Group 4 metals cyclopentadienyl complexes

The five molecular 7r-orbitals of an isolated cyclopentadienyl group are constructed from the five 2/77i-atomic orbitals and are pictorially represented in Figure 22. Since in the bis-7r-cyclopentadienyl complexes, the metal atom... 

Kopf-Maier P, Kopf H (1988) Transition and Main-Group Metal Cyclopentadienyl Complexes Preclinical Studies on a Series of Antitumor Agnets of Different Structural... 

Viewed as a group, aU of the lanthanide cyclopentadienyl complexes show the behavior expected if the metal-ring interaction were purely electrostatic in nature. 

Two communications on propene polymerization by non-metallocene catalysts that include DFT/MM calculations have been recently published [60, 61]. They deal with group 4 bidentate non-cyclopentadienyl complexes. In the first communication [60], the topic addressed is the fact that a C2-symmetric precatalyst of titanium leads to a syndiotactic polymer, contrary to observations of metallocene catalysts. The chirality at the metal center is found to play a key role in the stereocontrol of the process. The second communication [61] addresses the fact that a C2-symmetric precatalyst of zirconium very similar to the previous one produces an isotactic polymer, finds out that it is due to a complicated concourse of synergic steric and electronic effects, and emphasizes the key role that serendipity still plays in the design of new catalysts. 

Germyl metallocene derivatives of the early transition metals, Ti, Zr and Hf (M) have been synthesized by the combination of metallocene halides and germyl anions (equations 4 and 5)38-40. The cyclopentadienyl complexes Cp2M(GePh3)Cl39 are difficult to purify due to their instability in solution however, use of the pentamethylcyclopen-tadienyl group(Cp ) enhances both the solubility and stability of these complexes40,41. 

A. R. Kudinov, M. I. Rybinskaya, Yu. T. Struchkov, A. I. Yanovskii, and P. V. Petrovskii, Synthesis of the First 30-Electron Triple-Decker Complexes of the Iron Group Metals with Cyclopentadienyl Ligands. X-Ray Structure of [(/ -C5H5)Ru (p,jy-C5Me5)Ru(jy-C5Me5)]PF6, J. Organomet. Chem. 336, 187-197 (1987). 

Considerable effort has been directed to the stereoselective synthesis of aimulated cyclopentadienyl complexes of the group 4 transition metals. Ci asymmetric and C2 synunetric aimulated cyclopentadienyl ligands have been used to prepare chiral organotitanium complexes, which are now being actively studied as catalysts for asymmetric synthesis and olefin polymerization see Asymmetric Synthesis by Homogeneous Catalysis). 

Transition-metal 7i-cyclopentadienyl complexes are known for all of the naturally occurring transition and inner transition metals and representative examples are grouped below according to their positions in the periodic chart. For the sake of brevity, most of the work involving monosubstituted cyclopentadienyl and mixed-sandwich (i.e., cyclopentadienyl-metalloboranes ) complexes has been excluded. 

The 7c-complexing ability of the carbon groups can affect the thermodynamics of Eq. (a). While 7t-complexation by alkenes and alkynes with main-group metals is weak, that with transition metals leads to stable n-complexes. With such transition-metal n-complexes as products, transmetallation can be favorable, e.g., in the reaction of cyclopentadienyldiethylalane with FeClj the cyclopentadienyl rather than the Et group is transferred ... 

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