Electronic States and Metal–Ligand Bonding of Gadolinium Complexes of Benzene and Cyclooctatetraene

Cation Complexes of Uranium and Thorium with Cyclooctatetraene: Photochemistry and Decomposition Products

Ion-molecule complexes of uranium or thorium singly-charged positive ions bound to cyclooctatetraene (COT), i.e., M⁺(COT)_1,2, are produced by laser ablation and studied with UV laser photodissociation. The ions are selected by mass and excited at 355 or 532 nm, and the ionized dissociation products are detected using a reflectron time-of-flight mass spectrometer. The abundant fragments M⁺(C₆H₆), M⁺(C₄H₄), and M⁺(C₂H₂) occur for complexes of both metals, whereas the M⁺(C₄H₂), M⁺(C₃H₃), and M⁺(C₅H₅) fragments are prominent for uranium complexes but not for thorium. Additional experiments investigate the dissociation of M⁺(benzene)_1,2 ions which may be intermediates in the fragmentation of the COT ions. The experiments are complemented by computational quantum chemistry to investigate the structures and energetics of fragment ions. Various cation-π and metallacycle structures are indicated for different fragment ions. The metal ion-ligand bond energies for corresponding complex ions are systematically greater for the thorium species. The computed thermochemistry makes it possible to explain the mechanistic details of the photochemical fragmentation processes and to reveal new actinide organometallic structures.

Low-Energy Photoelectron Imaging Spectroscopy of Lan(benzene) (n = 1 and 2)

La_n(benzene) (n = 1 and 2) are formed in a pulsed laser-ablation molecular beam source and characterized by low-energy photoelectron imaging spectroscopy. The photoelectron spectrum of La₂(benzene) displays a strong origin band, a short metal-ligand stretching progression, and a weak ring deformation band. Four isomers are considered for La₂(benzene), and the preferred structure is an inverse sandwich with two La atoms residing on the opposite sides of the benzene ring. The ground electronic state of the inverse sandwich is ¹A_1g (D_3d) with (5d_xy,x²-y² + π*)⁴6s² electron configuration. Ionization removes a La-based 6s electron and yields a ²A_1g ion. The spectrum of La(benzene) is similar to the zero-electron kinetic energy spectrum reported previously by our group, although the spectral line width is somewhat broader. The measurement of the photoelectron angular distribution of La(benzene) confirms that the ejected electron has largely a p wave character. The metal-ligand bonding of La₂(benzene) is considerably stronger than that of La(benzene) due to the threefold binding of each La atom in the dilanthanum species and the twofold binding in the monolanthanum complex.

Theoretical Study on the Photoelectron Spectra of Ln(COT)2-: Lanthanide Dependence of the Metal-Ligand Interaction

We have performed a theoretical analysis of the recently reported photoelectron (PE) spectra of the series of sandwich complex anions Ln(COT)₂^- (Ln = La-Lu, COT = 1,3,5,7-cyclooctatetraene), focusing on the Ln dependence of the vertical detachment energies. For most Ln, the π molecular orbitals, largely localized on the COT ligands, have the energy order of e_1g < e_1u < e_2g < e_2u as in the actinide analogues, reflecting the substantial orbital interaction with the Ln 5d and 5p orbitals. Thus, it would be expected that the lanthanide contraction would increase the orbital interaction so that the overlaps between the COT π and Ln atomic orbitals tend to increase across the series. However, the PE spectra and theoretical calculations were not consistent with this expectation, and the details have been clarified in this study. Furthermore, the energy level splitting patterns of the anion and neutral complexes have been studied by multireference ab initio methods, and the X peak splittings observed in the PE spectra only for the middle-range Ln complexes were found to be due to the specific interaction between the Ln 4f and ligand π orbitals of the neutral complexes in e_2u symmetry. Because the magnitude of this 4f-ligand interaction depends critically on the final state 4f electron configuration and the spin state, a significant Ln dependence in the PE spectra is explained.

Hexahapto-lanthanide interconnects between the conjugated surfaces of single-walled carbon nanotubes

Organometallic bonding of lanthanide metals to the graphene sidewalls in single-walled carbon nanotube networks enhances the electrical conductivity, particularly in the case of Sm and Eu.