Bonding in a binuclear metal carbonyl: experimental charge density in Mn2(CO)10

Abstract

X-ray diffraction data (Ag Ka and Mo Net radiation) have been collected at 74 K on dimanganese decacarbonyl Mn2(CO)I 0, and the crystal structure was refined. The atomic positional and thermal parameters were determined from high-angle AgKa data to avoid systematic errors due to bonding effects. The molecular geometry is discussed. The distortions are larger than at room temperature: for example, the torsion angle of the two Mn(CO) 5 fragments is 50.2 °, compared to 47.4 ° at room temperature, and 45 ° for an ideal D4a symmetry. 'X-X' deformation-density maps were computed and averaged over chemically equivalent sites. No significant charge-density accumulation is observed on the Mn-Mn bond. The configuration around the Mn atoms is essentially octahedral, with about 75% of the 3d electrons in the diagonal orbitals (dxy,d~z,dy ~) and the remaining 25% in d: and dx2_/. Atomic charges were determined both by direct integration and by least-squares refinement. Directly integrated charges are very small. Mn seems slightly negative. Both methods of charge integration show a clear difference between axial and equatorial carbonyls, which is confirmed by comparing their electron density with that of free carbon monoxide: the differences are characteristic of a a-bonding n-backbonding mechanism, and are larger for the axial carbonyls than for the equatorial carbonyls. This confirms the stronger bonding of the axial carbonyls, also responsible for the longer C-O bond and the shorter Mn-C bond.