Cyclooctatetraene uncomplexed

Witli cyclooctatetraene, the complex formed above —20°C lias the stoichiometry (CaH )Ni. The black lustrous crystals are only slightly soluble. Although the infrared spectrum of the solid shows no evidence of uncomplexed double bonds, the NMR of a benzene solution shows two different types of protons leading to the suggestion 73) that the solid compound has a polymeric structure (196) which is dissociated upon dissolution. 

A wide variety of other conjugated diolefins have been shown to form (C4)M-type bonding systems. These include cyclopentadiene, cyclopenta-dienone, cyclohexadiene, cycloheptatrienone, and cyclooctatetraene. The bands attributed to the free olefin, appearing at 1575-1600 cm Mn the free olefins, display a typical bathochromic shift (to 1500-1460 cm" ) in their complexes. If one double bond of the diolefin is complexed while the second remains free, often two well-separated absorptions are observed, one being in the region of an uncomplex olefinic linkage and the other at a lower frequency (Table 4-5). 

Two distinct observations are made with respect to the hydrogenation of olefin 7i-complexes with catalysts such as raney nickel. When the olefin is firmly bound to a metal, complete catalytic hydrogenation is prohibited. Thus 7i-cyclopentadienylcobalt cyclooctatetraene (6-22), 7r-cyclopentadienyl tt-cyclopentadiene rhenium dicarbonyl (6-23), and Tr-cycloheptatriene iron tricarbonyl (6-24), each of which contains at least one uncomplexed double bond, readily undergo hydrogenation of the uncomplexed bonds only. The metal-olefin bond is preserved in each case. 

Hartmann et al. studied the effect of ACN concentration [ACN] and the presence of cyclooctatetraene on direct or Rose Bengal-sensitized excitation in 16 solvents of varying polarity. The logarithmic cisoid/ transoid ratio of 1 showed a linear relationship with the dielectric constant of the solvent. Cyclooctatetraene suppresses the formation of transoid-l, whereas sensitization by Rose Bengal results in a decreased cisoid/transoid ratio. In fine with these results, the authors made some important inferences on the reaction mechanism that have currently been proven to be correct in essence. Thus, an association of two ground state ACN molecules can form a molecular complex (van der Waals complex), which is stabilized in more polar solvents. The Sj state of the molecular complex (an excimer) may give cisoid-l exclusively, this is in contrast to the Tj state of the uncomplexed ACN molecule, which leads to both cisoid- and transoid-l, whose distribution depends on the nature of the solvent. 

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