Bivalent spectator oxo bonds in metathesis and epoxidation alkenes

Several studies suggest the importance of oxygen in the reactive intermediates for alkene metathesis, M=CH₂ + H₂C= C*H₂ ⇌ M=C*H₂ + H₂C=CH ₂ (1) and of additional metal=oxo bonds in epoxidations, For example: (1) catalytic amounts of oxygen were found to be essential for the formation of a stable metathesis catalyst in the WCl₆/C₂H ₅AlCl₂ system¹; (2) alkene epoxidation (and general oxidation) reactants such as CrCl₂O₂, OsO ₄, and SeO₂ all have at least two oxygen ligands ²; and (3) recently Osborn (personal communication) has described an Mo=oxo carbene as an effective metathesis catalyst. From extensive ab initio theoretical studies of Cr and Mo complexes, we show here that the presence of spectator metal-oxo bonds drives the formation of metallocycles, thereby playing a crucial role in the chemistry.