La atom reactions with 1-pentene and 2-pentene are carried out in a laser-vaporization molecular beam source. The two reactions yield the same metal-hydrocarbon products from the dehydrogenation and carbon-carbon bond cleavage of the pentene molecules. The dehydrogenated species La(CH) is the major product, whereas the carbon-carbon bond cleaved species La(CH) and La(CH) are the minor ones. La(CH) is also observed and is presumably formed by La(CH) addition to a second pentene molecule. La(CH) and La(CH) are characterized with mass-analyzed threshold ionization (MATI) spectroscopy and quantum chemical computations. The MATI spectra of each species from the two reactions exhibit the same transitions. Adiabatic ionization energies and metal-ligand stretching frequencies are determined for the two species, and additional methyl bending and torsional frequencies are measured for the larger one. Five possible isomers are considered for La(CH), and a C metallacyclopentene (Iso A) is identified as the most possible isomer. La(CH) is confirmed to be a C metallacyclopropene. The ground electronic state of each species is a doublet with a La 6s-based electron configuration, and ionization yields a singlet state. The formation of the lanthanacyclopentene includes La addition to the C=C double bond, La insertion into two C(sp)-H bonds, and concerted dehydrogenation. For the 2-pentene reaction, the formation of the five-membered ring may also involve 2-pentene to 1-pentene isomerization. In addition to the metal addition and insertion, the formation of the three-membered metallacycle from 1-pentene includes C(sp)-C(sp) bond breakage and hydrogen migration from La to C(sp), whereas its formation from 2-pentene may involve the ligand isomerization.