14822 Phys. Chem. Chem. Phys., 2012, 14, 14822–14831 This journal is c the Owner Societies 2012 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 14822–14831 Electronic structure and bonding of lanthanoid(III) carbonatesw Yannick Jeanvoine, a Pere Miro´, b Fausto Martelli, a Christopher J. Cramer* b and Riccardo Spezia* a Received 14th June 2012, Accepted 31st July 2012 DOI: 10.1039/c2cp41996c Quantum chemical calculations were employed to elucidate the structural and bonding properties of La(III) and Lu(III) carbonates. These elements are found at the beginning and end of the lanthanoid series, respectively, and we investigate two possible metal-carbonate stoichiometries (tri- and tetracarbonates) considering all possible carbonate binding motifs, i.e., combinations of mono- and bidentate coordination. In the gas phase, the most stable tricarbonate complexes coordinate all carbonates in a bidentate fashion, while the most stable tetracarbonate complexes incorporate entirely monodentate carbonate ligands. When continuum aqueous solvation effects are included, structures having fully bidentate coordination are the most favorable in each instance. Investigation of the electronic structures of these species reveals the metal–ligand interactions to be essentially devoid of covalent character. 1. Introduction The hydration properties of lanthanoids (Ln) in aqueous solution have been widely studied both experimentally and theoretically. 1–5 Such studies have primarily focused on lanthanoids in their 3+ oxidation state, which are important in nuclear waste remediation and medical imaging. 6–8 In the context of nuclear waste, these ions are relevant because of the challenge associated with separating them from actinide ions (An). 9 Ln(III) ions in deposited nuclear waste are expected to interact with carbonate as a counterion in so far as the presence of carbonates in geological media is ubiquitous. Interestingly, reliance on differential lanthanide-carbonate interactions has been proposed as a possible separation procedure for Ln(III) and An(III) ions in solution. 10 Consequently, the characterization of lanthanoid carbonate structures is central to understanding how lanthanoid ions will behave in aqueous solutions with available carbonate counterions that may act as supporting ligands. Crystallographic data for Ln 3+ carbonate hydrates are available for tri-carbonate ligands, 11 and for Nd(III) Runde et al. 12 have suggested the formation of a [Nd(CO 3 ) 4 H 2 O] 5À structure at high carbonate concentrations. Recently Philippini et al. have studied several Ln(III)-carbonate complexes in solution using electrophoretic mobility measurements and time- resolved laser-induced fluorescence spectroscopy (TRLFS). 13–15 They concluded that light Ln(III) ions coordinate four carbonate ligands while heavier ones coordinate only three ligands. In contrast, considering available crystallographic and spectroscopic data (including UV-vis, near infrared, and infrared), Janicki et al. concluded that in aqueous solution all Ln(III) ions form tetra- carbonates when carbonate ions are not limited. 16 These authors also performed a set of theoretical calculations that suggest that there is partial charge transfer between the Ln(III) ion and the carbonate ligand that introduces a degree of covalency to the metal–ligand bonding. Another recent theoretical contribution in this area was a report by Sinha et al. on [Nd(CO 3 ) 4 ] 5À using the Parameterized Model 3 (PM3) semi-empirical method. 17 Notwithstanding these two studies, no systematic, quantitative theoretical study has been undertaken in order to characterize the structures and bonding of lanthanoid(III) tri- and tetra- carbonates, while, e.g., such kinds of studies were performed on actinyl carbonate complexes. 18,19 Among the questions that remain open: (i) what is the coordination geometry of the carbonate ligands for Ln(III) complexes in water?; (ii) which stoichiometry dominates? and (iii) what is the degree of ionic vs. covalent bonding for the Ln(III)-carbonate interaction? Electronic structure methods, and in particular density- functional theory (DFT), have proven to be valuable tools for the study of heavy elements. Increasingly accurate lantha- noid and actinoid pseudo-potentials 20 have been particularly useful in this regard. In the present study, we focus on tri- and tetracarbonates ([Ln(CO 3 ) 3 ] 3À and [Ln(CO 3 ) 4 ] 5À , respectively) considering the Ln(III) ions lanthanum (La) and lutetium (Lu). As these two elements begin and end the lanthanoid series, respectively, they should establish limiting behavior with respect to forming complexes with carbonates. In aqueous solution with non-coordinating counterions, the difference in a Universite ´ d’Evry Val d’Essonne, CNRS UMR 8587 LAMBE, Bd F. Mitterrand, 91025 Evry Cedex, France. E-mail: riccardo.spezia@univ-evry.fr b Department of Chemistry, Supercomputing Institute, and Chemical Theory Center, University of Minnesota, 207 Pleasant St. SE, Minneapolis, MN 55455-0431, USA. E-mail: cramer@umn.edu w Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cp41996c PCCP Dynamic Article Links www.rsc.org/pccp PAPER Published on 01 August 2012. Downloaded by Princeton University on 28/01/2015 19:34:06. 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