Nature of the Transition Metal-Carbene Bond in Grubbs Olefin Metathesis Catalysts

A comparative density functional theory study of tungsten-based Fischer carbenes, tungsten- and molybdenum-based Schrock carbenes, and ruthenium- and osmium-based Grubbs-type olefin metathesis catalysts reveals that the last group of catalysts should not be taken to be intermediate between the two established categories of carbenes. Rather, the Grubbs catalysts have characteristic properties similar to those of the Schrock family of carbenes and can be viewed as an extension of the latter class. Whereas an appreciable fraction of the electrons involved in the carbene bond of Fischer carbenes are unshared, and these compounds may be classified as ionic or donor-acceptor carbenes, a distinct covalent character is observed for both Schrock carbenes and Grubbs catalysts, which thus together fall into a larger category that may be termed covalent or electron-sharing carbenes. For these compounds, the energies of the atomic valence d orbitals determine the polarization of the sigma and pi components of the carbene bond. Whereas the early metals (as reflected in valence d orbitals of relatively high energies) W and Mo render Schrock carbenes weakly nucleophilic, the later metals Ru and Os form weakly to moderately electrophilic carbenes. Schrock carbenes may thus, more accurately, be classified as nucleophilic covalent or nucleophilic electron-sharing carbenes, while Grubbs catalysts could be termed electrophilic covalent or electrophilic electron-sharing carbenes.