The known bis(butadiene)(cyclopentadienyl)niobium complex (C5H4R)Nb(C4H& (1, R = H) exists in solution as a 1: 1 mixture of a bis(cis-butadiene) isomer 1 a and a (cis-butadiene)(transbutadiene) isomer 1 b. In solution the barrier to isomerisation is 80 f 2 kJ/mol. New derivatives 2 (R = Me) and 3 (R = SiMe3) of 1 are described. One of the butadiene ligands in 1 -3 is readily substituted with CO, CNfBu, P(OMe)3, and PMe3. CpNb(PMe3),(C4H6) (?) reacts with disubstituted acetylenes to form acetylene complexes CpNb(PMe3)(C,HG)(RC = CR') ( 8 R, R' = Ph, 9 R = Me, R' = Ph, and 10: R = Ph, R = SiMe3). The structure of 9 resembles that of bent metallocene derivatives with the acetylene acting as a 2-e ligand.(Butadien)(cyclopentadienyl)metall-Komplexe friiher dMetalle waren in den letzten Jahren Gegenstand intensiver systematischer Erforschung*). Schwerpunkte lagen bei Komplexen des Typs C~,Zr(butadien)~), CpTa(butadien);) und C~Mo(NO)(butadien)~). Komplexe des Typs CpNb(butadienh sind weniger gut bekannt6
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