CHARGE DENSITY ANALYSIS IN THE 3d-METAL SANDWICH COMPLEXES WITH THE MIXED-RING [[pi]]-LIGANDS. K.A.Lyssenko*, M.Yu. Antipin*, S.Yu.Ketkov** Institute of Organoelement Compounds (INEOS), Vavilov St. 28, B-334,Moscow, 117813, Russia. ** Institute of Organometallic Chemistry, N.Novgorod, 603600, Russia.

Electron density distribution using single crystal X-ray diffraction data was studied for the the similar [[pi]]-complexes ([[eta]]⁵-C₅H₅)M([[eta]]⁷-C₇H₇) where M=Ti, V and Cr. This series represents the 16-, 17- and 18-electron complexes, respectively. Crystal structures studied are isomorphous and compounds are isostructural (space group Pmna, Z=4, molecules in crystals have m-symmetry), that allows to compare carefully their molecular geometry and electron density distribution features caused mainly by the different nature of the transition metal. All experimental data were obtained at low temperatures (100-150K) with the "Siemens P3/PC" diffractometer using Mo-radiation, for each crystal ca. 15000 reflections were collected, and R values were 0.025-0.030. Similar crystal structure has the 17- electron complex ([[eta]]⁵-C₅H₅)Ti([[eta]]⁸-C₈H₈) that was also studied (data collection at 203K because of the phase transition, ca 9000 reflections, R=0.025). All complexes have the sandwich-like structure with the planar cyclic ligands that are symmetrically bonded with the metal atoms. A typical asymmetry of the electron density distribution picture near the metals was observed in the deformation electron density maps for all compounds studied that was attributed to the different occupancies of the 3d-metal orbitals. Numerical values of these occupancies were estimated using a multipole refinement procedure. In the series of the 16-,17-,18-electron complexes a gradual occupancy of the different metal orbitals was observed that was in agreement with the performed ab initio calculations and measured vapor-phase electronic absorption spectra of the compounds studied. It was found for Ti-complexes with the C₇H₇- and C₈H₈-ligands that due to the diffrerent size of the ligand ring the energy sequence of the molecular e₁, a₁ and e₂ orbitals has been changed, and this effect is clearly seen in the deformation electron density maps.