Key questions for the study of chemical bonding in actinide compounds are the degree of covalency that can be realized in the bonds to different donor atoms and the relative participation of 5f and 6d orbitals. A manifold of theoretical approaches is available to address these questions, but hitherto no comprehensive assessments are available. Here, we present an in-depth analysis of the metal–ligand bond in a series of actinide metal–organic compounds of the [M­(salen)2] type (M = Ce, Th, Pa, U, Np, Pu) with the Schiff base N,N′-bis­(salicylidene)­ethylenediamine (salen). All compounds except the Pa complex (only included in the calculations) have been synthesized and characterized experimentally. The experimental data are then used as a basis to quantify the covalency of bonds to both N- and O-donor atoms using simple electron-density differences and the quantum theory of atoms in molecules (QTAIM) with interacting quantum atoms. In addition, the orbital origin of any covalent contributions was studied via natural population analysis (NPA). The results clearly show that the bond to the hard, charged O-donor atoms of salen is consistently not only stronger but also more covalent than bonds to the softer N-donor atoms. On the other hand, in a comparison of the metals, Th shows the most ionic bond character even compared to its 4f analogue Ce. A maximum of the covalency is found for Pa or Np by their absolute and relative covalent bond energies, respectively. This trend also correlates with a significant f- and d-orbital occupation for Pa and Np. These results underline that only a comprehensive computational approach is capable of fully characterizing the covalency in actinide complexes.

Two series of isostructural tetravalent actinide amidinates [AnX((S)-PEBA) 3 ] (An = Th, U, Np; X = Cl, N 3 ) bearing the chiral (S,S)-N,N′-bis(1-phenylethyl)benzamidinate ((S)-PEBA) ligand have been synthesized and thoroughly characterized in solid and in solution. This study expands the already reported tetravalent neptunium complexes to the lighter actinides thorium and uranium. Furthermore, a rare Ce(IV) amidinate [CeCl((S)-PEBA) 3 ] was synthesized to compare its properties to those of the analogous tetravalent actinide complexes. All compounds were characterized in the solid state using singlecrystal XRD and infrared spectroscopy and in solution using NMR spectroscopy. Quantum chemical bonding analysis including also the isostructural Pa and Pu complexes was used to characterize the covalent contributions to any bond involving the metal cation. Th shows the least covalent character throughout the series, even substantially smaller than for the Ce complex. For U, Np, and Pu, similar covalent bonding contributions are found, but a natural population analysis reveals different origins. The 6d participation is the highest for U and decreases afterward, whereas the 5f participation increases continuously from Pa to Pu.

Tetradentate N2O2-type Schiff base complexes with tetravalent 4f- and 5f-block metals, [M(salpn)2] (H2salpn = N,N′-disalicylidene-1,3-diaminopropane; M = Ce, Th, U, Np, and Pu), were characterised both in the solid state and in solution.

This study elucidates the mutual influence of the interaction of ISA with UO22+ on their speciation, based on spectroscopic techniques.

Complexation by small organic ligands controls the bioavailability of contaminants and influences their mobility in the geosphere. We have studied the interactions of Cm3+, as a representative of the trivalent actinides, and Eu3+, as an inactive homologue, with glucuronic acid (GlcA) a simple sugar acid. Time-resolved laser-induced luminescence spectroscopy (TRLFS) shows that complexation at pH 5.0 occurs only at high ligand to metal ratios in the form of 1:1 complexes with standard formation constants log β0 = 1.84 ± 0.22 for Eu3+ and log β0 = 2.39 ± 0.19 for Cm3+. A combination of NMR, QMMM, and TRLFS reveals the structure of the complex to be a half-sandwich structure wherein the ligand binds through its carboxylic group, the ring oxygen, and a hydroxyl group in addition to five to six water molecules. Surprisingly, Y3+, which was used as a diamagnetic reference in NMR, prefers a different coordination geometry with bonding through at least two hydroxyl groups on the opposite side of a distorted GlcA molecule. QMMM simulations indicate that the differences in stability among Cm, Eu, and Y are related to ring strain induced by smaller cations. At higher pH a stronger complex was detected, most likely due to deprotonation of a coordinating OH group.