Electronic structure of chromium hexacarbonyl at 78 K. I. Neutron diffraction study

Abstract

The distribution of the nuclei of chromium hexacarbonyl Cr(CO)6 in the crystalline state was refined from neutron diffraction data at 78K. It was found that diffraction by the aluminum walls of the cryostat used for low-temperature measurements was superimposed on the diffraction peaks of the crystal, and a method was derived to correct for this effect. Extinction was rather severe, and the correction formulae recently derived by Becker & Coppens [Acta Cryst. (1974). A30, 129-153] were applied successfully in the least-squares refinements. The Cr(CO)6 octahedron is significantly distorted in the crystal, but no significant differences were found between chemically equivalent bond lengths. The average values, directly derived by a least-squares procedure from the observed structure factors, and corrected for riding motion around chromium, are: C r -C= 1.918 /~ and C O = 1.141 A. These values show that the carbonyl groups are less strongly bonded to chromium than in the related complex benzene chromium tricarbonyl, which is consistent with the larger electronegativity of the carbonyl groups, compared to benzene.