Orbital interactions in the ruthenium olefin metathesis catalysts

The structural and bonding interactions of the ruthenacyclobutane 4 suggested its nonclassical nature. A strong bonding interaction between the orbitals of the -CH2CH2CH2- fragment and the d(y)(2) orbital on the metal in 4 was found that indicated the presence of two alpha-CC agostic interactions in the complex. Further, in the transition state corresponding to the rate-determining step of the metathesis, mutually benefiting pi interactions exist between the Ru=CH2 and the olefin fragments. These features are important in the metathesis activity of Grubbs first-generation catalyst because they make the potential energy surface of the M(=CH2)(C2H4) <----> M(C2H4)(=CH2) region flat.