After generation of Fe(CO)(3) by 308-nm gas-phase photolysis of Fe(CO)(5), 1-pentene adds to Fe-(CO)(3) to form Fe(CO)(3)(eta(2)-1-pentene) with a bimolecular rate constant of k(a) = (4 +/- 1) x 10(-10) cm(3) molecule(-1) (-1). Rapid beta-hydrogen transfer, by way of intramolecular C-H bond insertion to form HFe(CO)(3)(eta(3)-C5H9), follows rate-limiting addition of 1-pentene to Fe(CO)(3) and proceeds with a lower bound of k(1) greater than or equal to 10(9) s(-1) Under experimental conditions, HFe(CO)(3)(eta(3)-C5H9) decays on a millisecond time scale with concurrent formation of Fe(CO)(3)(eta(2)-pentene)(2) by addition of 1-pentene to an Fe(CO)(3)(eta(2)-pentene) intermediate. It is Fe(CO)(3)(eta(2)-pentene) that is in equilibrium with HFe(CO)(3)(eta(3)-C5H9) that adds 1-pentene to form Fe(CO)(3)(eta(2)-pentene), which may contain an isomerized olefin. CO may add to Fe(CO)(3)(eta(2)-pentene) that is in equilibrium with HFe(CO)(3)(eta(3)-C5H9) to form Fe(CO)(4)(eta(2)-pentene). Fe(CO)(4)(eta(2)-pentene) remains stable on the time scale of catalytic turnover and its formation serves as a termination pathway for thermal catalysis. This system is compared to the analogous propene system (Long, G. T.; Wang, W.; Weitz, E. J. Am. Chern. Sec. 1995, 117, 12810). The major difference in behavior between these systems is attributed to an similar to 3 orders of magnitude shift in the equilibrium constant toward HFe(CO)(3)(pi-allyl) relative to Fe(CO)(3)(olefin) when the starting olefin is l-pentene instead of propene. The magnitude of the equilibrium constants indicates that there is an similar to 4 kcal mol(-1) greater enthalpy difference between HFe(CO)(3)(eta(3)-C5H9) and Fe(CO)(3)(eta(2)-pentene) than for the corresponding species in the propene system.