Bond characterization of chromium-Fischer carbene complexes: A combined study of experiment and theory

Four pentacarbonyl chromium-carbene complexes, (CO)(5)CrC(XR')R, have been studied via X-ray diffraction and molecular orbital calculations. One of the carbene complexes (XR' = OCH3, R = -(C=CPh)) has been investigated extensively at 110 K by X-ray diffraction using Mo K alpha radiation. The electron density distribution of this compound as well as the asphericity in electron density around the Cr atom is clearly demonstrated by deformation density and the Laplacian of electron density. The comparison between experiment and theory is made in terms of deformation density, topological properties, and d orbital populations of Cr. Further chemical bond characterization is based on quantum mechanical molecular orbital calculation and properties associated with bond critical points. The pi bond character of a metal carbene can be best represented by a Cr-C-X three-centered four-electron bond with the pi electron density mainly located at either the d(yz) orbital of Cr or the p(z) orbital of X in the carbene ligand. This makes the carbene carbon an electrophilic site in the p, direction. The resemblances and differences between amino-(X=N) and alkoxy-(X=O) carbene complexes are of special interest. Because the energy of pi*(C-N) orbital is fairly close to that of the pi*(C=O) (carbonyl on Cr), the pi bond character is delocalized toward the M-(carbonyl) at the trans position i.e., O=C=M-C=N in the amino carbene case; this is in accord with the shortening of bond lengths of the trans M-C-carbonyl for many amino-carbene complexes. The pi bond-delocalization is also illustrated by the Fermi hole distribution. The bond dissociation energies of these carbene complexes are calculated at the CASSCF level and with the density functional method (DFT). The relative orbital energies are also compared with photoelectron spectroscopy (PES) data. The values based on DFT using the transition-state approximation give the best agreement with the experimental results.