Actinide(III) and lanthanide(III) complexes with nitrogen ligands: Counterions and ligand substituent effects on the metal-ligand bond

Relativistic density functional theory calculations are reported for actinide(III) (An=U, Am, Cm) and lanthanide(III) (Ln=La, Nd) complexes with tridentate aromatic nitrogen ligands. Calculations have been performed on 2,2′:6′,2″-terpyridine (Terpy) complexes with various species in the first coordination sphere of the metal ion besides one Terpy ligand (H2O, Cl− and NO3−) and on bis(1,2-4-triazin-3-yl)pyridine (Btp) complexes with various susbtituents attached to the triazine rings (CH3, OCH3 and CN). Metal-ligand bonds in trivalent lanthanum, neodynium, americium and curium complexes are predominantly ionic, and significant variations are found in metal-nitrogen bond lengths when the total charge of the complexes is changed by the number of counterions or when the charges on the coordinating nitrogen atoms are altered by substituents. Uranium-ligand bonds are more covalent and smaller variations are found in uranium-nitrogen bond distances. The most remarkable effect on uranium-ligand bonds is obtained by attaching CN groups to the Btp, uranium(5f)-to-Btp(π*) back-bonding is enhanced and uranium-nitrogen bonds are shortened.