Electronic structure of benzene chromium tricarbonyl by X-ray and neutron diffraction at 78 K

Abstract

The crystal structure of C6H6Cr(CO)3 has been studied at 78 °K by both X-ray and neutron diffraction. The studies indicate a lowering of the benzene-ring symmetry to C3v. The C-C bonds in the ring alternate in length, the shortest bonds being trans to the carbonyl groups. The average difference is 0-017 (2)/~,. The observed bond lengths correlate well with electron overlap populations obtained in semi-empirical molecular orbital calculations. The neutron results show the hydrogen atom to be displaced by an average of 0.03 A from the plane of the benzene ring in a direction towards the chromium atom. X-N maps show large residual density features near the chromium nucleus, which are to be confirmed by further studies on related compounds. Other residual peaks correspond to the overlap density in the Cr-C, C-C, C-O and C-H bonds and to the lone-pair electrons on the oxygen atoms. The Cr-C(O) bond peaks are closer to the carbon atom, in agreement with theoretical predictions. This is the first combined X-ray and neutron diffraction study of a compound containing third-row atoms, and it is concluded that the bonding in transition metal complexes can now be studied by this technique. A spherical charge refinement of the X-ray data did not lead to satisfactory results. This is partly due to the lack of an appropriate X-ray form factor for chromium.

Theoretical calculations of deformation densities in some transition metal complexes

“…Further investigations are required to test this hypothesis. The peaks in this theoretical study are located at shorter distances from the metal atoms and are higher than those found in analogous experimental investigations (Rees & Coppens, 1973;Iwata & Saito, 1973;Marumo et al, 1974). The main reason for this discrepancy is probably the inadequate description of the thermal motion.…”

“…From the deformation, or residual, density (defined here as the difference between the electron density distribution in a molecular system and the corresponding density obtained from a superposition of the constituent spherically averaged atoms) a-bonds, ~-bonds, bent-bonds, lone-pair electrons and the like may be recognized. Some recent investigations (Rees & Coppens, 1973;Iwata & Saito, 1973;Marumo, Isobe, Saito, Yagi & Akimoto, 1974) have also demonstrated deformation density features in the vicinity of transition metal atoms. These departures from sphericity are probably very common and should be observable from the results of many crystal structure determinations.…”

The structure of tricarbonyl(styrene)chromium, a crystal active in second-harmonic generation

“…Given the extent of overlapping of the two fractional ligands, the description of the disorder seems reasonably satisfactory. The geometry of the Cr(CO) 3 group and the relation of the group to each of the fractional styrene ligands is generally consistent with the structure of tricarbonly(benzene)-chromium as found by Bailey & Dahl (1965) at room temperature and by Rees & Coppens (1973) at 78 K, except that for each of the two fractional styrene C 6 rings three of the Cr--Cring bonds almost eclipse the three Cr--Cco bonds. In contrast, the two kinds of (29) * Centroids of the C 6 rings.…”

Hydrogen bond studies. CX. A neutron diffraction and deformation electron density study of sodium hydrogen oxalate monohydrate, NaHC2O4.H2O

“…the Na ion directed towards the lone-pair region of the water O atoms. A similar electron distribution has been obtained around the Cr atom in C6H6Cr(CO) 3 (Rees & Coppens, 1973). It should be remembered, however, that the largest errors in X-N maps arise in the vicinity of the nuclei.…”