What's the longest C=C bond?

Question

There's been ample work and media coverage on how to make a long C-C single bond. To my knowledge, the longest stable C-C bond is currently 1.704 Å, prepared by Schreiner Nature 477, 308–311 in 2011.

All around us, there's considerable plastics, made from polymerizing alkenes like polyethylene. Typical bond lengths for alkenes are ~1.33 Å, with bond dissociation energies around 611 kJ/mol, vs ~347 kJ/mol for C-C.[ref] Clearly, longer C=C bonds could decrease the energy required to polymerize the alkane.

Using either chemical databases, like the Crystallographic Open Database, or computational methods, what's the longest C=C bond you can find?

Please include in your answer:

The molecular structure, with the C=C bond either obvious or highlighted (e.g., if it's a conjugated bond)
The method used to determine the bond length (e.g., DFT geometry optimization using program X, found in database Y, etc.)
Proof that the bond is really a double bond (e.g., clear evidence of substituents, planarity, etc.)
Any appropriate literature citations

Feel free to use Henry's blog as a starting point. (Consider that I might ping him to add an answer too. :-) — Geoff Hutchison, Commented Jan 9, 2018 at 18:00
tbh; this is a lot more challenging than I initially thought. C=C does not like to be stretched. — Martin - マーチン, Commented Jan 15, 2018 at 13:06
I also must admit that Henry Rzepa did a great job fetching those structures from CCDC. I followed his approach, and oh boy, I haven't seen so many garbage crystal structures with C=C bonds from 1.7 up to 2 Å in years. — andselisk, Commented Jan 17, 2018 at 18:26
Not directly related to the topic, but since you mentioned the longest C-C single bond, the record has now been drastically increased to 1.806(2) Å - "Longest C–C Single Bond among Neutral Hydrocarbons with a Bond Length beyond 1.8 Å". — Nicolau Saker Neto, Commented Mar 10, 2018 at 23:23

EJC · Accepted Answer · 2018-01-19 11:39:08Z

16

+200

I thought about investigating push-pull alkenes. The explosive 1,1–diamino—2,2—dinitroethylene also known as FOX-7 has a double bond with the length 145.6 pm determined by X-ray crystallography:

The evidence is in this publication and CIF.

Calculations on the B97D3/def2-TZVPP level of theory (Gaussian 09 Rev. E.01) reveal the bond length in the gas phase to be 142.4 pm, which is in quite good agreement of the crystal structure.

E(RB97D3/def2-TZVPP)       =       -598.257208193 
C          0.04916        0.00000       -0.00001
C         -1.37453        0.00031       -0.00000
N          0.80303       -1.22125        0.11237
O          0.24490       -2.28958       -0.23784
O          1.92916       -1.18705        0.58174
N         -2.07141       -1.13253       -0.20314
H         -3.03559       -1.17626        0.08434
H         -1.51303       -1.97791       -0.31069
N         -2.07092        1.13344        0.20315
H         -3.03510        1.17757       -0.08431
H         -1.51218        1.97858        0.31060
N          0.80359        1.22090       -0.11241
O          0.24599        2.28948        0.23787
O          1.92972        1.18619       -0.58172

edited Jan 19, 2018 at 11:39

answered Jan 17, 2018 at 18:23

EJC

14.4k15 gold badges68 silver badges157 bronze badges

$\begingroup$ @Martin-マーチン strictly speaking, this crystal structure has been measured at -100 °C. Even though it shouldn't affect the bond distances much, but this is one thing to keep in mind when replicating the geometry with computational methods. Also, there is one weak spot in this crystal structure: G00F is way off (1.655 vs 1, ideally). $\endgroup$
– andselisk ♦
Commented Jan 17, 2018 at 21:29
1

$\begingroup$ I think this is the way more impressive find than the desperate attempt I came up with... you'll get my bounty tomorrow ;) $\endgroup$
– Martin - マーチン ♦
Commented Jan 29, 2018 at 16:38
$\begingroup$ @Martin-マーチン Too bad that no one else participated in this. $\endgroup$
– EJC
Commented Jan 29, 2018 at 21:39

Add a comment |

Martin - マーチン · Accepted Answer · 2018-01-19 05:09:00Z

I hope that by providing one data-point I might get the ball rolling. Most of my attempts have been unfruitful, and I suspect that stretching a carbon-carbon double bond is difficult in general.

