Origin of Selectivity of a Triazinyl Ligand for Americium(III) over Neodymium(III).

[M(EtBTP)3 ][BPh4 ]3 ⋅3 CH3 CN (M=Nd, Am; EtBTP=2,6-bis(5,6-diethyl-1,2,4-triazin-3-yl)pyridine) have been synthesized from reactions of MCl3 ⋅n H2 O with EtBTP in acetonitrile followed by anion metathesis. Structural analysis reveals that these compounds contain M3+ cations bound by tridentate EtBTP ligands to create a tricapped trigonal prismatic geometry around the metal centers. Collection of high-resolution, single-crystal X-ray diffraction data also allowed reduction in bond lengths esd's, such that a slight contraction of Δ=0.0158(18) Å in the Am-N versus Nd-N bond lengths was observed, even though these cations ostensibly have matching ionic radii. Theoretical evaluation revealed enhanced metal-ligand bonding through back donation in the [Am(EtBTP)3 ]3+ complex that is absent in [Nd(EtBTP)3 ]3+ .