi$ bonding orbitals and a bond order of two. "The bond dissociation energies (BDE) of $\ce{B2, C2}$, and $\ce{N2}$ show increasing BDE consistent with single, double, and triple bonds." (Ref) So this model of the $\ce{C2}$ molecule seems quite reasonable.

My questions, since this is most definitely not my area of expertise:

  • Is dicarbon found naturally in any quantity and how stable is it? Is it easy to make in the lab? (The Wikipedia article reports it in stellar atmospheres, electric arcs, etc.)
  • Is there good evidence for the presence of a quadruple bond in $\ce{C2}$ that wouldn't be equally well explained by double bonding?
  • You may be interested in this blog post by Rzepa on the $\ce{CN+}$ cation, which putatively contains a $\ce{CN}$ quadruple bond and is isoelectronic with $\ce{C2}$ – Richard Terrett Jun 5 '12 at 2:56
  • @Richard Terrett Thanks for the reference...it's one I hadn't found. So, the quadruple bond is plausible from a calculation stand point (if I'm reading that right). Is there experimental evidence that could/would support one view or the other? As I said, I'm "a bit" out of my field here. – Janice DelMar Jun 5 '12 at 5:39
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    @JaniceDelMar There is no evidence, and there never will be. The C2 molecule looks like any other homodiatomic: two fluffy balls of electron density pushed together. Where are the four ropes in that picture? – Eric Brown May 4 '13 at 6:31
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    It would not necessarily form higher clusters, because maybe 2 C-C -> C-C-C-C is an endothermic reaction. The product, too, is a diradical! It's a non-explanation. – Eric Brown May 4 '13 at 6:37
up vote 37 down vote accepted

Okay, this is not so much of an answer as it is a summary of my own progress on this topic after giving it some thought. I don't think it's a settled debate in the community yet, so I don't feel so much ashamed about it :)

A few of the things worthy of note are:

  • The bond energy found by the authors for this fourth bond is 13.2 kcal/mol, i.e. about 55 kJ/mol. This is very weak for a covalent bond. You can compare it to other values here, or to the energies of the first three bonds in triple-bonded carbon, which are respectively 348, 266 and 225 kJ/mol. This fourth bond is actually even weaker than the strongest of hydrogen bonds ($\ce{F\bond{...}H–F}$, at 160 kJ/mol). Another point of view on this article could thus be: “valence bond necessarily predicts a quadruple bond, and it was now precisely calculated and found to be quite weak”.

  • The findings of this article are consistent with earlier calculations using other quantum chemistry methods (e.g. the DFT calculations in ref. 48 of the Nature Chemistry paper) which have found a bond order between 3 and 4 for molecular dicarbon.

  • However, the existence of this quadruple bonds is somewhat at odds with the cohesive energy of gas-phase dicarbon, which according to Wikipedia is 6.32 eV, i.e. 609 kJ/mol. This latter value is much more in line with typical double bonds, reported at an average of 614 kJ/mol. This is still a bit of a mistery to me…

The real issue is that no one has ever taken a picture (i.e. electron density) of genuine, unambigious, cases of a single, double, triple, quadruple??? bonds. And they never will, because these concepts are not based on quantum mechanics.

Two atoms reside next to each other, and if they have a favorable electrostatic interaction, then a certain type of topology arises in their electron density. (q.v. Quantum Theory of Atoms in Molecules)

You might as well say that every "bond" is a single-bond, or, equivalently, and infinity-bond.

These types of articles are bogus, as they can not be confirmed experimentally. They got lucky with the reviewers, and/or an editor who knows their readership is just dying to hear news of a quadruple bond, having heard for so many years that triple is the highest you can go.

I mean, what are people looking for? Four "ropes" that link between the two carbon atoms? Where is the unambiguous, unbiased dividing line between bond energies of a single/double/triple/quadruple bond?

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