A compound of the formula I as claimed in claim 1, in which, in formula I,M1 is zirconium or hafnium, R1 and R2 are identical and are a C1 -C3 -alkyl group or a halogen atom, the radicals R3 are identical and are a C1 -C4 -alkyl group, R4 to R20 are identical or different and are hydrogen or are a C1 -C4 -alkyl group and ##STR15## in which M2 is silicon and R12 and R13 are identical or different a