The reactions of phosphido complexes [Nb(eta(5)-C(5)H(4)SiMe(3))(2)(L)(PPh(2))] [L = CO (1), CNxylyl (2), CNCy (3)] with alkynes have been carried out. The new diphenylphosphinoalkenyl niobocene complexes [Nb(eta(5)-C(5)H(4)SiMe(3))(2)(eta(1)-C-C(CO(2)CH(3))=C(R)PPh(2))(CO)] [R = H (4), CH(3) (5)] and [Nb(eta(5)-C(5)H(4)SiMe(3))(2)(eta(1)-C-C(CO(2)R)=C(CO(2)R)PPh(2))(CO)] [R = CH(3), (6), R = (t)Bu, (7)] were successfully synthesized by the reaction of with methyl propiolate (HC[triple bond]CCO(2)CH(3)) or methyl 2-butynoate (CH(3)C[triple bond]CCO(2)CH(3)) and dimethyl 2-butynedioate [(CH(3) O(2)C)C[triple bond]C(CO(2)CH(3))] or di(tert-butyl) 2-butynedioate [((t)BuO(2)C)C[triple bond]C(CO(2)(t)Bu)], respectively. However, reaction was not observed with more electron-rich alkynes. Complex reacted with methyl propiolate, methyl 2-butynoate (MeC[triple bond]CCO(2)Me) or di(tert-butyl) 2-butynedioate to give surprising new heteroniobacycle complexes [Nb(eta(5)-C(5)H(4)SiMe(3))(2)(eta(1)-C-C(=NXylyl)C(R(1))=C(R(2))PPh(2)-kappa(1)-P)] [R(1) = H, R(2) = CO(2)Me (8); R(1) = Me, R(2) = CO(2)Me (9); R(1) = CO(2)(t)Bu, R(2) = CO(2)(t)Bu (10)]. Finally, the phosphido complexes and reacted with phenylacetylene (PhC[triple bond]CH) to give new diphenylphosphinoalkenyl niobocene derivatives [Nb(eta(5)-C(5)H(4)SiMe(3))(2)(eta(1)-C-C(C(6)H(5))=C(H)PPh(2))(CNR)] [R = xylyl (11), Cy (12)]. All of these compounds were characterized by NMR spectroscopy and the molecular structure of was determined by single-crystal X-ray diffraction studies. Theoretical studies were also carried out by means of density functional theory (DFT) calculations on the insertions of alkynes into the Nb-P bond in the phosphido niobocenes.