(Butadien)(cyclopentadienyl)niob‐Komplexe

(Butadiene)(cyclopentadienyl)niobium Complexes The known bis(butadiene)(cyclopentadienyl)niobium complex (C 5 H 4 R)Nb(C 4 H 6 ) 2 ( 1 , RH) exists in solution as a 1 : 1 mixture of a bis( cis ‐butadiene) isomer 1a and a ( cis ‐butadiene)( trans ‐butadiene) isomer 1b . In solution the barrier to isomerisation is 80 ± 2 kJ/mol. New derivatives 2 (RMe) and 3 (RSiMe 3 ) of 1 are described. One of the butadiene ligands in 1 – 3 is readily substituted with CO, CN t Bu, P(OMe) 3 , and PMe 3 . CpNb(PMe 3 ) 2 (C 4 H 6 ) ( 7 ) reacts with disubstituted acetylenes to form acetylene complexes CpNb(PMe 3 )(C 4 H 6 )(RCCR') ( 8 : R, R'=Ph, 9 : RMe, R'=Ph, and 10 : RPh, R'=SiMe 3 ), The structure of 9 resembles that of bent metallocene derivatives with the acetylene acting as a 2‐e ligand.