Structure and conformations of heteroatom-substituted free carbenes and their group 6 transition metal analogues

A systematic density functional theory (DFT) study of free and Cr- and W-coordinated alkoxy-carbenes has been carried out. The structure of the free carbenes and their relative energies remains unaltered by coordination to the metal. A clear bias for the anti-isomer is observed in both the free and coordinated carbenes. The exception is complexes having alkynyl substituents. In this case, the metal complex prefers the syn-disposition. The theoretical model explains the observed bias for the anti-isomer by the stabilizing contribution of a stereoelectronic effect due to the two-electron donation from the sigma(CH) orbital of the O-Me group to the pi*(CO) orbital of the CO ligand and feedback donation of the pi(CO) orbital to the sigma*(CH) orbital and the destabilizing repulsion between the groups linked to the carbene carbon. This last factor is the determinant one since the linear triple bond produces a clear bias for the syn-isomer only in the coordinated carbene, showing that the steric contribution rather than the electronic repulsion is responsible for the structure of group 6 Fischer carbenes. The anti-syn isomerization in chromium(0)-carbene complexes has also been studied. This process occurs without affecting either the carbene ligand or the coordination sphere of the metal by rotation of the C(Cr)-O-Me bonds. The developed model is fully applicable to tungsten-carbene complexes.