Probing the Chemical Bond between Lanthanides and Carbon: CeC, PrC, NdC, LuC, and TmC2

Resonanttwo-photon ionization experiments have been conductedto probe the bond dissociation energy (BDE) of the lanthanide-carbonbond, allowing the BDEs of CeC, PrC, NdC, LuC, and Tm-C-2 to be measured to high precision. Values of D (0)(CeC) = 4.893(3) eV, D (0)(PrC)= 4.052(3) eV, D (0)(NdC) = 3.596(3) eV, D (0)(LuC) = 3.685(4) eV, and D (0)(Tm-C-2) = 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 investigatedusing quantum chemical calculations. Despite LaC, CeC, PrC, and NdChaving ground electronic configurations that differ only in the numberof 4f electrons present and have virtually identical bond orders,bond lengths, fundamental stretching frequencies, and metallic oxidationstates, a peculiar 1.30 eV range in bond dissociation energies existsfor these molecules. A natural bond orbital analysis shows that themetal atoms in these molecules have a natural charge of +1 with a5d(2) 4f( n ) 6s(0) configurationwhile the carbon atom has a natural charge of -1 and a 2p(3) configuration. The diabatic bond dissociation energies, calculatedwith respect to the lowest energy level of this separated ion configuration,show a greatly reduced energy range of 0.32 eV, with the diabaticBDE decreasing as the amount of 4f character in the sigma-bond increases.Thus, the wide range of measured BDEs for these molecules is a consequenceof the variation in atomic promotion energies at the separated ionlimit. TmC2 has a smaller BDE than the other LnC(2) molecules, due to the tiny amount of 5d participation in the valencemolecular orbitals. Thefigure on the left illustrates how the observation ofa sharp predissociation threshold is used to measure the bond dissociationenergies of the LnC and Tm-C-2 molecules. The figureon the right illustrates the 4f orbital character on Tm combiningwith the 2 sigma(g) orbital of C-2, leading tothe transfer of electron density into the otherwise vacant 2 sigma(g) bonding orbital of the C-2 unit.

Automated summary

Resonant two-photon ionization experiments were conducted to measure the bond dissociation energy (BDE) of lanthanide-carbon bonds. Quantum chemical calculations were used to investigate the electronic structure of these compounds. Despite having similar ground electronic configurations, LaC, CeC, PrC, and NdC exhibit a peculiar range of 1.30 eV in their BDEs.