Abstract
The stereochemistry of the olefin metathesis has been the subject of numerous publications which tried to rationalize the behaviour of various catalysts with acyclic olefins [1-5]. It had been noticed during these studies that a cis olefin gave preferentially a cis olefin and that a trans olefin led essentially to a trans olefin. This was explained in terms of stability of the metallacyclobutanes involved in the catalytic cycle (Scheme 1): the most favoured metallacyclobutanes are those where the substituents in positions 1 and 3 are equatorial. This simple rule allowed the explanation of most experimental results and the configuration retention.