Comprehensive Bonding Analysis of Tetravalent f-Element Complexes of the Type [M(salen)₂]

Abstract

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.

Covalency in actinide(iv) hexachlorides in relation to the chlorine K-edge X-ray absorption structure

“…No clear variation is seen in the MBOs, which remain about the same from U to Pu (about 0.64 and 1.04 with MCWFT and KST/PBE0 respectively). Additionally, we noted that the positive NPA metal charge is lowest at U (Table S3), in line with the NPA charge trend found in other complexes of the tetravalent actinides, 125 underlining that there is likely more pronounced donation bonding in [UCl 6 ] 2compared to the other hexachlorides. Thus, the takeaway from the NPA and BO analyses is that the An-Cl covalency peaks at [UCl 6 ] 2and remains about the same, perhaps slightly decreasing, towards [PuCl 6 ] 2-.…”

“…Note that An IV covalency that is not genuinely increasing in complexes with An ranging from Th/Pa-Pu is has been assigned in other complex series as well, e.g. An(C 8 H 8 ) 2 , 137,138 An(C 5 H 5 ) 4 , 139,140 An(salen) 2 , 125 and An( Ar acnac) 4 . 141 We conclude this subsection by noting that the appearance of the core RAS 𝑡 2𝑢 and 𝑡 1𝑢 natural orbitals of Figure S4, becoming more localized and contracted towards Pu, reflect the slightly decreasing trend in 5f covalency.…”

“…Calculated SF GS configurations (cfg.) with various approaches, f-shell (n f ) and d-shell (n d ) populations from NPA/Mulliken analyses, and Mayer/Wiberg bond orders (MBO/WBO) for [AnCl 6 ] 2À (An ¼ Th-Pu) Following ref 125. and 126, n f counts 5f and 6f AO populations (see TableS2).…”

How 5 f Electron Polarisability Drives Covalency and Selectivity in Actinide N‐Donor Complexes

“…Moreover, the linear trends in Figure 2 indicate the expected predominantly ionic bonds AnÀ N pyrrolide and AnÀ N imine . [7] A more profound investigation of the AnÀ N bond character can however only be based on additional quantum chemical data.…”

“…A prominent backbonding effect from Pa to the ligand-based orbitals leads to enhanced bond strength and increased covalent character in [Pa(salen) 2 ], whereas for the other metal centres similar bond characteristics could be observed. [7] However, in the salen system An bonding was dominated by the strong, hard O-donor, which ultimately limits the flexibility of the AnÀ N bond to react to changes in electronic situation, for example, caused by backbonding. Studies of pure N donors remain scarce, especially for the transuranium elements.…”