Modern ruthenium based olefin catalysts encompass a range of complexes able to readily generate a 14 electron catalytic active species of the generic formula (L)(X2)Ru=CRR’ (L being a phosphine ligand in first generation catalysts).
The first well-defined catalysts of this type, discovered by Grubbs in the early 90’s, feature a ruthenium metal center flanked with two donor ligands, two anionic ligands and an alkylidene moiety. Important examples of such catalysts include the phosphine/benzylidene catalyst (known as the 1st generation Grubbs catalyst, I), and its dimethylvinyl alkylidene analogue II. These catalysts tolerate a wide variety of functional groups and are soluble in arenes and chlorinated solvents. For these catalysts, the active catalytic species is generated by dissociation of one phosphine ligand. In the case of the 1st generation Hoveyda-Grubbs’ catalyst (III, an even more robust catalyst), the dissociative ligand is a chelating ether.
All the previous catalysts are ML5 complexes that have a distorted square pyramidal geometry with the alkylidene moiety in the apical position. More recently, ML4 complexes have become known. The cationic 1st generation Piers catalyst (IV) is an example. This catalyst doesn’t require ligand dissociation prior to activation because it starts out with a vacant coordination site.