A general now metal-mediatad [5 + 2] cycloadclition reaction of eta(5)-pentadienyl cobalt complexes and alkynes is reported, the first such cycloaddition reaction to provide controlled incorporation of 1 equiv of alkyne. The reaction of terminally substituted pentadienyl! complexes proceeds thermally under exceptionally mild conditions, affording cycloheptadianyl complexes cleanly and in high yield. As a consequence of the unusually low kinetic barrier to the cycloaddition, reactions with acetylene at low temperature produce the cycloheptadienyl ring system exclusively as the eta(2),eta(3)-T,isomer, an unprecedented coordination mode for unbridged seven-membered ring complexes. lsomerization of the kinetic product to the fully conjugated eta(5)-cycloheptadienyl isomer is observed quantitatively upon heating. Disubstituted alkynes, as represented by 2-butyne, proceed through the eta(2),eta(3)-intermediate to the thermodynamic eta(5)-complex, but the rate of initial alkyne incorporation is attenuated to the extent that isomerization is competitive with the cycloaddition, preventing isolation of the eta(2),eta(3)-intermediates. The reaction with terminal alkynes is relatively insensitive to modest steric or electronic influences, providing mixtures of regioisomeric products. Consistent with these results, a dissociative mechanism is proposed, initiated by the unusually facile eta(5)->eta(3) isomerization of the pentadienyl ligand. The pentadienyl complexes themselves arise from protonolysis of an organic 1,4-pentadien-3-ol in the presence of the labile Co(l) complex, (eta(5)-pentamethylcyclopentadieno)cobalt(ethylene)(2), suggesting that the reaction can be considered a novel "interrupted Nazarov" cyclization, amenable to further development in an explicitly organic context.