BOND CHARACTERIZATION OF CHROMIUM-FISCHER CARBENE COMPLEXES. Yu Wang, C.C. Wang, H.J. Liu, K.J. Lin, L.K. Chou, K.S. Chan, Department of Chemistry, National Taiwan University, Taipei, Taiwan, ROC
Four pentacarbonyl chromium carbene complexes, (CO)5CrC(X)R, have been studied via X-ray diffraction and molecular orbital calculations. One of the carbene complexes (X=OCH3, R=-C(CPh) has been investigated extensively at 110K by X-ray diffraction using MoK( radiation. The electron density distribution of this compound as well as the asphericity in electron density around the Cr atom are clearly demonstrated. The comparison between experiment and theory is made in terms of deformation density, net atomic charge, and d-orbital populations of Cr. Further chemical bond characterization is based on quantum mechanical molecular orbital calculation. The resemblances and differences between amino-(X=NR'R") and oxy-(X=OR') carbene complexes are of special interest. The results indicate that the competition between the (-characters of Cr-Ccarbene and Ccarbene-X bonds always exists, however the majority contribution comes from either Cr dxz-orbital or the pz orbital of X in carbene ligand. This makes the carbene carbon an electrophilic site at the p( direction. Since the energy of (* orbital of C-N is much higher than that of C-O, the amino-carbene is a poorer (-acceptor than the oxy-carbene. Furthermore, the energy of (*C-N is fairly close to that of (*C(O, the ( bond character is even found on the M-Ccarbonyl at the trans position. Therefore the axial carbonyl bond is a double bond i.e. N=C-M=C=O. This result is in accord with the differences found on bond lengths of axial carbonyl for many amino-carbene complexes. The bond dissociation energies of these carbene complexes are calculated at CASSCF level. The relative orbital energies are also checked upon with photoelectron spectroscopy(PES). The linear relationship exists between experiment and theory for all the MO calculations, however the result based on the density functional method (DFT) using the transition state approximation gives the best agreement with the experimental result.