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

Abstract

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 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–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 4f n 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.

High-Resolution Photoelectron Spectroscopy of the Ground and First Excited Electronic States of MgXe+

“…The photoionization spectrum of 24 Mg 132 Xe in the vicinity of the X + 2 Σ + (v + ) ← a 3 Π 0 (v″ = 0) ionizing transition of MgXe is compared with the PFI-ZEKE-PE spectrum of a natural sample of MgXe in Figure 2. The photoionization spectrum exhibits a sharp onset at the position corresponding to the adiabatic ionization energy of the a 3 Π 0 metastable state of 24 Mg 132 Xe [E I (v + = 0)/(hc) = 37,468.3(6) cm −1 , see below]. After a strong autoionization resonance at the position of the v + = 0 ionization threshold, the ion signal decreases.…”

“…The rotational constant for the a 3 Π 0 +/− state was taken from ref 3 as well as the 3 Π 0 + / 3 Π 0 − splitting of 1.46 cm −1 (the 0 − component lies below the 0 + one 3 ). The rotational constants of the X + state of 24 Mg 132 Xe + were obtained from the potential-energy function derived in this work (see Section 3.1.2). The vibrational and rotational constants of the other isotopomers were calculated using the appropriate mass-scaling laws.…”

“…The calculated spectra of all of the isotopomers were scaled according to their relative abundances and summed. The fitted band origins E I (v + )/ (hc) of 24 Mg 132 Xe, corrected for the field-induced shifts, are summarized in Table 1, which also lists the term values T(v + ) relative to the X + (0) level. Vibrational constants were derived for the X + ground state of 24 Mg 132 Xe + from the observed 24 Mg 132 Xe in the vicinity of the MgXe + X + 2 Σ + ←MgXe a 3 Π 0 ionization threshold.…”

“…The fitted band origins E I (v + )/ (hc) of 24 Mg 132 Xe, corrected for the field-induced shifts, are summarized in Table 1, which also lists the term values T(v + ) relative to the X + (0) level. Vibrational constants were derived for the X + ground state of 24 Mg 132 Xe + from the observed 24 Mg 132 Xe in the vicinity of the MgXe + X + 2 Σ + ←MgXe a 3 Π 0 ionization threshold. (b) Measured and (c) calculated overview PFI-ZEKE-PE spectra of the X + 2 Σ + (v + ) ←a 3 Π 0 (v″ = 0) ionizing transition of MgXe.…”

Organolanthanide Complexes Containing Ln–CH3 σ-bonds: Unexpectedly Similar Hydrolysis Rates for Trivalent and Tetravalent Organocerium

“…Since the isolation of the first π- and σ-bonded organometallic lanthanides in 1954 by Wilkinson and Birmingham, and in 1972 by Cotton et al, respectively, organolanthanides have found synthetic and catalytic relevance in methanol synthesis, polymerization, olefin hydrogenation, hydrosilylation, alkyne dimerization, and even metallobiochemistry. – The Ln–C bond has also become a focus of gas-phase studies, including laser-ablated lanthanide complexes probed via two-photon ionization or infrared spectroscopy. – Unsurprisingly, then, there are many reviews detailing aspects of organolanthanide bonding, spanning oxidation states II, III, and IV (Roman numerals implying formal oxidation states), as well as σ- and π-bonds. ,– Organolanthanides are categorically sensitive to air and moisture and involve electrostatic Ln–C bonds exhibiting fast ligand exchange (e.g., exchanging alkyls or cyclopentadienyl with halides in FeCl 3 and UCl 4 to form the more covalent Fe–C and U–C analogues or scrambling bridging and terminal alkyls intramolecularly). Thus, organolanthanides require steric bulk and coordinative saturation to stabilize molecular complexes.…”