What's the longest C=C bond? 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
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&nbsp;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&nbsp;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