7 of the Periodic Table of the Elements; optionally two R7 groups can also form an aliphatic ring having 3 to 8 atoms which can bear alkyl substituents, preferably said alkyl substituents are C1???C10-alkyl radicals; L is preferably CR7 2 or SiR7 2; preferably R7 is a hydrogen atom or a methyl or phenyl radical; more preferably L is Si(CH3)2 or C(CH3)2;