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

In the previous two posts, I noted the recent suggestion of how a stable frozen SN2 transition state might be made. This is characterised by a central carbon with five coordinated ligands. The original suggestion included two astatine atoms as ligands (X=At), but in my post I suggested an alternative which would have five carbon ligands instead (X=cyclopentadienyl anion).

The Sn2 transition state

The Sn2 transition state

However, these five ligands are not all equal; far from it. Three form normal strength bonds to the central carbon, and two very weak (deci)bonds. So, could a molecule be made with five equal bonds all coordinated to a central carbon atom? Well, the inspiration for designing such a molecule comes with the report of a remarkable compound of silicon by Jutzi and co-workers[1]. Examples with Ge, Sn and  Pb are also known.

The silyliumylidene cation

The silyliumylidene cation

Using a large non-coordinating anionic counterion, a crystal structure could be determined for the pentamethyl derivative (Refcode: BIDLEG), which reveals the five-fold symmetry of the silicon coordination. The obvious mutation therefore is to see if the corresponding carbon compound might be stable.  A B3LYP/6-311G(d,p) calculation (DOI: 10.14469/ch/2408) run with  C5 symmetry reveals this system to have only positive force constants, with five equal C-C bonds to the central carbon, each with the unusual length of 1.799Å. The bouncing castle vibrational mode involving the pentacoordinate carbon has a value of  767 cm-1

The carbyliumylidene Cation

The carbyliumylidene Cation

So, not only do we now have a clearly penta-coordinate carbon, all five bonds are of equal length! More unusual still, all five ligands occupy one hemisphere of the carbon coordination. Why might such a geometry be stable? Well, as with the silicon analogue, C2+ has only two valence electrons left. To elevate this to the standard octet, it must accept six electrons, and the cyclopentadienyl anion fulfils this role perfectly. The top three occupied molecular orbitals are shown below.

The HOMO orbital

The HOMO orbital. Click for 3D

The HOMO-1 (degenerate) orbital

The HOMO-1 (degenerate) orbital. Click for 3D

An AIM analysis (below) shows five equal bond critical points, with ρ(r) 0.13 au for each (see previous post for comparison), a value which probably can be described by the term bond. The ∇2ρ value of +0.07 au is similar to that quoted in the previous post. Noteworthy is the observation that no ring critical point (RCP, yellow dots) can be found for the cyclopentadienyl ring itself, only for the five three-membered rings to the pentacoordinate atom.

AIM (Atoms-in-Molecules) analysis

AIM (Atoms-in-Molecules) analysis

Can the species be made? Well, given that it seems the case that carbon and silicon chemistries are inverted, ie what is stable with silicon is unstable with carbon, and vice versa, the answer is probably no. But one never knows until one has tried!


  1. P. Jutzi, A. Mix, B. Rummel, W.W. Schoeller, B. Neumann, and H. Stammler, "The (Me 5 C 5 )Si + Cation: A Stable Derivative of HSi +", Science, vol. 305, pp. 849-851, 2004.

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3 Responses to “It’s penta-coordinate carbon Spock- but not as we know it!”

  1. Steven Bachrach says:

    These last three posts have been very provocative, and I’ve been silent in response mainly because I have been playing with a variety of ideas inspired buy these comments. Unfortunately, I’ve been unsuccessful in all my attempts to find additional representatives of the hypercoordinate carbon! So Bravo to Henry for locating these interesting structures!

    With regards to publishing these ideas in a more traditional venue – well, obviously you’ve placed all of the ACS journals out of the options available. But there are other journals with more liberal and forwardthinking philosophies. Perhaps the Beilstein Journal of Chemistry or Chemistry Central Journal– these should allow you to also incorporate Jmol representation within the article…

  2. Mark P. says:

    How close does this get to the type of bonding we see in carboranes? This almost looks to me like an all-carbon model of what boron does all the time in higher boranes. If that is the case carbyliumylidene (what a mouthful) could conceivably be stable.

  3. Henry Rzepa says:

    Ut would be worth identifying an analogue containing boron and comparing the two. Suggestions welcome. PS: I have identified CH12B11 as a reasonably close analogue. I will report a bonding analysis in a separate post.

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