The reactions of LnCl 3 with molten boric acid result in the formation of Ln[B 4O 6(OH) 2Cl] (Ln = La-Nd), Ln 4[B 18O 25(OH) 13Cl 3] (Ln = Sm, Eu), or Ln[B 6O 9(OH) 3] (Ln = Y, Eu-Lu). The reactions of AnCl 3 (An = Pu, Am, Cm) with molten boric acid under the same conditions yield Pu[B 4O 6(OH) 2Cl] and Pu 2[B 13O 19(OH) 5Cl 2(H 2O) 3], Am[B 9O 13(OH) 4]•H 2O, or Cm 2[B 14O 20(OH) 7(H 2O) 2Cl]. These compounds possess three-dimensional network structures where rare earth borate layers are joined together by BO 3 and/or BO 4 groups. There is a shift from 10-coordinate Ln 3+ and An 3+ cations with capped triangular cupola geometries for the early members of both series to 9-coordinate hula-hoop geometries for the later elements. Cm 3+ 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.