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  • 14822 Phys. Chem. Chem. Phys., 2012, 14, 1482214831 This journal is c the Owner Societies 2012

    Cite this: Phys. Chem. Chem. Phys., 2012, 14, 1482214831

    Electronic structure and bonding of lanthanoid(III) carbonatesw

    Yannick Jeanvoine,a Pere Miro,b Fausto Martelli,a Christopher J. Cramer*b andRiccardo 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 metalligand

    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.15 Such studies have primarily focused on

    lanthanoids in their 3+ oxidation state, which are important

    in nuclear waste remediation and medical imaging.68 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 Ln3+ carbonate hydrates are

    available for tri-carbonate ligands,11 and for Nd(III) Runde

    et al.12 have suggested the formation of a [Nd(CO3)4H2O]5

    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).1315

    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

    metalligand bonding. Another recent theoretical contribution in

    this area was a report by Sinha et al. on [Nd(CO3)4]5 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-potentials20 have been particularly

    useful in this regard. In the present study, we focus on tri- and

    tetracarbonates ([Ln(CO3)3]3 and [Ln(CO3)4]

    5, 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

    aUniversite dEvry Val dEssonne, CNRS UMR 8587 LAMBE,Bd F. Mitterrand, 91025 Evry Cedex, France.E-mail:

    bDepartment of Chemistry, Supercomputing Institute, and ChemicalTheory Center, University of Minnesota, 207 Pleasant St. SE,Minneapolis, MN 55455-0431, USA. E-mail:

    w Electronic supplementary information (ESI) available. See DOI:10.1039/c2cp41996c

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  • This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 1482214831 14823

    ionic radius for these two elements gives rise to a difference

    in hydration number (9-fold vs. 8-fold for La and Lu,

    respectively).21,22 Ln(III)-aquo interactions have been deter-

    mined to be mainly electrostatic in nature, as one might expect

    given the hard characters of both Ln(III) ions and water. As

    such, the variation in ionic radius is the main physical quantity

    that affects hydration properties.22,23 The fact that ionic radii

    can dictate the complexation properties has also been pointed

    out for the case of ligands that are potentially less hard than

    water, like hexacyanoferrate.24 Nevertheless, carbonates are

    softer ligands than water, and it is also possible that the

    metalligand interaction may change across the spectrum of

    the lanthanoid series. The difference between La and Lu offers

    insight into the extrema for the whole series if the interaction is

    mainly electrostatic and/or if the contribution of 4f orbitals is

    negligible to Ln/carbonate interaction. This last situation is to

    be expected since 4f orbitals are compact around lanthanoids

    and rarely invoked as contributing to valence bonding; indeed

    this behavior rationalizes the key role that ionic radius plays in

    dictating interactions with water as a ligand.25 As we will show

    in the present study, this is indeed the case for carbonate as

    well and thus the difference between La and Lu complexes

    does likely span the lanthanoid spectrum.

    We study differences in Ln-carbonate interactions as a

    function of the lanthanoid, focusing on the number and

    coordination geometries of the carbonate ligands. The influ-

    ence of aqueous solvation has been included through the use

    of implicit solvation methods, which are useful for predicting

    the electrostatic component that dominates the free energies of

    solvation for these highly charged species. Finally, topological

    analysis of the electron density and examination of valence

    natural orbitals are undertaken to address the nature of the

    various Ln-carbonate bonds.

    2. Computational details

    All geometries were fully optimized at the density functional

    theory level with the Gaussian 03 electronic structure program

    suite26 using the hybrid three parameter functional incorpor-

    ating Becke exchange and LeeYangParr correlation, also

    known as B3LYP.27 For La and Lu atoms, we have used the

    energy-consistent pseudopotentials (ECP) of the Stuttgart/

    Cologne group which are semi-local pseudopotentials adjusted

    to reproduce atomic valence-energy spectra.28,29 Amongst the

    available pseudopotentials, we have chosen the ECP28MWB

    small core with 28 core electrons, multi electron fit (M) and

    quasi relativistic reference data (WB) and we have used the

    ECP28MWB_SEG basis set for La and Lu. For carbon and

    oxygen atoms, we employ the 6-31+G(d) basis set and we have

    checked, by exploring the [LnCO3]+ energy surface, the utility

    of this basis (increasing the basis set to near triple zeta

    6-311+G(d), adding polarization functions 6-311++G(3df),

    or going to the still more complete basis set aug-cc-pVTZ all

    failed to significantly change the character of the surface (see

    Fig. S1 in ESIw)). Integral evaluation made use of the griddefined as ultrafine in the Gaussian 03 program. The natures

    of all stationary points were verified by analytic computa-

    tion of vibrational frequencies. Aqueous solvation effects were

    included with the PCM continuum solvation model.30


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