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

[M(EtBTP) ][BPh ] ⋅3 CH CN (M=Nd, Am; EtBTP=2,6-bis(5,6-diethyl-1,2,4-triazin-3-yl)pyridine) have been synthesized from reactions of MCl ⋅n H O with EtBTP in acetonitrile followed by anion metathesis. Structural analysis reveals that these compounds contain M 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) ] complex that is absent in [Nd(EtBTP) ] .