L is preferably a substituted or unsubstituted cyclopentadienyl group, T is preferably R2 2 B!n, where B is carbon, silicon, germanium or tin and the radicals R2 are identical or different and are each hydrogen or a C1 -C30 -hydrocarbon radical such as C1 -C10 -alkyl or C6 -C14 -aryl, and n is equal to 1, 2, 3 or 4.