Orientation of Coordinated Olefins

A coordinated olefin is often most stable in one orientation, relative to the other ligands at the metal. In many cases, the barrier to rotation about the metal-olefin bond axis can be measured by solution NMR methods. The rotational barriers (10-25 kcal/mol) arise from a combination of steric and electronic effects. 

A cr-bond lacks a nodal plane along the bond axis. One would expect, therefore, an absence of an electronic preference for orientation about the metal-olefin axis in a complex in which the olefin acts only as a cr-donor. In contrast, one would expect an electronic preference for the orientation of an olefin to a metal center that acts as a cr-donor and a ir-acceptor. This preferred orientation would place the olefin in position to maximize overlap between its empty tt orbital of the olefin and a filled d-orbital on the metal. 

Orientation of olefins in d trigonal planar, f trigonal-bipyramidal, and d trigonal bipyramidal complexes. 

Orbital diagram to predict the conformation of olefins in trigonal bipyramidal complexes. 

Infrared spectroscopy of olefin complexes is a less useful probe of n-bonding than infrared spectroscopy of CO complexes. Binding of an olefin to an electron-rich metal center does reduce the C-C stretching frequency, as one would expect from the reduction of the C-C bond order due to Ti-backbonding. However, the C-C stretch of a coordinated olefin is weaker than that of coordinated carbon monoxide because the vibration of the olefin creates a smaller change in the dipole moment. (Recall that symmetric vibrations are not observed in the infrared spectrum because of a lack of change in the dipole moment.) Thus, the olefin stretch is weak and lies at a frequency that overlaps with other bands. 

---

Several complexes arose due to different orientations of coordinated olefin and reacting group were characterized in the theoretical study of ethylene insertion in rhodium complexes (Fig. 4) [40]. The calculations were carried out for the model system using [Rh(ri -CH2=CH2)(Z)(PMe3)] complex, where Z = CH3, NH2, and OH. 

---