What's the longest C=C bond?
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