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Treatment of Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (Cl)(L) (1) with Mg(CtCR) 2 in toluene, under appropriate reaction conditions, leads to the alkynyl complexes Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (CtCR)(L) (2: L ) CO, R ) Ph (2a); LThe alkynyl-containing niobocene species 2 can be chemically or electrochemically oxidized to give the corresponding cation-radical alkynyl complexes [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (CtCR)(L)] •+ [BPh 4 ] -(3: L ) CO, R ) Ph (3a); L ) CO, R ) t Bu (3c); L ) PMe 2 Ph, R ) Ph (3d)). These complexes, under different experimental conditions, give rise to the mononuclear vinylidene d 2 niobocene species [Nb(η 5 -C 5 H 4 -SiMe 3 ) 2 (dCdCHR)(L)][BPh 4 ] (4: L ) CO, R ) Ph (4a); L ) CO, R ) t Bu (4c); L ) PMe 2 Ph, R ) Ph (4d)) with a hydrogen atom by abstraction from the solvent or, for 3a, the binuclear divinylidene d 2 niobocene complex [(η 5 -C 5 H 4 SiMe 3 ) 2 (CO)NbdCdC(Ph)(Ph)CdCdNb(CO)(η 5 -C 5 H 4 SiMe 3 ) 2 ][BPh 4 ] 2 (4a′) from a competitive ligand-ligand coupling process. Complexes 4 were also prepared by an alternative procedure in which the corresponding complexes 2 were reacted with HBF 4 . Finally, in solution the CO-containing vinylidene mononuclear complexes 4a and 4c undergo an unexpected isomerization process to give the η 2 -alkyne derivatives [Nb(η 5 -C 5 H 4 SiMe 3 ) 2 (η 2 (C,C)-HCtCR)(CO)] + (5: R ) Ph (5a); R ) t Bu (5c)). The structure of 5a was determined by single-crystal diffractometry. DFT calculations were carried out on [NbCp 2 (dCdCHCH 3 )(L)] + /[NbCp 2 (HCtCCH 3 )(L)] + (Cp ) η 5 -C 5 H 5 ; L ) CO, PH 3 ; exo, endo) model systems in order to explain the η 1 -vinylidene-η 2 -alkyne rearrangement observed. Calculations have shown that in both carbonyl-niobocene and phosphine-niobocene systems the η 1vinylidene and the η 2 -alkyne complexes are isoenergetic, in marked contrast with the systems previously considered in theoretical studies. The reaction takes place through an intraligand 1,2-hydrogen shift mechanism where η 2 (C,H)-alkyne species are involved. The energy barrier for the isomerization process in the phosphine-containing niobocene systems is almost 10 kcal mol -1 higher than in the analogous process for the carbonyl-containing niobocene system. This increase in activation barrier indicates that the different experimental behavior between 4a, 4c, and 4d has a kinetic rather than a thermodynamic origin. Finally, the interconversion between exo and endo isomers has been studied.