That is due to the fact that if introducing bulky substituents in most of the cases I looked at the planarity is lost to avoid stress, i.e. the methylene groups rotate and form an approximately 90 degree angle. Restricting this movement is more challenging than I previously thought.

In other cases the double bond is still typically around 134 pm, as the sum of their covalent radii would suggest.^[1]

Jumping off of that the only somewhat stretched bond I could find is within the following molecule, which I am not even attempting to name. The key features here are the alkynyl links to get the bulky diamantyl moieties far enough away from the double bond to simultaneously allow them to have positive dispersion interactions. Even with these rather extreme measures, I was only able to stretch the bond to 139.7 pm at the B97D3/def2-TZVPP level of theory, calculated with Gaussian 09 Rev. E.01. (The image is from the previous B97D3/def2-SVP optimisation and hence shows 140.8 pm bond distance. The overall change is too small to justify re-uploading a new image.)

I have yet to increase the level of theory and obtain a frequency calculation, but since this is quite the sizeable molecule it'll take some more time.

Pekka Pyykkö and Michiko Atsumi published covalent (single-, double-, triple-) bond radii for (almost) the entire periodic table in Chem. Eur. J. 2009, 15, 12770-12779. It is summarised in figure 3. These are obtained from experimental or theoretical data. The figure itself can be found on Pyykkö's homepage as a pdf-file.

