Spectroscopic characterization of lanthanum-mediated dehydrogenation and C-C bond coupling of ethylene

La(C₂H₂) and La(C₄H₆) are observed from the reaction of laser-vaporized La atoms with ethylene molecules by photoionization time-of-flight mass spectrometry and characterized by mass-analyzed threshold ionization spectroscopy. La(C₂H₂) is identified as a metallacyclopropene and La(C₄H₆) as a metallacyclopentene. The three-membered ring is formed by concerted H₂ elimination and the five-membered cycle by dehydrogenation and C-C bond coupling. Both metallacycles prefer a doublet ground state with a La 6s-based unpaired electron. Ionization of the neutral doublet state of either complex produces a singlet ion state by removing the La-based electron. The ionization allows accurate measurements of the adiabatic ionization energy of the neutral doublet state and metal-ligand and ligand-based vibrational frequencies of the neutral and ionic states. Although the La atom is in a formal oxidation state of +2, the ionization energies of these metal-hydrocarbon cycles are lower than that of the neutral La atom. Deuteration has a small effect on the ionization energies of the two cyclic radicals but distinctive effects on their vibrational frequencies.