It’s Hexa-coordinate carbon Spock – but not as we know it!

Science is about making connections. And these can often be made between the most unlikely concepts. Thus in the posts I have made about pentavalent carbon, one can identify a series of conceptual connections. The first, by Matthias Bickelhaupt and co, resulted in the suggestion of a possible frozen SN2 transition state. They used astatine, and this enabled a connection to be made between another good nucleophile/nucleofuge, cyclopentadienyl anion. This too seems to lead to a frozen Sn2 transition state. The cyclopentadienyl theme then asks whether this anion can coordinate a much simpler unit, a C2+ dication (rather than Bickelhaupt’s suggestion of a (NC)3C+ cation/radical) and indeed that complex is also frozen, again with 5-coordinate carbon, and this time with five equal C-C bonds. So here, the perhaps inevitable progression of ideas moves on to examining the properties of this complex, the outcome being a quite counter-intuitive suggestion which moves us into new territory.

The journey starts with the previous observation that the HOMO of the carbyliumylidene cation, shown in the previous post, has prominent electron density along the five-fold symmetry axis of the molecule;

The HOMO orbital

The HOMO orbital. Click for 3D

This suggests that the apical 5-coordinate carbon might actually be basic, and hence coordinate a proton to form a di-cation (below). So adding a proton results in the following stable (in the sense of having all positive force constants) structure, with apical C-C bond lengths of 1.7Å (compared to 1.8Å for the unprotonated system) and the bouncing castle/trampoline mode of 875 cm-1 (DOI: 10.14469/ch/2410) is likewise increased (for the pentamethyl derivative of the structure shown below). The apical C-H stretch has the highest value of all the CHs in the molecule, 3208 cm-1. The calculated proton affinity of the parent compound is 134.2 kcal/mol. To put this into context, we can compare this value with a range of first and second proton affinities reported for carbon bases by Frenking[1]. The highest second proton affinity there reported (ie protonation of an already positive system) is around 106 kcal/mol, which is a good deal less than that found here! So we might conclude that our value must be a candidate for highest second proton affinity ever proposed for a carbon base.

Hexa-coordinate Carbon?

Hexa-coordinate Carbon?

The value of ρ(r) for the AIM bond critical point located for each of the five apical C-C bonds is 0.156 au, again up from the value for the unprotonated species. As before, the Cp ring itself shows no ring critical point. An ELF analysis (below) shows five disynaptic basins in the C-C bond region, with the basin integrating to 0.75 electrons each. Together with the electrons in the apical C-H bond, 6.09 electrons are associated with basins surrounding this carbon atom. Both the AIM and the ELF concur in describing this carbon as not only hexa-coordinated, but hexavalent (although the bonds are not the conventional two-electron type, but perhaps more akin to a six-centre-four-electron interaction).

ELF Basins

ELF Basins. Click for 3D

So I suggest that simple protonation of a highly basic cation has resulted in a six-coordinate carbon atom, which exhibits six strong bonds coordinated around it. I suppose it is inevitable again that one ends this post with the question whether this species too might one day be made.


  1. R. Tonner, G. Heydenrych, and G. Frenking, "First and Second Proton Affinities of Carbon Bases", ChemPhysChem, vol. 9, pp. 1474-1481, 2008.

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6 Responses to “It’s Hexa-coordinate carbon Spock – but not as we know it!”

  1. Steven Bachrach says:

    Fantastic find! I was inspired by your last few blog posts to do some searching for unusual hypercoordinate species, like the methyl analogue of the above (replacing H with Me, though as the monocation) and related things and they all lead to either dissocciated species or more conventionally bonded molecules.

    I would suggest these posts be combined into a paper – and hopefully some progressive publisher will recognize the value of the science and the role the blog has played in pursuing this avenue.

  2. […] developed in this series of posts, and following each leads in rather different directions.  In this previous post the comment was made that the coordinating a carbon dication to the face of a cyclopentadienyl […]

  3. […] This claim was supported by an unusual measured property, the electron density ρ(r) and its Laplacian in the putative O…C region. These two properties are one of those (relatively rare) meetings between experiment and quantum mechanics, and their usefulness has been noted in this blog on previous occasions. However, note that in this designed structure, the O…C distances are merely 2.65-2.7Å, significantly longer than in solid carbon dioxide! So carbon dioxide, in a form many of us are familiar with (solid), can certainly be justified as being described as having a hexacoordinate carbon (although we might draw the line at describing it as having hexavalent carbon). […]

  4. […] carbon revisited. A little while ago, I speculated (blogs are good for that sort of thing) about hexavalent carbon, and noted how one often needs to […]

  5. Henry Rzepa says:

    A crystalline version of the system reported above (with methyl groups replacing the H) has recently been reported, DOI: 10.1002/anie.201608795

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