xyz coordinates

142
symmetry c2h
C        0.000000000      0.000000000      0.703991000
C        0.000000000      0.000000000     -0.703991000
C       -0.000036000      1.228392000     -1.422011000
C       -0.000036000      1.228392000      1.422011000
C        0.000036000     -1.228392000      1.422011000
C        0.000036000     -1.228392000     -1.422011000
C       -0.000039000      2.307702000     -2.004101000
C       -0.000039000      2.307702000      2.004101000
C        0.000039000     -2.307702000      2.004101000
C        0.000039000     -2.307702000     -2.004101000
C        0.000007000      3.632781000      2.620363000
C       -0.000321000      3.543471000      4.170911000
H       -0.890097000      2.977508000      4.502792000
H        0.889220000      2.977332000      4.503111000
C       -1.260688000      4.427875000      2.173610000
H       -2.166884000      3.872289000      2.476464000
H       -1.273063000      4.492650000      1.070965000
C        1.261096000      4.427671000      2.174190000
H        2.167022000      3.871915000      2.477502000
H        1.273950000      4.492403000      1.071573000
C        0.000007000      3.632781000     -2.620363000
C        1.261096000      4.427671000     -2.174190000
H        1.273950000      4.492403000     -1.071573000
H        2.167022000      3.871915000     -2.477502000
C       -1.260688000      4.427875000     -2.173610000
H       -2.166884000      3.872289000     -2.476464000
H       -1.273063000      4.492650000     -1.070965000
C       -0.000321000      3.543471000     -4.170911000
H        0.889220000      2.977332000     -4.503111000
H       -0.890097000      2.977508000     -4.502792000
C       -0.000007000     -3.632781000     -2.620363000
C       -1.261096000     -4.427671000     -2.174190000
H       -1.273950000     -4.492403000     -1.071573000
H       -2.167022000     -3.871915000     -2.477502000
C        1.260688000     -4.427875000     -2.173610000
H        2.166884000     -3.872289000     -2.476464000
H        1.273063000     -4.492650000     -1.070965000
C        0.000321000     -3.543471000     -4.170911000
H       -0.889220000     -2.977332000     -4.503111000
H        0.890097000     -2.977508000     -4.502792000
C       -0.000007000     -3.632781000      2.620363000
C        0.000321000     -3.543471000      4.170911000
H        0.890097000     -2.977508000      4.502792000
H       -0.889220000     -2.977332000      4.503111000
C        1.260688000     -4.427875000      2.173610000
H        2.166884000     -3.872289000      2.476464000
H        1.273063000     -4.492650000      1.070965000
C       -1.261096000     -4.427671000      2.174190000
H       -2.167022000     -3.871915000      2.477502000
H       -1.273950000     -4.492403000      1.071573000
C       -1.256835000      5.742791000     -4.340720000
C       -1.254127000      5.830583000     -2.794873000
C        0.000330000      6.620635000     -2.348365000
C        1.254467000      5.830385000     -2.795414000
C        1.256518000      5.742596000     -4.341263000
C       -0.000316000      4.952467000     -4.782788000
H       -2.160732000      5.192099000     -4.666973000
C       -1.255695000      7.150419000     -4.960009000
H       -2.159549000      6.376170000     -2.463344000
H        0.000576000      6.699088000     -1.243023000
C        0.000320000      8.027542000     -2.968740000
H        2.160119000      6.375816000     -2.464258000
H        2.160191000      5.191761000     -4.667890000
C        1.255319000      7.150218000     -4.960570000
H       -0.000550000      4.866220000     -5.887095000
H       -1.277972000      7.076821000     -6.064351000
H       -2.168257000      7.697354000     -4.653970000
H        0.890296000      8.590343000     -2.626973000
H       -0.889406000      8.590507000     -2.626592000
H        1.277086000      7.076611000     -6.064921000
C       -0.000024000      7.918685000     -4.505704000
H        2.168108000      7.697008000     -4.654946000
H       -0.000037000      8.931342000     -4.951455000
C        1.256518000      5.742596000      4.341263000
C        1.254467000      5.830385000      2.795414000
C        0.000330000      6.620635000      2.348365000
C       -1.254127000      5.830583000      2.794873000
C       -1.256835000      5.742791000      4.340720000
C       -0.000316000      4.952467000      4.782788000
H        2.160191000      5.191761000      4.667890000
C        1.255319000      7.150218000      4.960570000
H        2.160119000      6.375816000      2.464258000
H        0.000576000      6.699088000      1.243023000
C        0.000320000      8.027542000      2.968740000
H       -2.159549000      6.376170000      2.463344000
H       -2.160732000      5.192099000      4.666973000
C       -1.255695000      7.150419000      4.960009000
H       -0.000550000      4.866220000      5.887095000
H        1.277086000      7.076611000      6.064921000
H        2.168108000      7.697008000      4.654946000
H       -0.889406000      8.590507000      2.626592000
H        0.890296000      8.590343000      2.626973000
H       -1.277972000      7.076821000      6.064351000
C       -0.000024000      7.918685000      4.505704000
H       -2.168257000      7.697354000      4.653970000
H       -0.000037000      8.931342000      4.951455000
C       -0.000330000     -6.620635000      2.348365000
C        1.254127000     -5.830583000      2.794873000
C        1.256835000     -5.742791000      4.340720000
C        0.000316000     -4.952467000      4.782788000
C       -1.256518000     -5.742596000      4.341263000
C       -1.254467000     -5.830385000      2.795414000
H       -0.000576000     -6.699088000      1.243023000
C       -0.000320000     -8.027542000      2.968740000
H        2.159549000     -6.376170000      2.463344000
H        2.160732000     -5.192099000      4.666973000
C        1.255695000     -7.150419000      4.960009000
H        0.000550000     -4.866220000      5.887095000
H       -2.160191000     -5.191761000      4.667890000
C       -1.255319000     -7.150218000      4.960570000
H       -2.160119000     -6.375816000      2.464258000
H       -0.890296000     -8.590343000      2.626973000
H        0.889406000     -8.590507000      2.626592000
H        1.277972000     -7.076821000      6.064351000
H        2.168257000     -7.697354000      4.653970000
H       -2.168108000     -7.697008000      4.654946000
C        0.000024000     -7.918685000      4.505704000
H       -1.277086000     -7.076611000      6.064921000
H        0.000037000     -8.931342000      4.951455000
C        1.256835000     -5.742791000     -4.340720000
C        1.254127000     -5.830583000     -2.794873000
C       -0.000330000     -6.620635000     -2.348365000
C       -1.254467000     -5.830385000     -2.795414000
C       -1.256518000     -5.742596000     -4.341263000
C        0.000316000     -4.952467000     -4.782788000
H        2.160732000     -5.192099000     -4.666973000
C        1.255695000     -7.150419000     -4.960009000
H        2.159549000     -6.376170000     -2.463344000
H       -0.000576000     -6.699088000     -1.243023000
C       -0.000320000     -8.027542000     -2.968740000
H       -2.160119000     -6.375816000     -2.464258000
H       -2.160191000     -5.191761000     -4.667890000
C       -1.255319000     -7.150218000     -4.960570000
H        0.000550000     -4.866220000     -5.887095000
H        1.277972000     -7.076821000     -6.064351000
H        2.168257000     -7.697354000     -4.653970000
H       -0.890296000     -8.590343000     -2.626973000
H        0.889406000     -8.590507000     -2.626592000
H       -1.277086000     -7.076611000     -6.064921000
C        0.000024000     -7.918685000     -4.505704000
H       -2.168108000     -7.697008000     -4.654946000
H        0.000037000     -8.931342000     -4.951455000

