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 (C1P2)
in the presence of NaOH or NaNO3 result in the crystallization
of three structure types: RE­[CH2(PO3H0.5)2] (RE = La, Ce, Pr, Nd, Sm; Pu) (A type), NaRE­(H2O)­[CH2(PO3)2] (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy; Am) (B type), or NaLn­[CH2(PO3H0.5)2]·(H2O) (Ln = Yb and Lu) (C type). 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 A and 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 A favors a D2d dodecahedron
while B adopts a C2v geometry. In C, 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.