CHEMICAL BONDING IN METALAHETEROCUMULENE COMPLEXES .2. DEMONSTRATION OF A TEMPERATURE-DEPENDENT CONFORMATIONAL DISORDER OF THE METHYL-GROUPS IN [(CO)(5)CRNCN(C2H5)(2)]

During the determination of the crystal structure of [(CO)(5)CrNCN(C2H 5)(2)] by neutron diffraction, a remarkable temperature-dependance of the intensity of three reference reflections between 100 and 300 K was accidentally observed. In order to try to understand the nature of th is phenomenon, the behaviour of this compound as a function of the tem perature has been studied by differential calorimetry, and by powder a nd single crystal X-ray diffraction methods. These experiments show ne ither a phase transition nor a crystal space group modification. The e xperimental results, supported by a simplified model, suggest a rotati onal. disorder of the methyl groups near room temperature.