Great insightful answer! I'm wondering is there a good reason that the bond is shown with a single stick? — andselisk, Commented Jan 17, 2018 at 18:17
"Rotated" double bonds (i.e., so-called "tictoid" $\pi$ conjugation) is a known, accepted way to increase bond length in conjugated systems. ;-) — Geoff Hutchison, Commented Jan 17, 2018 at 18:52
@andselisk Yes, there is: I prefer it that way. In all my pictures you'll never see me use a double or even triple rod. — Martin - マーチン, Commented Jan 17, 2018 at 19:04
@Martin-マーチン I can tell:) But why is it so, if I may ask? Is it because the bond order is often fractional and the integer values denoted via multiple-stick bonds bother you? — andselisk, Commented Jan 17, 2018 at 19:11

pH13 - Yet another Philipp · Accepted Answer · 2018-02-08 09:55:51Z

While the two other (so far) presented molecules look realistic (Martin) or are real (Marko), I couldn't find the constraint, that the molecules actually need to be realistic, so here we go.

This is Gaussian 16 Rev. A.03 using (again, why do we use this niveau?) B97D3/def2TZVPP with Opt=(VeryTight) and SCF=(VeryTight) on to-be-named-by-Loong I (TBNBL I) with some nice double (or so) bond length of $\pu{143.5 pm}$. (At least formally, it needs to be a double bond.)

E(RB97D3) =  -692.564719674
C                  0.00000000    0.71742600    0.00000100
C                  0.00000000   -0.71742800    0.00000100
C                 -1.26096700    1.35299700    0.00000100
C                  1.26096700    1.35299700    0.00000100
C                 -1.26096700   -1.35299900    0.00000100
C                  1.26096700   -1.35299900    0.00000100
C                 -2.47714700    1.44346000    0.00000000
C                 -2.47714700   -1.44345900    0.00000100
C                  2.47714700   -1.44345900    0.00000000
C                  2.47714600    1.44345900    0.00000100
C                 -3.88898600    1.10111200   -0.00000100
C                  3.88898600    1.10111300    0.00000000
C                  4.18780000    0.00000100   -1.08412100
C                  3.88898700   -1.10111100   -0.00000100
C                  4.18780200    0.00000000    1.08411900
C                 -3.88898700   -1.10111100    0.00000000
C                 -4.18780000    0.00000000   -1.08412100
C                 -4.18780200    0.00000100    1.08411900
H                 -4.55239300   -1.97024400   -0.00000100
H                 -3.55876000    0.00000100    1.97407800
H                 -5.24617200    0.00000100    1.36283000
H                 -5.24617000    0.00000000   -1.36283300
H                 -3.55875700    0.00000000   -1.97407800
H                 -4.55239300    1.97024500   -0.00000100
H                  4.55239300   -1.97024300   -0.00000100
H                  5.24617000    0.00000100   -1.36283300
H                  3.55875700    0.00000100   -1.97407800
H                  5.24617200    0.00000000    1.36283000
H                  3.55876000    0.00000000    1.97407800
H                  4.55239300    1.97024500   -0.00000100

It has no negative frequencies and thus is a local minimum on its potential energy hypersurface. This holds true for every following molecule.

