ReviewTheoretical insights into the chemical bonding in actinide complexes
Graphical abstract
Introduction
Because of the specific electronic configuration and unique chemical properties, the chemistry of the actinides thus emerges as a very multifold, lively, and scientifically challenging field of inorganic chemistry. As put by Pyykkö [1] “Theoretical chemistry could be seen as a bridge from the real physics of the physicists to the real chemistry of the experimental chemists. We hence expect that any measurable property of any chemical object could, in principle, be calculated to arbitrary accuracy, if the relevant physical laws are known.” The theoretical study of the chemical properties of actinide element complexes still remains a challenging task despite recent developments in relativistic quantum theory, computational algorithms, and techniques. One particular concern for theoretical work is that the large number of competing highly correlated electronic states are difficult to capture with computationally tractable methods.
Actinides are strong electron acceptors and can be considered as hard acids as defined by Pearson [2]. Consequently, they tend to interact with strong electron donors. The question of covalency in complexes of the 4f and 5f elements has been a source of intense research and controversy. In addition to academic interest in this debate, there is an industrial motivation for better understanding of bonding in f-element complexes with the aim to separate trivalent actinide from trivalent lanthanide in advanced nuclear fuel cycles [3]. This is a challenging task because of their similar charge and chemical behavior, e.g., with the hard oxygen donor ligands used in conventional separation processes. Hence, the nature of the chemical interactions in f-element complexes is a topic of considerable interest. The understanding and the interpretation of the chemical bond in terms of covalent and electrostatic (or ionic) interatomic interactions is fundamental in f-element chemistry. In the literature, the discussion is frequently based on electronegativities of the atoms, or on calculated atomic partial charges. Frenking et al. [4] have showed clearly that the use of the latter can be misleading and an estimate of the electrostatic contribution to a chemical bond necessitates a more detailed analysis of the interatomic interactions. The covalent bond in actinide compounds is usually analyzed by inspecting shape and occupation of the orbitals or by calculating bond orders which are based on orbital overlap and occupation numbers. However, this may not give a definite answer because the choice of the partitioning method may strongly influence the result and, even qualitatively, different answers may be found [4]. These last years, progress has been made in the development of methods which give insights into the nature of the metal–ligand bonding. The molecular orbital theory suggested by Dewar 60 years ago can be quantitatively supported by quantum chemical topology and energy decomposition analyses.
There have been several recent reviews of actinide chemistry and physics [3], [5], [6], [7], [8], [9]. Morss et al. surveyed many of the experimental issues associated with the actinide in The Chemistry of the Actinide and Transactinide Elements [10].
In the following sections, the discussion is focused to the theoretical tools useful to investigate the nature of chemical bond in actinide complexes and reviews several applications and what has been learned from them. Relativistic effects can strongly influence the chemical and physical properties of heavy elements and their compounds. Here, this aspect will not be discussed, but these effects have been taken into account in the calculations considered in Section 3. Detailed informations can be found in recent publications, reviews and references therein [1], [11], [12], [13], [14], [15], [16].
Section snippets
Molecular wavefunction analysis
The concept of partial atomic charge and bond order has been utilized since the early years of computational chemistry for analyzing chemical bonding. Several popular orbital based techniques may be used including Mulliken population analysis [17], [18], [19], [20], [21], [22], Löwdin partitioning [23], [24] and Natural Population Analysis (NPA) [25], [26].
In the Mulliken population analysis, the electrons are distributed into the atomic basis functions. This method partitions the D · S matrix
Actinide minor partitioning
Actinide recycling by separation and transmutation is considered worldwide as one of the most promising strategies to reduce the inventory of radioactive waste [124]. A liquid–liquid extraction and treatment strategy is currently implemented [125]. One of the most challenging task is the partitioning of trivalent actinides, Am3+ and Cm3+ from trivalent lanthanides. The two series of ions are both hard lewis acids, with similar charges and ionic radii. Few years ago, it was discovered that
Concluding remarks
In this review, we have discussed different ways to analyze the nature of chemical bond in actinide compounds. These methods are well established for organic, inorganic or organometallic compounds. However, their applications for actinide bonding remain sparse and original. Coverage has been extensive but not exhaustive. Indeed, the joint and complementary charge, orbital, quantum chemical topology and energy decomposition analyses are very powerful tools to understand chemical bonding in the
Acknowledgements
The author gratefully acknowledges Dr. Carine Clavaguéra for a careful reading of the manuscript and helpful discussions. This work was granted access to the HPC resources of [CCRT/CINES/IDRIS] under the allocation c2013086146 made by GENCI (Grand Equipement National de Calcul Intensif).
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