Electron delocal ization

During our discussion of benzene and its derivatives it may have occurred to you that cyclobutadiene and cyclooctatetraene might be stabilized by cyclic rr electron delocal ization m a manner analogous to that of benzene... 

Comparison of the calculated C—C bond distances (MP2) of 105 [C(1)=C(2) 1.357, C(2)—C(3) 1.439, C(3)=C(4) 1.371, C(l)—C(7) 1.497 A] with tran -butadiene [C(1)=C(2) 1.341, C(2)—C(3) 1.461 A] shows that a considerable degree of bond equal-ization is present in 105. Cremer and Kraka suggest that this is the consequence of homo-conjugative electron delocalization. This conclusion was confirmed by these authors from the values of the calculated bond orders and 7r-character indices. These results are contrary to predictions obtained at lower, less reliable levels of theory " ". ... 

EPR spectroscopy suggests that part of this stabilization arises from Jt-delocal-ization. g values in [Co(9S3)2] (Table 5) deviate little from g, a reflection of appreciable delocalization of metal electron density onto the ligands [22, 99]. 

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