Cyclooctatetraene, bond lengths

Cyclononane, strain energy of, 114 Cyclooctane, strain energy of, 114 Cyclooctatetraene, bond lengths in, 524... 

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. 

Fig. 4. Calculated eneigies (kcal/mole) of planar forms of cyclooctatetraene with alternating and equal bond lengths relative to the tub form...

Cyclooctatetraene, CgHg, has a D2g tublike structure with four rather isolated double bonds. Cyclooctatetraenes can be the products of the catalytic cyclotetramerization of alkynes, and cyclobutadienes may be the intermediates. The BN homologues of cyclooctatetraenes have been known since 1962 (67). Like cyclooctatetraene, molecules of [(SCN)BNtBu]4 were shown to have a tublike ring structure of S4 symmetry with alternating bond lengths of 140 and 146 pm, the shorter ones perpendicular to the direction of the S4 axis (68). 

A comparison of the metal-carbon bond lengths, ionic radii and formal coordination numbers of these compounds is summarized in Table 2. The formalism used in estabhshing coordination number assumes that a -cyclooctatetraene ligand is a 5 electron-pair donor. The ionic radii have been adjusted for both the charge of the central metal and coordination number (50). When the ionic radius for a given coordination number is not available, it has been estimated by interpolation from radii of other coordination numbers. It will be seen that the differ-... 

Table 2. Comparison of M—C bond lengths in several cyclooctatetraene complexes...

In accord with the Hiickel rule of 4/t + 2 electrons, both cyciobutadiene and cyclooctatetraene (cot) are nonaromatic. Cyclooctatetraene contains alternating bond lengths and has a tub-shaped conformation ... 

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 ... 

In the end of the 1920s, the structure of hexam-ethylbenzene was solved by Lonsdale53 by means of X-ray diffraction and found to have identical CC bond lengths. Thus, a correspondence has been established between the real geometric structure and the empirical equivalence of the molecular positions deduced from the early experiments of the Kekule era. Still, the difference between benzene and cyclooctatetraene defied all existing hypotheses. The concept of delocalization has been latent still, and its connection to stability is not at all apparent. 

Next we apply this simple model to the annulenes. These compounds are monocyclic conjugated polyenes with the general molecular formula (CH) with n even. Thus benzene may be considered as [6]-annulene. As n increases, essentially all [ ]-annulenes are non-planar. For instance, cyclooctatetraene, (CH)s, has the well-know tub structure. However, [18]-annulene is nearly planar, as shown below (bond lengths in picometers are displayed in bold italic font). 

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. 

You will probably be surprised to find cyclooctatetraene (COT for short), unlike benzene, is not planar. Also none of the double bonds are conjugated—there are indeed alternate double and single bonds in the structure but conj ugation is possible only if the p orbitals of the double bonds can overlap here they do not. Since there is no conjugation, there are two C-C bond lengths in cyclooctatetraene—146.2 and 133.4 pm—which are typical for single and double C-C bonds. If possible, make a model of cyclooctatetraene for yourself—you will find the compound naturally adopts the shape below. This shape is often called a tub . 

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. 

Moreover, this molecule is believed to have the ability of turning inside out via a transitional plane structure. At the same time it was demonstrated that in a plane cyclooctatetraene molecule the bond lengths do not get equalized, the double bonds retaining their distinctive features. Even if this molecule were plane, it would still be nonaromatic. 

In the tetrameric chloride crystal, the ring is puckered, with S symmetry, though with equal bond lengths round the ring (43). This may be contrasted with the most recent data for cyclooctatetraene, which has... 

The vanadium—vanadium bond length in the organometallic complex ( 5-C5H5)2V2(C4H8)2 (Figure 9) of 2.315(1) A is somewhat controversial.31 32 Similar to /i-( 5 5-cyclooctatetraene)bis[( p-cyclopentadienyl)divanadium]33,34 with a metal-metal distance of 2.439(1) A, the interpretation of the interaction between the two vanadium centers has been proposed to be a triple bond, a very short single bond,33 or a weak double bond.35... 

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. 

---