Aspects of the latter topic were reviewed recently by Cavallo alkylidene.
This process has been studied heavily, particularly in recent years.
A wide range of pre-catalysts are known, the majority of which bear N-heterocyclic carbene (NHC) ligands, Metathesis pre-catalysts typically feature a so-called "throw-away" ligand which is not present in the active catalyst; this is typically a chelating alkoxystyrene group or a phosphine, although a wide range of such ligands have been employed.
Work continues in this field, to elucidate the finer mechanistic details.
The first detailed study of the initiation mechanism of this class of catalysts was carried out by Plenio, who studied the initiation of GH2 and Grela2 using UV/visible spectroscopy.