Tetracarbonylchloroethylidynechromium: electron deformation density revisited. Joint multipole model refinement and ab initio CASSCF study
Résumé
The X-ray diffraction measurements obtained by Goddard and Krüger for Cl(CO)4Cr triple CH3 (crystallizing in P21/c; a = 6.504 (2) Å. b = 1 1 .261 (5) Å c, = 12.756 (6) Å, = 105.58 (3) 0 . Z = 4) have been refined by using the multipole model proposed by Hansen and Coppens. Static and dynamic model maps are obtained that provide suitable information concerning the reorganization of the electron density induced by the CFC and Cr—(CO)4 bonding. The observed alternation of the Cr—Cco bond lengths does not seem to bc the consequence of a delocalized electronic effect originating in the methyl group. The static maps are discussed in connection with a theoretical density distribution obtained from ab initio CASSCF calculations. Both the theoretical and the nwltipole model maps show the marked covalent character of the Cr=C T bonds, compared to the back-donation occurring in thc plane. The presence of a negative net charge on the carbynic carbon is confirmed. and the corresponding density accumulation is shown to lie in the region of the triple bond, facing the metal. This charge distribution is consistent with u 6 2 7 2 formal electronic configuration for the carbynic carbon. The metal d-orbital populations deduced from the static model distribution are in quite good agreement with the theoretical predictions.