η5-Cyclopentadienyl-stabilized metallacyclopentane and -cycloheptane derivatives of chromium have been isolated, and their chemical behavior lends support to a metallacyclic mechanism for the Cr-catalyzed selective trimerization of ethylene to 1-hexene. In the presence of MAO, the amino-substituted cyclopentadienyl−chromium derivatives show a remarkable activity for the catalytic polymerization of ethylene.

Me 2 NC 2 H 4 C 5 Me 4 Li reacts with Cr(THF) 3 Cl 3 to give (η 1 :η 5 -Me 2 NC 2 H 4 C 5 Me 4 )CrCl 2 , in which the complexation of the N-donor atom to the metal atom has been confirmed by X-ray crystallography. A series of related compounds, e.g. (η 1 :η 5 -cyclo-C 4 H 8 NSiMe 2 OSiMe 2 C 5 H 4 )-CrCl 2 , has been prepared by varying the substituents on the organic ligand. Further reaction with organomagnesium reagents leads to formation of the corresponding dialkyl-Cr complexes. Related species have been prepared containing imine-, alkoxy-, and alkylthiosubstituted cyclopentadienyl groups as well as the C-donor ligand tetramethylimidazol-2ylidene. Treatment of these compounds with methylalumoxane (MAO) leads to the formation of highly active catalysts for the oligomerization, polymerization, and copolymerization of ethylene.

A series of phosphinoalkyl-substituted cyclopentadienylchromium complexes, e.g., (η 1 :η 5 -R 2 PC 2 H 4 C 5 H 4 )CrCl 2 , have been synthesized. The complexation of the P atom to the chromium has been confirmed by crystal structure determinations. Activated by methylalumoxane (MAO), these compounds catalyze the oligomerization and polymerization of ethylene. The catalytic activity and the degree of oligomerization are controlled by the steric properties of the substituents on the donor atom: the smaller the substituent, the greater the activity and the lower the molecular weight of the oligomer. Density functional calculations indicate that propagation and termination through β-hydrogen transfer are competitive for small substituents and that the latter process, which involves spin inversion, becomes selectively disfavored with increasing size of the substituents. For large substituents β-hydrogen elimination to the chromium atom is found to be the preferred route for termination, although this reaction is less facile than insertion.

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