Rotational barriers in a series of centrally bound benzocyclobutanechromium tricarbonyl compounds have been calculated according to the extended Hackel method. The difference of bond populations (ABP) between the long and short C-C bonds in the distorted arene ring correlates well with the rotational barrier. ABP measures directly the decrease in the ability of arene to bond with chromium during rotation of Cr(CO)3; its magnitude depends on the extent of distortion of the arene and the strength of the antiaromatic effect from the ?r bond of annelated four-membered rings.
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