Ethyl-, n-propyl- and [(trimethylsilyl)methyl]niobium (4e)-alkyne complexes Tp*Nb(Cl)(CH(2)R)(PhC=CR’) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate; R = Me, R’ = Me (2a), Et (2b), n-Pr (2c); R = Et, R’ = Me (3a), Et (3b); R = SiMe(3), R’ = Me (4a)) exhibit alpha-agostic structures in solution (H-1 and C-13 NMR) and in the solid state (X-ray structure at 173 K for 4a). The phenylpropyne complexes 2a, 3a, and 4a undergo a thermally induced rearrangement which yields niobium methyl complexes Tp*Nb(Cl)(Me)(PhC=CEt), Tp*Nb(Cl)(Me)(PhC=C-n-Pr), and Tp*Nb(Cl)(Me)(PhC=CCH(2)SiMe(3)), respectively. This reaction follows a first-order rate law. The rate decreases with increased steric bulk of the alkyl group in the starting complex. For 2a, a full kinetic analysis yields a high enthalpic barrier and an entropy of activation close to zero. Heating a toluene solution of Tp*Nb(Cl)(n-Pr)-(PhC=CEt) (3b) establishes a 1:1 equilibrium between 3b itself and Tp*Nb(Cl)(Et)(PhC=C-n-Pr) (2c). Heating a solution of Tp*Nb(Cl)(Et)(PhC=CEt) (2b) and PhC=CMe yields the eta(1)-alkenyl (4e)-phenylpropyne complex Tp*Nb(Cl)(eta(1)-CPh=CEt(2))(PhC=CMe) (6), demonstrating that alkyl migration is a key step of the rearrangement. Under pseudo-first-order conditions, this reaction does not depend on the phenylpropyne concentration. Potential 2e-donor ligands fail to act as efficient traps. Accordingly, a successful trapping experiment involves dinitrogen extrusion from N3P(N-i-Pr-2)(2) in the presence of 2b which yields an unprecedented (phosphinoimido)niobium complex : Tp*Nb(Cl)(eta(1)-CPh=CEt(2))[=N-P(N-i-Pr-2)(2)] (7) The crystal structure of 7 confirms the eta(1)-alkenyl formulation and reveals a formal Nb-N triple bond, a linear Nb-N-P link, and a pyramidal P. Deoxygenation of propene oxide and desulfurization of ethylene sulfide gives, respectively, the oxo and sulfide eta(1)-alkenyl complexes Tp*Nb(Cl)(eta(1)-CPh=CEt(2))(=X) (X = 0 (8), S (9)). The observed thermal rearrangements are unique examples of reversible migratory insertion/beta-alkyl elimination in transition metal alkyl alkyne complexes. In the absence of efficient trapping agent, the unsaturated eta(2)-alkenyl intermediate that is first generated reversibly ring opens and ring closes. This realizes the intramolecular C-C bond activation which leads to the observed alkyl switch. Kinetic and trapping data indicate that the migratory insertion of the alkyl group on to the alkyne is the rate-determining step of the whole process.