The reactions of phosphido complexes [Nb(η5-C5H4SiMe3)2(L)(PPh2)] [L = CO (1), CNxylyl (2), CNCy (3)] with alkynes have been carried out. The new diphenylphosphinoalkenyl niobocene complexes [Nb(η5-C5H4SiMe3)2(η1-C-C(CO2CH3)=C(R)PPh2)(CO)] [R = H (4), CH3 (5)] and [Nb(η5-C5H4SiMe3)2(η1-C-C(CO2R)=C(CO2R)PPh2)(CO)] [R = CH3, (6), R = tBu, (7)] were successfully synthesized by the reaction of 1 with methyl propiolate (HC≡CCO2CH3) or methyl 2-butynoate (CH3C≡CCO2CH3) and dimethyl 2-butynedioate [(CH3≡O2C)C≡C(CO2CH3)] or di(tert-butyl) 2-butynedioate [(tBuO2C)C≡C(CO2tBu)], respectively. However, reaction was not observed with more electron-rich alkynes. Complex 2 reacted with methyl propiolate, methyl 2-butynoate (MeC≡CCO2Me) or di(tert-butyl) 2-butynedioate to give surprising new heteroniobacycle complexes [Nb(η5-C5H4SiMe3)2(η1-C-C(=NXylyl)C(R1)=C(R2)PPh2-κ1-P)] [R1 = H, R2 = CO2Me (8); R1 = Me, R2 = CO2Me (9); R1 = CO2tBu, R2 = CO2tBu (10)]. Finally, the phosphido complexes 2 and 3 reacted with phenylacetylene (PhC≡CH) to give new diphenylphosphinoalkenyl niobocene derivatives [Nb(η5-C5H4SiMe3)2(η1-C-C(C6H5)=C(H)PPh2)(CNR)] [R = xylyl (11), Cy (12)]. All of these compounds were characterized by NMR spectroscopy and the molecular structure of 8 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.
