prov:value
| - or R3 is a halogen atom or an NR2 2, PR2 2, B(OR2)2, SiR2 3 or SnR2 3 group, where R2 are identical or different and are each a C1-C20-hydrocarbon group, which may each bear one or more identical or different halogen, OH, OR2, SR2, NR2 2???, PR2 2???, ???SiR2 3 or ???OSiR2 3 substituents, or two radicals R2 may be joined to form a ring system, or R3 is a C1-C20-heterocyclic group which is bound v
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