Some additional evidence for the sixfold symmetry of benzene in dibenzenechromium in the vapor phase

Abstract

Some time ago an IR-spectroscopic study of gaseous dibenzenechromium involving, one of us’ produced the fust experimental proof of the sixfold symmetry (Dsh) of benzene in this complex in the vapor state. This ended a long controversy in the history of which some papers were published which claimed (X ray* and IR studies3-‘) and other papers which denied (X ray”’ and IR”r) that benzene in Cr(C6H6)2 had a symmetry which was lower (D3d) than that of the free molecule (D&). We were interested in additional aspects of the matter and it occurred to us that an elegant possibility was offered by the electron diffraction study”. The electron diffraction study had yielded two solutions (the first being the more likely one): benzene as a ligand in Cr(&Hs)s was found to have only one type of carboncarbon bond (1.423 A) and hence sixfold symmetry, if the mean C-C vibrational amplitude was the same (0.045 & as that of free benzene (0.0454 A)ll. On the other hand, two different types of carbon-carbon bonds were possible (hence threefold symmetry), but then the mean C-C vibrational amplitude had to be smaller (0.0435 A) than that of