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 R12 are identical or different and are hydrogen or a C1-C4-alkyl group, and R13 is where M2 is silicon or germanium and R14 and R15 are identical or different and are a C1-C4-alkyl group or a C6-C10-aryl group. 6.