Olefin metathesis (equation 1; = some ligand coordination sphere) has been known for -35 years (ref. l), but until a few years ago the only catalysts available were "classical" catalysts (usually containing Mo, W, or Re), i.e., catalysts that were simple to make, but whose nature was not known in detail and whose activity therefore could not be controlled to a degree to which we have become accustomed in modern day catalysis chemistry. Such "black box" catalysts in general will not tolerate

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... donor functionalities, perhaps in part because little metal is actually active in such systems and absolute activity therefore has to be exceedingly high. Side reactions associated with "co-catalysts" such as aluminum halides are another significant complication.