Dramatic enhancement of the alkene metathesis activity of Mo imido alkylidene complexes upon replacement of one tBuO by a surface siloxy ligand

[(equivalent to SiO) Mo(equivalent to NAr)(= CHCMe2R)(OtBu)](2) a well-defined silica supported alkene metathesis catalyst precursor, shows a dramatic enhancement of activity and selectivity compared to [ Mo( NAr)(= CHCMe2R)(OtBu)(2)] and [( SiO) Mo( NAr)(= CHCMe2R)(CH(2)tBu)], respectively.