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The Creutz-Taube complex [(NH3)5-Ru-pyrazine-Ru-(NH3)5]5+ is studied using wave function-based methods, namely the CASSCF/MS-CASPT2 method. Spin-orbit effects have been calculated with the SO-RASSl program. The nature of the ground state is analyzed, and all the excited states up to 50 000 cm-1 are calculated. They form a quasi continuum from 25 000 cm-1, theoretical bands are assigned to UV-visible spectra, and MCD bands are assigned by calculating transition moments from first principles for both spectroscopies, g-Factors are calculated from first principles and modeled by a model Hamiltonian: they compare well to experimental values and are shown to be the same as in the monomeric species. [ABSTRACT FROM AUTHOR]

Actinide +IV complexes with six nitrates [An IV (NO 3 ) 6 ] 2- (An = Th, U, Np, and Pu) have been studied by 15 N and 17 O NMR spectroscopy in solution and first-principles calculations. Magnetic susceptibilities were evaluated experimentally using the Evans method and are in good agreement with the ab initio values. The evolution in the series of the crystal field parameters deduced from ab initio calculations is discussed. The NMR paramagnetic shifts are analyzed based on ab initio calculations. Because the cubic symmetry of the complex quenches the dipolar contribution, they are only of Fermi contact origin. They are evaluated from first-principles based on a complete active space/density functional theory (DFT) strategy, in good accordance with the experimental one. The ligand hyperfine coupling constants are deduced from paramagnetic shifts and calculated using unrestricted DFT. The latter are decomposed in terms of the contribution of molecular orbitals. It highlights two pathways for the delocalization of the spin density from the metallic open-shell 5f orbitals to the NMR active nuclei, either through the valence 5f hybridized with 6d to the valence 2p molecular orbitals of the ligands, or by spin polarization of the metallic 6p orbitals which interact with the 2s-based molecular orbitals of the ligands.

The crystal field parameters are determined from first-principles calculations in the [An III (DPA) 3 ] 3- series, completing previous work on the [Ln III (DPA) 3 ] 3- and [An IV (DPA) 3 ] 2- series. The crystal field strength parameter follows the Ln(III) < An(III) < An(IV) trend. The parameters deduced at the orbital level decrease along the series, while J-mixing strongly impacts the many-electron parameters, especially for the Pu(III) complex. We further compile the available data for the three series. In some aspects, An(III) complexes are closer to Ln(III) than to An(IV) complexes with regard to the geometrical structure and bonding descriptors. At the beginning of the series, up to Pu(III), there is a quantitative departure from the free ion, especially for the Pa(III) complex. The magnetic properties of the actinides keep the trends of the lanthanides; in particular, the axial magnetic susceptibility follows Bleaney's theory qualitatively.

Actinide +IV complexes (An IV = Th IV , U IV , Np IV , and Pu IV ) with two dipicolinic acid derivatives (DPA and Et-DPA) have been studied by 1 H and 13 C NMR spectroscopies and first-principles calculations. The Fermi contact and dipolar contributions to the actinide-induced shifts (AIS) are evaluated from a temperature dependence analysis, combined with ab initio results. It allows an experimental estimation of the axial anisotropy of the magnetic susceptibility Δχ ax and of the hyperfine coupling constants of the NMR-active nuclei. Due to the compactness of the coordination sphere, the magnetic anisotropy of the paramagnetic center is small, and this makes the contact contribution to be the dominant one, even on the remote atoms. The sign of the hyperfine coupling constants and related spin densities is alternating on the nuclei of the ligand cycle, denoting a preponderant spin polarization mechanism. This is well reproduced by unrestricted density functional theory (DFT) calculations. Those values are furthermore slightly decreasing in the actinide series, which indicates a small decrease of the covalency from U IV to Pu IV .

The monomer [Ce(COT) 2 ] - and the dimer [Ce 2 (COT) 3 ], with Ce(III) and COT = 1,3,5,7-cyclooctatetraenide, are studied by quantum chemistry calculations. Due to the large spin-orbit coupling, the ground state of the monomer is a strong mixing of σ and π states. The experimental isotropic coupling in the dimer was evaluated by Walter et al. to be J = -7 cm -1 (with a Heisenberg Hamiltonian [Formula: see text]) with a small anisotropic coupling of 0.02 cm -1 . The coupling between the two Ce(III) in the dimer is calculated using CI methods. The low energy part of the spectra are modeled by spin Hamiltonians. All spin Hamiltonians parameters are deduced from ab initio calculations. g factors are calculated for both the pseudodoublet of the monomer and the pseudotriplet of the dimer and their sign have been determined. The magnetic coupling in the dimer is rationalized by a model based on crystal field theory. The kinetic and exchange contributions arising from the different configurations to the isotropic and anisotropic couplings are evaluated. It is shown that the main contribution to isotropic coupling is kinetic and originates from the f σ -f σ interaction due to the large transfer integral between those orbitals. However, the f π -f π interaction plays a non-negligible role. The anisotropic coupling originates from the difference of exchange energy of states arising from the f σ f π configuration and is, in no matter, related to the anisotropy of the local magnetic moments as already pointed by van Vleck for a fictitious s-p system. The analysis of the natural orbitals evidences a superexchange mechanism through a σ CH * orbital of the bridging cycle favored by a local 4f σ /5d σ hybridization and that the δ type orbitals, both the HOMOs of the ligands and the virtual f δ orbitals of the cerium atoms play an important polarization role, and to a less extend the π type orbitals, the HOMOs-1 of the ligands, and the metal f π orbitals.