Does π−σ−π Through-Bond Coupling Significantly Increase C−C Bond Lengths?

Orbital effects ascribed to π−σ−π through-bond coupling are the result primarily of destabilizing filled−filled interactions; little to no mixing occurs between the antisymmetric π combination and σ*. The magnitude of through-bond coupling does not correlate with bond lengthening. Previous conclusions that through-bond coupling induces long bonds relied in some cases on poor X-ray data and/or inappropriate molecular orbital theories to deduce geometries. Comparison of semiempirical and empirical force field data as a test for through-bond coupling is unwarranted. Standard force fields underestimate the steric bulk of the benzene face, but addition of so-called “through-bond coupling” terms does not provide a physically significant measure of through-bond coupling.