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 AI2 (CH3)6' The results indicate that 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 cyclopentadienylligands, especially for those containing a dimethylsilyl bridge. Only one methyl group is transferred to a zirconocene dichloride even by a large excess of AI2(CH3 )6' while the uptake of two alkyl groups, followed by alkane evolution, is observed with triethyl and triisobutyl aluminum.