Ethylene complexes rotation

Fig. 3. Important valence orbitals of some metal fragments. The energy scale markings are eV. (Reprinted, with permission, from Ethylene Complexes, Bonding, Rotational Barriers, and Conformational Preferences, Albright, T. A. et al. J. Am. Chem. Soc. 101, 3802, Fig. 1, copyright, 1979, by the American Chemical Society)...

Fig. 5 Selected geometric parameters (A) of the optimized ri -s/n,ri (C ) transition-state structure TSefc[2] for allylic enantioface conversion and of the optimized rotational q -syn,r (C ) transition-state structure TSiso[3] for allylic isomerization occurring in the [Ni (octadienediyl)(ethylene)] complex. Activation free energies (kcal mob ) are given relative to the [Ni"(q -syn,q (C ),A-cis,-octadienediyl)(ethylene)] isomer of 2...

A similar series of five-coordinate rhodium(I) molecules has been examined. [RhCl(CO)(i7 -C2H4)(biL)] (biL = diimine) are trigonal bipyramidal with axial Cl and CO in CDCI3. The ethylene (which rotates on the nmr time scale) is readily lost. The related compounds [RhCl(CO)2(biL)] and [RhCl(PF3)2(biL)] also contain trans-axial Cl and CO or Cl and PF3. Intramolecular exchange of the CO sites is rapid, however, suggesting an easier pseudorotation type of motion for these complexes. 

In order to account for the large positive entropy of activation for the reaction, it is necessary to assume that there is virtually free rotation of the incipient ethylene molecules in the complex. The second possible transition complex involves the complete rupture of one carbon-carbon bond to give the tetramethylene biradical, and the reaction path may be envisaged as shown below ... 

These results can be summarized as follows The triplet carbene ( 5i) adds nonstereospecifically because its complex and a ground state ethylene correlate with the triplet state of an excited trimethylene configuration, which has no barriers to rotation around terminal bonds. 

The perpendicular orientation of the alkene in such complexes is favored because it maximizes the overlap of the bond with the LUMO (dx2 — y2, Figure 13.7) and minimizes 4e repulsive interactions with the HOMO (ndz2). The in-plane orientation is not expected to be strongly disfavored, however, because of the secondary interaction between the orbital and the dxy orbital. The rotational barrier of ethylene in Zeise s anion was theoretically estimated to be 55 kj/mol [282], within the range 42-63 kj/mol measured by NMR for related complexes [286]. 

---