Mechanistic investigation of contemporary metathesis catalysts

Akeypart in the analysis and proposal was electron accountancy - on the basis of the usual propensity for the adoption of a noble gas outer shell configuration - and all of the proposed intermediates have 16- or 18-electron configurations except the metallocyclobutane (26) in pathway II which has 14. On this basis, one might expect that pathway I, the non-dissociative pathway, would predominate over pathway II. In the presence of a large excess of Cy3P, which was used only to simplify the kinetic analysis, this is clearly the case. However, under the 

14-electron intermediates (26 and 27) and was not considered viable in the initial analysis inset the mechanism-derived rate equation. 

There is an alternative pathway to II, in which the phosphine dissociates before the alkene group coordinates pathway III. On the basis of electron accountancy alone, this should be viewed as unfavourable as it involves two 14-electron intermediates (26 and 27). However, it should be noted that the mechanism-derived rate equation for reaction via pathways I/III rather than I/II would be equally consistent with the empirical rate equation. 

---