BDE according to [1] is 553 kJ/mol.

This led me to change the cyclobutyl rings into double bonds (TBNBL II), but that was not productive, as the bond length is reduced to $\pu{140.4 pm}$. BDE is $\pu{606 kJ/mol}$.

E(RB97D3) =  -535.325912063
C                  0.00000000    0.70224700    0.00000000
C                  0.00000000   -0.70224700    0.00000000
C                  1.29152400    1.32362500    0.00000000
C                 -1.29152400    1.32362500    0.00000000
C                  1.29152400   -1.32362500    0.00000000
C                 -1.29152400   -1.32362500    0.00000000
C                  2.51038500    1.30761800    0.00000000
C                  2.51038500   -1.30761700    0.00000000
C                 -2.51038500   -1.30761700    0.00000000
C                 -2.51038500    1.30761700    0.00000000
C                  3.80696800    0.68032900    0.00000000
C                 -3.80696800    0.68032900    0.00000000
C                 -3.80696800   -0.68032900    0.00000000
C                  3.80696800   -0.68032900    0.00000000
H                  4.73157700   -1.24777100    0.00000000
H                  4.73157700    1.24777100    0.00000000
H                 -4.73157700   -1.24777100    0.00000000
H                 -4.73157700    1.24777100    0.00000000

Ok, if getting smaller does not help, maybe getting bigger again? What about changing the cyclobutyl* into cyclohexadiene*? We get larger again, but not larger as my first try. TBNBL III: $\pu{141.5 pm}$ with BDE of $\pu{587 kJ/mol}$.

E(RB97D3) =  -844.915342833
C          0.00000        0.70775        0.00000
C         -0.00000       -0.70775        0.00000
C          1.25608        1.36005        0.00000
C         -1.25608        1.36005       -0.00000
C          1.25608       -1.36005       -0.00000
C         -1.25608       -1.36005        0.00000
C          2.45242        1.57986        0.00000
C          2.45242       -1.57986       -0.00000
C         -2.45242       -1.57986        0.00000
C         -2.45242        1.57986       -0.00000
C         -3.91584        1.44760       -0.00000
C         -4.32828        0.66483        1.24739
C         -4.32828        0.66483       -1.24739
C         -4.32828       -0.66483        1.24739
H         -4.57791        1.23705        2.13578
C         -4.32828       -0.66483       -1.24739
H         -4.57791        1.23705       -2.13578
C         -3.91584       -1.44760       -0.00000
H         -4.57791       -1.23705        2.13578
H         -4.57791       -1.23705       -2.13578
H         -4.39998       -2.43128       -0.00000
C          3.91584        1.44760        0.00000
C          4.32828        0.66483       -1.24739
C          4.32828        0.66483        1.24739
C          4.32828       -0.66483       -1.24739
H          4.57791        1.23705       -2.13578
C          4.32828       -0.66483        1.24739
H          4.57791        1.23705        2.13578
C          3.91584       -1.44760       -0.00000
H          4.57791       -1.23705       -2.13578
H          4.57791       -1.23705        2.13578
H          4.39998       -2.43128        0.00000
H          4.39998        2.43128       -0.00000
H         -4.39998        2.43128        0.00000

There is also the cyclohexyl analogue (TBNBL IV) which has a little longer bond with $\pu{142.4 pm}$ and a BDE of $\pu{571 kJ/mol}$.

