The roles of 4f- and 5f-orbitals in bonding: a magnetochemical, crystal field, density functional theory, and multi-reference wavefunction study

Abstract

The electronic structures of 4f(3)/5f(3) Cp''3M and Cp''3M·alkylisocyanide complexes, where Cp'' is 1,3-bis-(trimethylsilyl)cyclopentadienyl, are explored with a focus on the splitting of the f-orbitals, which provides information about the strengths of the metal-ligand interactions. While the f-orbital splitting in many lanthanide complexes has been reported in detail, experimental determination of the f-orbital splitting in actinide complexes remains rare in systems other than halide and oxide compounds, since the experimental approach, crystal field analysis, is generally significantly more difficult for actinide complexes than for lanthanide complexes. In this study, a set of analogous neodymium(iii) and uranium(iii) tris-cyclopentadienyl complexes and their isocyanide adducts was characterized by electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility. The crystal field model was parameterized by combined fitting of EPR and susceptibility data, yielding an accurate description of f-orbital splitting. The isocyanide derivatives were also studied using density functional theory, resulting in f-orbital splitting that is consistent with crystal field fitting, and by multi-reference wavefunction calculations that support the electronic structure analysis derived from the crystal-field calculations. The results highlight that the 5f-orbitals, but not the 4f-orbitals, are significantly involved in bonding to the isocyanide ligands. The main interaction between isocyanide ligand and the metal center is a σ-bond, with additional 5f to π* donation for the uranium complexes. While interaction with the isocyanide π*-orbitals lowers the energies of the 5fxz(2) and 5fyz(2)-orbitals, spin-orbit coupling greatly reduces the population of 5fxz(2) and 5fyz(2) in the ground state.

DFT Investigations of the Magnetic Properties of Actinide Complexes

“…Several uranium-radical systems emerged in the mid-2000s as mononuclear complexes exhibiting magnetic properties [62,66,74,[129][130][131][158][159][160][161][162][163][164][165][166][167][168][169][170]. Structural, spectroscopic and magnetic properties of various mononuclear uranium(IV)-benzophenone radical complexes; e.g., the ketyl [(( tBu ArO) 3 tacn)U IV (OC• tBu Ph 2 )] complex (2) (Figure 26), were investigated by O.P.…”

CONDON 3.0: An Updated Software Package for Magnetochemical Analysis‐All the Way to Polynuclear Actinide Complexes

“…To emphasize the subtleties in the various approaches to model exchange‐coupled actinide systems, the basics are briefly recapitulated. From a computational point of view, most of the essential information has been given in multiple works concerning mononuclear and polynuclear 5 f ‐electron systems …”

“…The reason for this is that spin‐orbit coupling mixes the excited‐state wave functions into the ground state (2.6% 6 P 7/2 ) . In contrast, the ground terms for Cm III and Bk IV are only 78% and 74% of S 7/2 character as spin‐orbit coupling here mixes in substantial amounts of the 6 P 7/2 , 6 D 7/2 , and higher terms that result in a commensurately reduced μ eff value of 7.64 μ B or 7.59 μ B at room temperature.…”

Decomposition of d- and f-Shell Contributions to Uranium Bonding from the Quantum Theory of Atoms in Molecules: Application to Uranium and Uranyl Halides

“…The quantification of the covalent contribution to bonding in complexes of the f-elements is an area of great current research being explored via X-ray absorption [1][2][3][4][5][6][7][8][9][10][11], electron paramagnetic [12][13][14][15], nuclear magnetic resonance [16,17], emission [18,19], and photoelectron [20] spectroscopies as well as X-ray diffraction [21] and structural studies [22][23][24][25][26][27][28][29][30]. The latter have often been carried out in combination with theoretical studies and a wealth of purely theoretical data also exists .…”