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.