The half-sandwich methylimido complexes [Nb(q-C,R,)(NMe) CI,] (R = H l a , or M e l b ) have been prepared by treatment of [Nb(q-C,R,) CI,] with NMe(SiMe,), in acetonitrile. The sterically hindered alkyland aryl-imido analogues [Nb(q-C,H,)(NR)CI,] (R = But lc, or C6H,Pri2-2,6 Id), [Nb(q-C,Me,)-(N-C6H,Pri,-2,6)CI,] l e and the tantalum compound [Ta(q-C,Me,)(N-C,H,Pri,-2,6)C12] 1 f are obtained by treatment of [M(q-C,R,)CI,] with t w o equivalents of NHR(SiMe,) in chlorocarbon solvent. Crystal structures of l a , lc, I d and I f show that these complexes are mononuclear with quasi-linear imido ligands. The metal-nitrogen bond distances range from 1.744(3) t o 1.780(5) A consistent with pseudo triple bonds. Compounds la-le react with tertiary phosphines t o give eighteen-electron adducts of the type [Nb(q-C,H,)(NR)CI,(PR',)] 2a-2e. The crystal structure of [Nb(q-C,H,)(NMe)CI,(PMe,)]shows a distorted four-legged piano-stool geometry in which the phosphine binds cis to the methylimido and cis t o one of the chloride ligands. Alkoxide and aryloxide derivatives of the type [Nb(q-C,R,) (NR')(OR"),] may be prepared via reactions of the dichlorides 1 with 2 equivalents of LiOR" in Et,O. Fenske-Hall quantumchemical calculations indicate that there exists a close relationship between the Nb(q-C,H,) (NR) fragment and the bent metallocene moiety M(T~-C,H,)~ for the Group 4 elements.