Slackening a chromium carousel with a manganese bridle: selective C-13 isotopic labelling applied to the determination of the steric barrier to rotation of a Cr(CO)(3) group in a syn-facial hetero-bimetallic (Cr, Mn) cyclohexadienylbenzylidene complex

The steric barrier to rotation Delta G double dagger of the Cr(CO)(3) rotor of a syn-facial heterobimetallic cyclohexadienylbenzylidene (Cr, Mn) compl ex has been determined by C-13-NMR low-temperature experiments. The latter compound was prepared from a (eta(6)-arene)tricarbonylchromium complex isot opically enriched in C-13 by photolytic mono-decarbonylation and carbonylat ion with labelled CO. This C-13 dynamic NMR study presents the first exampl e of the freezing of the rotational motion of Cr(CO)(3) that takes place at moderately low temperatures (Delta G double dagger(253 K) = 15 kcal.mol(-1 )). (C) 1999 Academie des sciences/Editions scientifiques et medicales Else vier SAS.