The complexes (Hal)Nb(CO) 3(PR 3) 3 (PR 3 PEt 3, Hal I; PR 3 PMe 2Ph, Hal Cl, Br, I) and (Hal)Nb(CO) 4/2(dppe) 1/2 (Hal Br, I) have been prepared by oxidative halogenation of carbonylniobate with pyridinium halides (Hal Cl, Br) or iodine (Hal I). In the tricarbonyls, one CO and one PR 3 are labile and can be displaced by a four-electron donating alkyne to give all-trans-[(Hal)Nb(CO) 2(RCCR′)(PR 3) 2] (PR 3 = PMe 2Ph; Hal Cl, Br, I: R, R′ H, Et, Ph; R H, R′ Ph. PR 3 PEt 3, Hal I: R, R′ Pr; R H, R′ Bu, Ph; R Me, R′ Et). In the case of acetylene, INb(CO)(HCCH) 2(PEt 3) 2 is also formed. PR 3 can be displaced by P(OMe) 3. In the tetracarbonyls, two CO ligands are replaced by two isonitriles to form INb(CO) 2(CNR) 2dppe (R tBu, Cy), or by one alkyne to form (Hal)Nb(CO) 2(PhCCPh)dppe (Hal Br, I). In these complexes, the remaining CO ligands occupy cis positions. The structure of BrNb(CO) 2(dppe) 2·THF, INb(CO) 2(dppe) 2·hexane and INb(CO) 2(PEt 3) 2(MeCCEt) have been determined by a single crystal X-ray diffraction study. The alkyne complexes are best regarded as octahedral with the centre of the alkyne ligand occupying the positions trans to the halide and the C C axis aligned with the OCNbCO axis. The complexes (Hal)Nb(CO) 2(dppe) 2 adopt a trigonal prismatic structure with the halide capping the tetragonal face spanned by the four phosphorus functions. The crystal structure of a by-product, Br 2Nb(CO)(H 2CPhPCH 2CH 2PPh 2) 2·1/2THF has also been determined. The geometry is pentagonal bipyramidal, with one of the bromine atoms and the CO on the axis. Some 93 Nb NMR data for the Nb I complexes are presented, and preliminary observations on the reactions between the π-alkyne complexes and H 2 or H − are reported.