. . . . . "R5 and R6 are preferably (C1 -C4)-alkyl, which may be halogenated, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl or trifluoromethyl, in particular methyl. ##STR4## ???BR11, ???AlR11, --Ge--, --Sn--, --O--, --S--, ???SO, ???SO2, ???NR11, ???CO, ???Pr11 or ???P(O)R11, where R11, R12 and R13 are identical or different and are a hydrogen atom, a halogen atom, a C1 -C10 -, preferably C1 -C4" . . . .