Mono(cyclopentadienyl) and ansa-bis(cyclopentadienyl)imido derivatives of group 5 and 6 transition metals; crystal structures of [M{(eta-C5H4)CMe2(eta-C5H4)}(NR)X] (M = Nb, R = SiMe3, X = Cl; R = Bu-t, X = Cl, Br or I; M = Ta, R = Bu-t, X = Cl)

The new half-sandwich mono(cyclopentadienyl)tungsten compounds [W(eta-C5H4Me)(NBut)Cl-3] and [W(eta-C5H4Me)(NBut)(L)Cl] (L=eta-C2H4 or eta-MeC2Me) have been synthesized. In addition, the ansa-bis(cyclopentadienyl) Group 5 compounds [M{(eta-C5H4)CMe2(eta-C5H4)}(NR)X] (M=V, R=Bu-t, X=Cl; M=Nb, R=SiMe3, X=Cl;* R=Bu-t, X=Cl,* Br,* I* or Me; M=Ta, R=Bu-t, X=Cl* or Me) are reported. An asterisk indicates that the compound has been structurally characterized. Attempts to prepare the related dichloride compounds by replacement of the imido functionality in [M{(eta-C5H4)CMe2(eta-C5H4)}(NBut)Cl] (M=V, Nb or Ta) using HCl led to decomposition, suggestive of a labilising of the cyclopentadienyl rings by the strongly bonding imido ligand.