E(RB97D3) =  -849.855064121
C          0.00000       -0.71210        0.00000
C         -0.00000        0.71210        0.00000
C         -1.25688       -1.35508       -0.01308
C          1.25688       -1.35508        0.01309
C         -1.25688        1.35508        0.01309
C          1.25688        1.35508       -0.01308
C         -2.46545       -1.49799       -0.06310
C         -2.46545        1.49799        0.06310
C          2.46545        1.49799       -0.06310
C          2.46545       -1.49799        0.06310
C          3.90819       -1.43687        0.24464
C          4.25416       -0.48923        1.44910
C          4.64897       -0.93627       -1.02206
C          4.64897        0.93627        1.02206
H          5.08644       -0.93074        2.00793
H          3.39579       -0.45561        2.12315
C          4.25416        0.48923       -1.44910
H          5.72105       -0.97274       -0.79444
H          4.47838       -1.63040       -1.84994
C          3.90818        1.43687       -0.24464
H          5.72105        0.97275        0.79444
H          4.47838        1.63040        1.84994
H          3.39579        0.45561       -2.12315
H          5.08644        0.93074       -2.00793
H          4.28399        2.44120       -0.47664
C         -3.90819       -1.43687       -0.24464
C         -4.25416       -0.48923       -1.44910
C         -4.64898       -0.93627        1.02206
C         -4.64897        0.93627       -1.02206
H         -5.08644       -0.93074       -2.00793
H         -3.39579       -0.45561       -2.12314
C         -4.25416        0.48923        1.44910
H         -5.72105       -0.97274        0.79444
H         -4.47838       -1.63040        1.84994
C         -3.90819        1.43687        0.24464
H         -5.72105        0.97275       -0.79444
H         -4.47838        1.63040       -1.84994
H         -3.39579        0.45561        2.12315
H         -5.08644        0.93074        2.00793
H         -4.28399        2.44120        0.47664
H         -4.28399       -2.44120       -0.47665
H          4.28399       -2.44120        0.47664

So, this also did not help. Then I thought about going back to TBNBL I and modify it. Let's add Chlorine looking into the rings, which should push the triple bonds apart. That does not change anything with regard to the first molecule ... TBNBL V has $\pu{143.4 pm}$ with a BDE of $\pu{555 kJ/mol}$.

E(RB97D3) =  -2531.12566838
C         -0.00000       -0.00000        0.71686
C          0.00000       -0.00000       -0.71686
C         -1.25995        0.00000        1.35010
C          1.25995       -0.00000        1.35010
C         -1.25995        0.00000       -1.35010
C          1.25995        0.00000       -1.35010
C         -2.47385        0.00000        1.43772
C         -2.47385        0.00000       -1.43772
C          2.47385        0.00000       -1.43772
C          2.47385       -0.00000        1.43772
C         -3.88167        0.00000        1.11523
C          3.88167       -0.00000        1.11523
C          4.23226        1.05834        0.00000
C          3.88167        0.00000       -1.11523
C          4.23226       -1.05834       -0.00000
C         -3.88167        0.00000       -1.11523
C         -4.23226        1.05834       -0.00000
C         -4.23226       -1.05834        0.00000
H         -4.54203        0.00000       -1.98574
H         -5.30546       -1.25818        0.00000
H         -5.30546        1.25818        0.00000
H         -4.54203        0.00000        1.98574
H          4.54203       -0.00000       -1.98574
H          5.30546        1.25818       -0.00000
H          5.30546       -1.25818        0.00000
H          4.54203        0.00000        1.98574
Cl        -3.41634       -2.62778        0.00000
Cl        -3.41634        2.62778        0.00000
Cl         3.41634        2.62778       -0.00000
Cl         3.41634       -2.62778        0.00000

What about adding more chlorine atoms? Seems to work; TBNBL VI: $\pu{144.1 pm}$ with a BDE of $\pu{542 kJ/mol}$.

E(RB97D3) =  -4369.66038511
C          0.00000        0.00000        0.72068
C          0.00000        0.00000       -0.72068
C         -1.25395        0.00000        1.35718
C          1.25395       -0.00000        1.35718
C         -1.25395        0.00000       -1.35718
C          1.25395        0.00000       -1.35718
C         -2.46546        0.00000        1.48386
C         -2.46546        0.00000       -1.48386
C          2.46546        0.00000       -1.48386
C          2.46546       -0.00000        1.48386
C         -3.85845        0.00000        1.12299
C          3.85845       -0.00000        1.12299
C          4.13677        1.09997        0.00000
C          3.85845        0.00000       -1.12299
C          4.13677       -1.09997       -0.00000
C         -3.85845        0.00000       -1.12299
C         -4.13677        1.09997       -0.00000
C         -4.13677       -1.09997        0.00000
H         -4.57107        0.00000       -1.94770
H         -4.57107        0.00000        1.94770
H          4.57107       -0.00000       -1.94770
H          4.57107        0.00000        1.94770
Cl        -3.09602       -2.53375        0.00000
Cl        -3.09602        2.53375        0.00000
Cl         3.09602        2.53375        0.00000
Cl         3.09602       -2.53375        0.00000
Cl        -5.83899        1.64902        0.00000
Cl        -5.83899       -1.64902       -0.00000
Cl         5.83899       -1.64902        0.00000
Cl         5.83899        1.64902        0.00000

