Cyclopentadienyl ligand substituent effects

The scope of the present paper is limited to those cyclopentadienyl ligands that contain more than two bulky substituents and transition metal complexes derived thereof in order to be able to focus on the specific effects of these ligand systems. A selection of some mono-substituted cyclopentadienyl ligands will be treated also. Among the numerous reviews highlighting special aspects of cyclopentadienyl... 

The electronic and steric effects of substituents on cyclopentadienyl ligands that affect the activity of the catalyst have been extensively and intensively... 

Fundamental studies have led to a detailed insight into the mechanism of the polymerization and the control of the microstructure through the substituents on the cyclopentadienyl ligands. For a survey of these studies and the dominant effects playing a role the reader is referred to recent contributions by Ewen [38] and Erker [48]. In the present chapter we will summarize a few of the features that play a role in determining the microstructure the field is still in development and obviously the definite answer to several questions has not yet been given. 

Saunders et al. have studied the G-G coupling in rhodium and iridium complexes that links fluorinated aromatic substituents on phosphine ligands to the methyl substituents of cyclopentadienyl ligands. These reactions involve HF elimination and can be induced by heat or base. The most effective bases are proton sponge (l,8-bis(dimethyl-amino)naphthalene) and BU4NF. One example is shown in Scheme ZZ ... 

In particular the last observation is easily understood by the molecular orbital picture given above. The 7z>orbitals of the monomers are required for a stabilization of the otherwise anti-bonding cluster orbitals of t2 symmetry which must accept six electrons. If this mixing is prevented because these orbitals adopt the electron density of the ligands, e.g., 7z>electrons of the side-on coordinated cyclopentadienyl groups, their contribution to the cluster stability is minimized or in particular cases the formation of clusters does not occur at all. Thus, the substituents attached terminally to the clusters strongly influence their stability by the different donor or acceptor capabilities. A further effect may result from the different steric demand of the substituents which will be discussed below. 

---