Experimental X-ray charge density studies on the binary carbonyls Cr(CO)6, Fe(CO)5, and Ni(CO)4.

The experimental charge densities in the binary carbonyls Cr(CO)(6) (1), Fe(CO)(5) (2), and Ni(CO)(4) (3) have been investigated on the basis of high-resolution X-ray diffraction data collected at 100 K. The nature of the metal-ligand interactions has been studied by means of deformation densities and by topological analyses using the Atoms in Molecules (AIM) approach of Bader. A detailed comparison between the experimental results and theoretical results from previous work and from gas-phase and periodic DFT/B3LYP calculations shows excellent agreement, both on a qualitative and quantitative level. An examination of the kappa-restricted multipole model (KRMM) for Cr(CO)(6), using theoretically derived structure factors, showed it to provide a somewhat worse fit than a model with freely refined kappa' values. The experimental atomic graphs for the metal atoms in 2 and 3 were found to be dependent on the multipole model used for that atom. In the case of compound 2, restriction of the multipole populations according to idealized site symmetry of D(3h) gave an atomic graph in essential agreement with the theoretical gas-phase study. For compound 3, all multipole models fail to reproduce the atomic graph obtained from the theoretical gas-phase study. The atomic quadrupole moments for the C atoms in all compounds were consistent with significant pi back-donation from the metal atoms.