Cyclooctatetraene, bond lengths dianion

The crystal structure of the potassium salt of 1,3,5,7-tetramethylcyclootatetraene dianion has been determined by X-ray dififaction. ° The eight-membered ring is planar, with aromatic C—C bond lengths of about 1.41 A without significant alternation. The spectroscopic and structural studies lead to the conclusion that the cyclooctatetraene dianion is a stabilized delocalized structure. 

Examine the geometry of cyclooctatetraene dianion. Is it planar If not, describe its shape. Are all the carbon-carbon bond lengths the same If so, are they the same length as those in benzene If not, do they alternate between single and double bonds Do your observations suggest that cyclooctatetraene dianion is aromatic Is this in accord with Hiickel s rule ... 

The cyclooctatetraene dianion has a planar, regular octagonal structure with C — C bond lengths of 1.40 A close to the 1.397 A bond lengths in benzene. Cyclooctatetraene itself has eight pi electrons, so the dianion has ten (41V+2), with N = 2. The cyclooctatetraene dianion is easily prepared because it is aromatic. 

It is also possible to add electrons to cyclooctatetraene by treating it with alkali metals and a dinw-ion results. X-ray structures reveal this dianion to be planar, again with all C-C bond lengths the same (140.7 pm). The difference between the anion and cation of cyclooctatetraene on the one hand and cyclooctatetraene on the other is the number of electrons in the n system. The cation has six 71 electrons, the anion has ten, but neutral cyclooctatetraene has eight. 

Addition of two electrons to cyclooctatetraene would lead to a 10ji-electron system, the cyclooctatetraenyl dianion, Cyclooctatetraene, which is non-planar, reacts with potassium in tetrahydrofuran to give dipotassium cyclooctatetraenide, which is planar. The C-C bond lengths are all the same (141 pm). At low temperatures, a dication is formed when cyclooctatetraene reacts with SbF.. This also has some aromatic characteristics in keeping with a 6jt-electron system. 

Estimate interelectronic repulsion in the cyclooctatetraene dianion due to the interaction of the two charges only (use the benzene bond length and a reasonable average for the interelectronic distance). 

---