Differentiating between Trivalent Lanthanides and Actinides

The reactions of LnCl₃ with molten boric acid result in the formation of Ln­[B₄O₆(OH)₂Cl] (Ln = La–Nd), Ln₄[B₁₈O₂₅(OH)₁₃Cl₃] (Ln = Sm, Eu), or Ln­[B₆O₉(OH)₃] (Ln = Y, Eu–Lu). The reactions of AnCl₃ (An = Pu, Am, Cm) with molten boric acid under the same conditions yield Pu­[B₄O₆(OH)₂Cl] and Pu₂[B₁₃O₁₉(OH)₅Cl₂(H₂O)₃], Am­[B₉O₁₃(OH)₄]·H₂O, or Cm₂[B₁₄O₂₀(OH)₇(H₂O)₂Cl]. These compounds possess three-dimensional network structures where rare earth borate layers are joined together by BO₃ and/or BO₄ groups. There is a shift from 10-coordinate Ln³⁺ and An³⁺ cations with capped triangular cupola geometries for the early members of both series to 9-coordinate hula-hoop geometries for the later elements. Cm³⁺ is anomalous in that it contains both 9- and 10-coordinate metal ions. Despite these materials being synthesized under identical conditions, the two series do not parallel one another. Electronic structure calculations with multireference, CASSCF, and density functional theory (DFT) methods reveal the An 5f orbitals to be localized and predominately uninvolved in bonding. For the Pu­(III) borates, a Pu 6p orbital is observed with delocalized electron density on basal oxygen atoms contrasting the Am­(III) and Cm­(III) borates, where a basal O 2p orbital delocalizes to the An 6d orbital. The electronic structure of the Ce­(III) borate is similar to the Pu­(III) complexes in that the Ce 4f orbital is localized and noninteracting, but the Ce 5p orbital shows no interaction with the coordinating ligands. Natural bond orbital and natural population analyses at the DFT level illustrate distinctive larger Pu 5f atomic occupancy relative to Am and Cm 5f, as well as unique involvement and occupancy of the An 6d orbitals