ETHYLENE COMPLEXES - BONDING, ROTATIONAL BARRIERS, AND CONFORMATIONAL PREFERENCES

Rotational barriers and conformational preferences are a primary probe of bonding in olefin complexes. Such barriers in ethylene-ML2_5are analyzed in terms of differential interactions between the frontier orbitals of the MLn., fragment and the ethylene π and π. It is found that the large barrier to internal rotation about the M-ethylene axis in ethylene-ML2 complexes, favoring the in-plane orientation, is due to loss of overlap between ethylene π and an ML2b2orbital—the dominant interaction in these compounds. An analogous situation exists for rigid rotation in ethylene-ML4within the trigonal-bipyramidal geometry. A much lower energy pathway for this complex is found if rotation is accompanied by pseudorotation. The barrier in square-planar ethylene-ML3compounds of the Zeise's salt type, on the other hand, is largely set by steric factors which favor the upright geometry. Various strategies are devised to lower the barrier or reverse the conformational preference in these complexes. This may be accomplished by changing the electronic or steric properties of the ligands on the metal or the ethylene. Finally unsymmetrically substituted olefin complexes are examined. In the ML3 case the metal-carbon bond to the carbon bearing the weaker donor or weaker acceptor should be the stronger or shorter one. In the ML2 and ML4 complexes of ethylene the acceptor effect is accentuated, that of the donor less important. © 1979, American Chemical Society. All rights reserved.