Alkyl exchange between aluminum trialkyls and zirconocene dichloride complexes - a measure of electron densities at the Zr center

Equilibrium constants have been determined for the formation of 15 different zirconocene monomethyl monochloro complexes from the respective dichloride complexes and Al-2(CH3)(6). The results indicate chat the tendency of a particular zirconocene complex to exchange a chloride against a methyl ligand is a measure for the electron deficiency at its Zr center. The results indicate particularly low electron densities for complexes with indenyl instead of cyclopentadienyl ligands, especially for those containing a dimethylsilyl bridge. Only one methyl group is transferred to a zirconocene dichloride even by a large excess of Al-2(CH3)(6), while the uptake of two alkyl groups, followed by alkane evolution, is observed with triethyl and triisobutyl aluminum. (C) 1998 Elsevier Science S.A.