Periodic Trends in Lanthanide and Actinide Phosphonates: Discontinuity between Plutonium and Americium

The hydrothermal reactions of trivalent lanthanide and actinide chlorides with 1,2-methylenediphosphonic acid (<b>C1P2</b>) in the presence of NaOH or NaNO₃ result in the crystallization of three structure types: RE­[CH₂(PO₃H_0.5)₂] (RE = La, Ce, Pr, Nd, Sm; Pu) (<b>A type</b>), NaRE­(H₂O)­[CH₂(PO₃)₂] (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy; Am) (<b>B type</b>), or NaLn­[CH₂(PO₃H_0.5)₂]·(H₂O) (Ln = Yb and Lu) (<b>C type</b>). These crystals were analyzed using single crystal X-ray diffraction, and the structures were used directly for detailed bonding calculations. These phases form three-dimensional frameworks. In both <b>A</b> and <b>B</b>, the metal centers are found in REO₈ polyhedra as parts of edge-sharing chains or edge-sharing dimers, respectively. Polyhedron shape calculations reveal that <b>A</b> favors a <i>D</i>_2<i>d</i> dodecahedron while <b>B</b> adopts a <i>C</i>_2<i>v</i> geometry. In <b>C</b>, Yb and Lu only form isolated MO₆ octahedra. Such differences in terms of structure topology and coordination geometry are discussed in detail to reveal periodic deviations between the lanthanide and actinide series. Absorption spectra for the Pu­(III) and Am­(III) compounds are also reported. Electronic structure calculations with multireference methods, CASSCF, and density functional theory, DFT, reveal localization of the An 5f orbitals, but natural bond orbital and natural population analyses at the DFT level illustrate unique occupancy of the An 6d orbitals, as well as larger occupancy of the Pu 5f orbitals compared to the Am 5f orbitals