Probing the Chemical Bond between Lanthanides and Carbon: CeC, PrC, NdC, LuC, and TmC

Resonant two-photon ionization experiments have been conducted to probe the bond dissociation energy (BDE) of the lanthanide-carbon bond, allowing the BDEs of CeC, PrC, NdC, LuC, and Tm-C2 to be measured to high precision. Values of D0(CeC) = 4.893(3) eV, D0(PrC) = 4.052(3) eV, D0(NdC) = 3.596(3) eV, D0(LuC) = 3.685(4) eV, and D0(Tm-C2) = 4.797(6) eV are obtained. Additionally, the adiabatic ionization energy of LuC was measured, giving IE(LuC) = 7.05(3) eV. The electronic structure of these species, along with the previously measured LaC, has been further investigated using quantum chemical calculations. Despite LaC, CeC, PrC, and NdC having ground electronic configurations that differ only in the number of 4f electrons present and have virtually identical bond orders, bond lengths, fundamental stretching frequencies, and metallic oxidation states, a peculiar 1.30 eV range in bond dissociation energies exists for these molecules. A natural bond orbital analysis shows that the metal atoms in these molecules have a natural charge of +1 with a 5d2 4fn 6s0 configuration while the carbon atom has a natural charge of -1 and a 2p3 configuration. The diabatic bond dissociation energies, calculated with respect to the lowest energy level of this separated ion configuration, show a greatly reduced energy range of 0.32 eV, with the diabatic BDE decreasing as the amount of 4f character in the σ-bond increases. Thus, the wide range of measured BDEs for these molecules is a consequence of the variation in atomic promotion energies at the separated ion limit. TmC2 has a smaller BDE than the other LnC2 molecules, due to the tiny amount of 5d participation in the valence molecular orbitals.