Reversed Mills-Nixon effect

The ionic charge transfer structures involve the double bond coupling at meta positions occuring three times in contrast to ortho and para bonds, where 7r-electron perfect spin 

Structural parameters of benzoborirene and benzocyclopropenyl cation as obtained by the HF and MP2 models (distances in A and angles in degrees)0 

respectively. Another point of interest is the lengthening of the C(l)-C(2) bond in both small rings 35a and 36a upon H-C-C deformation, which mimicks annelation. This distance increases by 0.02 A in both molecular models. Additional stretching, which takes place in true molecules 33 and 34, is a consequence of the increased delocalization over 

Bonding parameters in 34 follow the same pattern with some notable quantitative differences. For example, the s-character in the fused bond drops to 19% only. The 7r-bond order of the C(l)-C(7) bond is higher than that in the ipso bond indicating increased resonance interaction between the aromatic moiety and the cationic center. 

A similar mechanism is operative in silacyclopropabenzenes and silacyclobutabenzenes 39 and 40, where Si atoms act as pseudo cationic centers. It appears that ortho bonds in both compounds are longer than that in free benzene - the amount of stretching being 0.025 A (MP3(fc)/6-31G model) and 0.013 A (MP2(fc)/6-31G model) in 39 and 40, 

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