As Geoff and I were curious to see what happens, when Bromine is used instead of Chlorine, I did the calculations (only geometry optimization so far) and they indeed yield a slightly longer bond but nothing is dramatically changed.

$144.3$, $144.3$ and $\pu{144.4 pm}$.

Iodine also adds a little to the bond length.

I asked a colleague and he said, what about about extending the lever (see law of the lever)? Indeed, it adds a little length.

His other thought was towards a BN triple bond. Not better than the CC triple bond in TBNBL I.

... and this is where I stop (again), because someone needs to beat my awesome DFT value of a super realistic molecule of 1445 pm now.

 EXPERIMENTALISTS THINK SILICON IS REALLY FUN TO USE
     ITS PLACE IN NOVEL COMPOUNDS IS CERTAIN TO AMUSE
 THEY SIT ALL DAY IN LABORATORIES MAKING ALL THIS SLUDGE
     "LOADED WITH THE SILICON THEY SAY", TO ME IT LOOKS LIKE FUDGE.
 FOR HAPPY THOUGH THEY BE WITH CRUD, I'D LIKE TO KNOW A LITTLE
      ABOUT THE PI BONDS ON THE EDGE AND SIGMAS IN THE MIDDLE.

 SO LETS DERIVE A WAVEFUNCTION.....6-31G*
     USE AN OPTIMAL GEOMETRY AND SEE WHERE ELECTRONS ARE.
 BUT WHAT OF CORRELATION?  ASKS THE WIRY LITTLE SKEPTIC.
     WE'LL THROW IN PERTURBATION AS AN ELECTRON ANTISEPTIC.
 AND WHEN THE PROGRAM GIVES US ANSWERS IN THEM WE CAN TRUST
     SINCE NOBODY CAN MAKE THE STUFF, WE HAVE NO CHOICE, WE MUST.

 SO THEORY GUYS HAVE GOT IT MADE, IN ROOMS FREE OF POLLUTION.
     INSTEAD OF PROBLEMS WITH THE REFLUX, THEY HAVE ONLY SOLUTIONS.
 AND WHEN THE FEDS ANNOUNCE THE LIST OF CARCINOGENIC TERRORS,
     THE THEORISTS SIT SAFELY AT THEIR TERMINALS FIXING ERRORS.
 IN OTHER WORDS, EXPERIMENTALISTS WILL LIKELY DIE OF CANCER
     FROM WORKING HARD YET FRUITLESSLY...TILL THEORY GIVES THE ANSWER.
          -- THOMAS A. HOLME, 1983

[1] Andreas A. Zavitsas, J. Phys. Chem. A, 2003, 107 (6), pp 897–898

[2] Images where created using CYLview, 1.0.564 BETA; Legault, C. Y., Université de Sherbrooke, 2009 (http://www.cylview.org)

I didn't check yesterday, but: TBNBL II "has an RHF->UHF instability". The rest "is stable under the perturbations considered." I was asked, if it is really a double bond or maybe a radical: HOMO and LUMO, pi and pi* of the DB, are separated by more than 2 eV (except for TBNBL II, where it is 0.23 eV). I'll leave TBNBL II in the answer, because it shows my working process, but I guess it is actually no DB. — pH13 - Yet another Philipp, Commented Feb 5, 2018 at 13:17
Out of curiosity.. what if you change the Cl to Br? Seems like the added radius would help the "ring push" aspect? — Geoff Hutchison, Commented Feb 6, 2018 at 19:14

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