Metallocenes structural energetics

Looking at C2-symmetric catalysts, there exist in principle two different metallocene-olefin complexes (Fig. 4). Structure a is energetically favored because the methyl group of the propene is in trans-position relative to the (3-methyl structure of the polymer chain. Both methyl groups in structure b are in cis position to each... 

Using the parent zirconocene-butadiene complex as a representative example, a typical bonding situation in these types of molecules is presented in Scheme 48. For 297, equilibration between the s-trans and the s-cis isomers occurs with a barrier of 23 kcal mol 1 at 283 K. The 72-olefin complex is believed to be a high-energy intermediate on the interconversion reaction surface. Significantly, structural data indicates that the s-cis complexes are best described as Zr(iv) compounds with a er2, ir ligand.158,175 The dynamic NMR measurements have also been extended to ansa-zirconocene and hafnocene butadiene complexes.176 Moreover, photoelectron spectroscopy has been used to determine the relative energetics of the two isomers for // -metallocenes.177... 

The stereorigid bridged metallocene dichloride 1, as depicted in Fig. 2, is a prochiral molecule and possesses bilateral symmetry. The activatirm of metallocene dichloride 1 leads to the formation of enantiomeric metallocenium-monoaUcyl cationic species that are both electron-deficient and coordinatively unsaturated and structurally very similar to the enantiomeric molecule depicted in Fig. 3. These enantiomeric metallocenium-monoalkyl species have the necessary vacant coordination position and energetically low lying, accessible empty fragment orbitals to be used for coordination and activation of incoming propylene molecules via interaction with their available n orbitals. 

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